FOUNDATION – PHYSIOLOGY

The glycocalyx is a carbohydrate-rich layer on the outer surface of the cell membrane. It consists of glycoproteins and glycolipids and plays an important role in cell-to-cell adhesion, cell recognition, and communication. The sticky nature of the glycocalyx allows cells to bind to each other, which is essential for tissue formation and immune responses.

Why the other options are incorrect:

• Integral protein:

  • These are embedded within the lipid bilayer and function mainly as channels, receptors, and transporters — while some contribute to adhesion, the glycocalyx is the primary structure responsible.

• Cytosol:

  • The fluid inside the cell — it doesn’t interact directly with the outer membrane or play a role in cell adhesion.

• Lipoprotein:

  • These are complexes of lipids and proteins, mainly involved in lipid transport in the bloodstream, not in cell-to-cell adhesion.

• Peripheral protein:

  • These are loosely attached to the inner or outer surface of the cell membrane and provide structural support and signaling — they do not mediate adhesion.

Red blood cells (RBCs) undergo osmosis when placed in solutions of different tonicity. A 0.1% sodium chloride (NaCl) solution is hypotonic relative to the normal physiological concentration of NaCl in the body (0.9% NaCl, or isotonic solution).

• In a hypotonic solution, there is a lower concentration of solutes outside the RBC than inside.
• Water moves into the RBC by osmosis to balance the solute concentration.
• This causes the RBCs to swell as they take in water.
• If excessive water enters, the RBCs burst (hemolysis) due to the increased internal pressure.

Why the Other Options Are Incorrect:

1. Shrink and burst

   • ❌ Incorrect because RBCs do not shrink in a hypotonic solution; instead, they swell and lyse. Shrinking (crenation) occurs in hypertonic solutions (e.g., >0.9% NaCl).

2. Remain same

   • ❌ Incorrect because water movement occurs, leading to swelling and bursting of RBCs in a hypotonic solution. The RBCs would remain the same only in an isotonic solution (0.9% NaCl).

3. Swell only

   • ❌ Incorrect because while RBCs initially swell, they eventually burst (lyse) due to excessive water intake.

4. Shrink

   • ❌ Incorrect because shrinking (crenation) occurs in hypertonic solutions, where water moves out of the cell. A 0.1% NaCl solution is hypotonic, causing water to enter instead.

• Total body water makes up about 50-60% of body weight in average adults.
• This percentage can vary based on age, sex, and body composition:
  • Men: Closer to 60% because they typically have more muscle mass, which holds more water.
  • Women: Closer to 50% because they generally have more fat tissue, which contains less water.
  • Infants: Can have up to 70-75% total body water, making them more susceptible to fluid imbalances.
  • Elderly: Total body water often decreases with age, sometimes dropping below 50%.

2. Breakdown of total body water:

• Intracellular fluid (ICF): About two-thirds (≈ 66%) of total body water is inside cells.
• Extracellular fluid (ECF): About one-third (≈ 33%) is outside cells, including:
  • Interstitial fluid: Surrounds cells.
  • Plasma: The fluid part of blood.
  • Transcellular fluid: In specialized spaces like CSF, synovial fluid, and peritoneal fluid.

3. Why the other options are incorrect:

• 60-70%:

  • This is too high for most adults but true for infants and newborns.

• 10-20%:

  • This would indicate severe dehydration and incompatible with life.
  • Even loss of 10% of body water can lead to serious physiological effects.

• 30-40%:

  • This is too low for a healthy adult but could be closer to elderly individuals with higher fat content.

• 70-80%:

  • Only seen in neonates, not in adults.

The question asks about the effects of parasympathetic stimulation. The parasympathetic nervous system is one of the two divisions of the autonomic nervous system and is responsible for the “rest and digest” response. It promotes activities that conserve and restore energy. Let’s analyze each option to determine why the correct answer is Pupillary constriction and why the others are incorrect.

Correct Answer: Pupillary constriction

– Why it’s correct: Parasympathetic stimulation causes pupillary constriction (miosis) via the activation of the oculomotor nerve (CN III). This occurs when the circular muscles of the iris (sphincter pupillae) contract, reducing the size of the pupil. This response is part of the parasympathetic system’s role in maintaining homeostasis and protecting the eye from excessive light.

Incorrect Answers:

Increased heart rate 
– Why it’s wrong: Parasympathetic stimulation decreases heart rate by activating the vagus nerve (CN X), which releases acetylcholine to slow down the sinoatrial (SA) node in the heart. Increased heart rate is an effect of sympathetic stimulation.

Vasoconstriction
– Why it’s wrong: Vasoconstriction (narrowing of blood vessels) is primarily mediated by the sympathetic nervous system, which releases norepinephrine to increase blood pressure and redirect blood flow. Parasympathetic stimulation does not cause vasoconstriction.

Increased blood pressure
– Why it’s wrong: Parasympathetic stimulation generally lowers blood pressure by decreasing heart rate and promoting vasodilation in certain areas (e.g., salivary glands, genitalia). Increased blood pressure is a result of sympathetic stimulation.

Vasodilation
– Why it’s wrong: While parasympathetic stimulation can cause vasodilation in specific regions (e.g., salivary glands, genitalia), it is not a systemic effect. The question does not specify a particular region, so this option is too broad and not the best answer.

Clathrin-mediated endocytosis is a specific type of endocytosis where clathrin proteins form a coated vesicle that helps internalize substances from the extracellular environment into the cell.

• Clathrin is a trimeric protein that assembles into a lattice-like structure, forming a clathrin-coated pit on the plasma membrane.
• These pits engulf extracellular molecules (e.g., nutrients, receptors, and viruses) and form vesicles that are transported into the cytoplasm.
• The vesicle later sheds its clathrin coat and fuses with endosomes for further processing.

Examples of Clathrin-Mediated Endocytosis:

✅ Receptor-mediated endocytosis (e.g., uptake of LDL cholesterol, transferrin, and growth factors).
✅ Synaptic vesicle recycling in neurons.
✅ Viral entry (some viruses use clathrin-mediated endocytosis to infect cells).

Why the Other Options Are Wrong:

1. Osmosis ❌

   • Osmosis is the passive movement of water across a semipermeable membrane due to a concentration gradient.
   • It does not involve clathrin or vesicles.

2. Diffusion ❌

   • Diffusion is the passive movement of molecules (e.g., gases like O₂, CO₂) across a membrane.
   • It does not require clathrin or vesicle formation.

3. Exocytosis ❌

   • Exocytosis is the release of substances from the cell via vesicle fusion with the membrane.
   • It is not dependent on clathrin but rather involves SNARE proteins and microtubules.

4. Facilitated Diffusion ❌

   • Facilitated diffusion involves carrier or channel proteins helping molecules cross the membrane without energy input.
   • It does not involve vesicles or clathrin.

The term “hypoesthesia” refers to a reduction in the sensation of touch or feeling, often associated with numbness or diminished tactile sensation in the skin. The question involves a paralyzed patient and asks which sense is affected when hypoesthesia is present.

Breakdown of the Answer Choices:

1. Hearing:

   • Explanation: Hearing is the ability to perceive sound and is mediated by the auditory system, including the ears and the auditory cortex of the brain. It is not directly related to touch sensation or hypoesthesia.
   • Why it’s incorrect: Hypoesthesia affects touch sensation, not hearing. Thus, hearing would not be directly impacted in this case.

2. Vision:

   • Explanation: Vision is the sense of sight, processed by the eyes and the visual cortex of the brain. It is unrelated to touch and hypoesthesia.
   • Why it’s incorrect: Hypoesthesia is about diminished touch sensation, not vision. Vision would not be affected by hypoesthesia.

3. Smell:

   • Explanation: Smell is mediated by the olfactory system and is distinct from tactile sensation. Hypoesthesia does not affect the olfactory system.
   • Why it’s incorrect: Hypoesthesia is not related to the sense of smell. This is a completely different sensory pathway.

4. Touch (Correct Answer):

   • Explanation: Touch refers to the ability to perceive physical sensations like pressure, temperature, pain, and vibration through the skin. Hypoesthesia specifically refers to reduced sensation of touch. This condition can result from damage to the sensory pathways in the nervous system, such as nerve injury or spinal cord issues, which may also contribute to paralysis.
   • Why it’s correct: Hypoesthesia directly refers to the reduction of touch sensation. Since it is described as hypoesthesia, the affected sense is touch.

5. Taste:

   • Explanation: Taste is mediated by the gustatory system, which involves the taste buds and the gustatory cortex in the brain. It is not typically associated with touch or hypoesthesia.
   • Why it’s incorrect: Hypoesthesia does not affect taste. The sense of taste relies on a different pathway and is unrelated to the sensory changes described in hypoesthesia.

The question is about identifying the most abundant cells in the human body. To answer this, we need to consider the relative numbers of different cell types and their roles in the body. Let’s analyze each option to determine which cell type is the most abundant.

Correct Answer: Red blood cells

Why are red blood cells correct?
– Red blood cells (RBCs), also known as erythrocytes, are the most abundant cells in the human body.
– They are produced in the bone marrow and are responsible for transporting oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.
– The average adult human has about 20–30 trillion red blood cells, making up approximately 70% of all cells in the body.
– Their high abundance is due to their critical role in oxygen transport and their relatively short lifespan (about 120 days), which necessitates continuous production.

Incorrect Options:

White blood cells:
– White blood cells (WBCs), or leukocytes, are part of the immune system and defend the body against infections and foreign invaders.
– Why it’s wrong: WBCs are far less abundant than RBCs, with only about 4,000–11,000 WBCs per microliter of blood, compared to 4–6 million RBCs per microliter.

Platelets:
– Platelets, or thrombocytes, are small cell fragments involved in blood clotting and wound healing.
– Why it’s wrong:Platelets are not true cells and are much less abundant than RBCs, with about 150,000–450,000 platelets per microliter of blood.

Neurons:- Neurons are the functional cells of the nervous system, responsible for transmitting electrical and chemical signals.
– Why it’s wrong:While the brain contains about 86 billion neurons, this number is far smaller than the number of RBCs in the body.

Cells of bone marrow:
– Bone marrow contains stem cells that produce blood cells (RBCs, WBCs, and platelets) and other cell types.
– Why it’s wrong: While bone marrow is a site of active cell production, the total number of bone marrow cells is much smaller than the number of RBCs.

Worn-out organelles or cellular debris are transported to lysosomes via autophagosomes. The process, called autophagy, involves the formation of double-membraned vesicles (autophagosomes) that engulf the damaged organelles. These vesicles then fuse with lysosomes, where the contents are broken down by enzymes.

Why the Other Options Are Wrong:

1. Mitochondria:

   • Mitochondria are not involved in transporting worn-out organelles to lysosomes; instead, they may be targeted by autophagosomes themselves in a process called mitophagy.

2. Granular endoplasmic reticulum:

   • The rough endoplasmic reticulum (granular) is involved in protein synthesis and has no role in the transport of worn-out organelles to lysosomes.

3. Residual bodies:

   • Residual bodies are the remnants of lysosomes after they have digested materials. They are not responsible for bringing organelles to the lysosome.

4. Agranular endoplasmic reticulum:

   • The smooth endoplasmic reticulum (agranular) is involved in lipid synthesis and detoxification but not in the transport of worn-out organelles to lysosomes.

The parasympathetic nervous system (PNS) is responsible for the “rest and digest” functions, promoting relaxation, digestion, and energy conservation. One of its effects is bronchoconstriction, which reduces airflow by narrowing the bronchi. This is mediated by the vagus nerve (CN X) through acetylcholine acting on muscarinic receptors (M3 receptors) in the lungs.

Why the Other Options Are Incorrect:

1. Ejaculation

   • ❌ Incorrect because ejaculation is controlled by the sympathetic nervous system (“fight or flight” response).
   • The mnemonic “Point and Shoot” helps remember that:
     • Parasympathetic controls erection (Point)
     • Sympathetic controls ejaculation (Shoot)

2. Dilation of pupil (Mydriasis)

   • ❌ Incorrect because pupil dilation is a sympathetic function.
   • The parasympathetic system causes pupil constriction (miosis) via the oculomotor nerve (CN III).

3. Relaxation of internal anal sphincter

   • ❌ Incorrect because the internal anal sphincter is primarily controlled by the autonomic nervous system.
   • Sympathetic stimulation keeps it contracted, while parasympathetic stimulation allows relaxation only when defecation is voluntarily initiated (not a direct effect).

4. Tachycardia (Increased heart rate)

   • ❌ Incorrect because tachycardia (increased heart rate) is a sympathetic effect.
   • The parasympathetic system decreases heart rate via the vagus nerve (CN X) by acting on M2 muscarinic receptors in the heart.

Why is this correct?
The sodium-potassium (Na+/K+) ATPase pump plays a critical role in maintaining cell volume by regulating the osmotic balance across the cell membrane. It does this by actively transporting:
– 3 sodium ions (Na+) out of the cell
– 2 potassium ions (K+) into the cell
This creates an electrochemical gradient that helps control the movement of water into and out of the cell, preventing cell swelling (edema) or shrinkage (crenation). Without this pump, cells would lose their ability to regulate osmotic pressure, leading to volume dysregulation.

Incorrect Answers
 It is an ionic channel passing ions through diffusion

– The Na+/K+ ATPase pump is not a passive ion channel. It is an active transporter that uses energy from ATP hydrolysis to move ions against their concentration gradients.
– Why it’s wrong: The pump actively transports ions, while ion channels allow passive diffusion.

 Uses the energy produced by the potassium
– The Na+/K+ ATPase pump uses energy from ATP hydrolysis, not from potassium ions. The energy derived from breaking the high-energy phosphate bond in ATP drives the conformational changes needed to transport Na+ and K+ against their gradients.
– Why it’s wrong: The energy source is ATP, not potassium.

It moves 3Na+ inside the cell and 2K+ outside the cell 
– This statement reverses the direction of ion movement. The pump moves 3 Na+ out of the cell and 2 K+ into the cell per ATP hydrolyzed.
– Why it’s wrong: The direction of ion transport is incorrect in this statement.

It is not inhibited by metabolic poisons 
– The Na+/K+ ATPase pump is highly sensitive to metabolic poisons such as ouabain and digitalis, which inhibit its function by binding to the extracellular side of the pump. These poisons block ATPase activity, disrupting ion transport.
– Why it’s wrong: The pump is indeed inhibited by metabolic poisons.

• Na⁺ will accumulate inside the cell because it is no longer being pumped out.
• K⁺ will leak out of the cell and not be replenished.
• Water follows sodium due to osmotic forces — where sodium goes, water follows!
• As a result:
  • The intracellular osmotic pressure increases.
  • Water rushes into the cell to balance this osmotic difference.
  • The cell swells due to water influx.
    

    ❌ Why Are the Other Options Incorrect?

    1. Cell necrosis:

       • While necrosis may eventually occur if the pump dysfunction persists, the first and primary event is swelling.
       • ❌ Incorrect as an immediate effect.

    2. DNA damage:

       • This pump dysfunction affects ion balance and osmotic pressure, not directly related to DNA damage.
       • ❌ Incorrect.

    3. Cell will shrink:

       • If the pump fails, Na⁺ accumulates inside, pulling water into the cell, causing swelling, not shrinkage.
       • ❌ Incorrect.

    4. Membrane damage:

       • Membrane damage might result secondarily from excessive swelling, but it is not the direct and first consequence of pump failure.
       • ❌ Incorrect.

In the immune response, epitopes are the specific parts of an antigen that are recognized by the immune system. The immune system has specialized cells that can recognize, process, and store information about these epitopes for future responses. Let’s break down the options:

1. Hematopoietic stem cells:

Hematopoietic stem cells are multipotent stem cells located in the bone marrow that give rise to all blood cells, including B cells and T cells. They do not store information about epitopes. Therefore, they are not responsible for storing information about antigens.

2. Plasma B cells:

Plasma B cells are the effector form of B cells that produce antibodies in response to an antigen. While plasma cells secrete antibodies that bind to antigens, they do not store information about the epitope. Instead, they respond actively to current infections by producing large amounts of antibodies. Therefore, they are not responsible for storing epitope information.

3. Helper T cells:

Helper T cells (CD4+ T cells) are involved in coordinating the immune response by activating other immune cells, including B cells and cytotoxic T cells. They do recognize antigens through their T cell receptors, but they do not store information about the epitopes of those antigens. Their main role is to activate and regulate other immune responses.

4. Cytotoxic T cells:

Cytotoxic T cells (CD8+ T cells) recognize infected cells displaying antigen fragments (epitopes) on their surface in association with MHC class I molecules. They are involved in killing infected or cancerous cells. While they are crucial in identifying and responding to epitopes, they do not store information about these epitopes for future responses.

5. Memory B cells: (Correct)

Memory B cells are specialized B cells that store information about the epitope of a previously encountered antigen. These cells are formed after an initial exposure to an antigen and “remember” the specific epitope. If the body encounters the same antigen again, memory B cells can quickly differentiate into plasma cells to produce antibodies, thus providing a faster and more efficient immune response. Therefore, memory B cells are responsible for storing information about the epitope of an antigen.

Nephrotic syndrome is the classical condition associated with significant proteinuria.

❌ Why Are the Other Options Incorrect?

1. Hepatic failure:

   • Liver failure affects protein synthesis, but it does not typically cause protein loss through urine.
   • ❌ Incorrect.

2. Down syndrome:

   • A genetic condition (trisomy 21) affecting development; not primarily a renal disease causing proteinuria.
   • ❌ Incorrect.

3. Kawasaki disease:

   • A vasculitis in children, affecting coronary arteries — not primarily associated with protein loss in urine.
   • ❌ Incorrect.

4. Diabetes:

   • While diabetic nephropathy (a complication of diabetes) can cause proteinuria, proteinuria is not universally present in all diabetes cases — it occurs only when kidney damage progresses.
   • So Diabetes can lead to proteinuria, but Nephrotic syndrome is the classical and direct cause of significant proteinuria.
   • ❌ Incorrect as per the most accurate and direct cause asked here.

Cilia and flagella are hair-like structures found on the surface of many cells, and they play a crucial role in movement and the movement of substances across cell surfaces. Cilia are found in respiratory epithelial cells and help to clear debris, while flagella are used by sperm cells for motility.

When there is a dysfunction in the movement of cilia and flagella, it can lead to several clinical conditions. Let’s break down the options:

Cystic fibrosis:

Cystic fibrosis is a genetic disorder that primarily affects the lungs, pancreas, and other organs. It leads to the production of thick, sticky mucus, which can clog airways and lead to respiratory problems. While cystic fibrosis is associated with abnormal ciliary function due to impaired mucociliary clearance, it is not specifically caused by immobile cilia. Therefore, this is not the correct answer.

Hurler disease:

Hurler disease (also known as mucopolysaccharidosis type I) is a rare genetic disorder caused by the deficiency of the enzyme alpha-L-iduronidase, leading to the accumulation of glycosaminoglycans in various tissues. This condition affects multiple organs and systems, but it is not related to immobile cilia or flagella. Therefore, this is not the correct answer.

Carcinoma:

Carcinoma refers to a type of cancer that originates in epithelial tissues. It has no direct relationship with the immobility of cilia or flagella. Therefore, carcinoma is not associated with this condition.

Immotile cilia syndrome:

Immotile cilia syndrome (also known as Primary ciliary dyskinesia) is a condition characterized by the lack of motility in cilia and flagella. This results in various symptoms, including respiratory tract infections due to the inability to clear mucus from the lungs, infertility in males due to immotile sperm flagella, and other problems. This disease is specifically caused by defects in the dynein arms or other components necessary for the movement of cilia and flagella, making option 4 the correct answer.

Pernicious anemia:

Pernicious anemia is a type of vitamin B12 deficiency anemia caused by the inability to absorb vitamin B12 from the digestive tract due to the lack of intrinsic factor. It is not related to the immobility of cilia or flagella. Therefore, this is not the correct

Adaptive control refers to the process by which the brain receives sensory feedback from movements and then modifies future movements to improve accuracy and efficiency.

• When a movement is performed, sensory receptors (muscle spindles, Golgi tendon organs, proprioceptors) send signals to the brain (cerebellum, motor cortex, basal ganglia) to evaluate its correctness.
• If the movement is inaccurate, the brain adapts and refines the motor commands for the next time the movement is executed.
• This is crucial for motor learning, coordination, and skill development.

Why not the other options?

• Stimulus enhancement → Refers to increased attention to a stimulus due to prior exposure, not movement correction.
• Rapid feedback mechanism → Describes immediate correction during movement (e.g., reflexes), not long-term learning.
• Rapid negative feedback mechanism → Involves immediate correction by inhibiting excessive action, but does not modify future movement.
• Positive feedback mechanism → Increases the intensity of a process (e.g., labor contractions), but does not refine movement.

Thus, the correct answer is “Adaptive control,” as it describes the brain’s ability to refine future movements based on past sensory feedback.

The answer is Shivering of skeletal muscles.

Here’s why:

When exposed to cold temperatures, the body’s primary goal is to generate heat and conserve the heat it has. Shivering is a rapid, involuntary contraction of skeletal muscles. This muscle activity generates heat, helping to raise the body’s core temperature.

Why the other options are incorrect:

• Increased sweating: Sweating is a mechanism for cooling the body, not warming it. It would be counterproductive in a cold environment.

• Vasodilation: Vasodilation is the widening of blood vessels. While it can increase blood flow to the skin, it also increases heat loss. In a cold environment, the body would primarily use vasoconstriction (narrowing of blood vessels) to conserve heat.

• Relaxation of piloerector muscles: Piloerector muscles are the small muscles attached to hair follicles. Their contraction causes “goosebumps.” While this can trap a thin layer of air for slight insulation, it’s a minor effect compared to shivering. Relaxation of these muscles would have the opposite effect – making you colder.

• Relaxation of skeletal muscles: Muscle relaxation would decrease heat production. This is the opposite of what the body needs in a cold environment.

Smooth muscle is found in the walls of blood vessels (among other structures like the digestive tract and airways). It is responsible for vasodilation (widening of blood vessels) and vasoconstriction (narrowing of blood vessels), which regulate blood flow and pressure. Smooth muscle is involuntary and controlled by the autonomic nervous system.

The other options are incorrect:

• Cardiac muscle: Found only in the heart and responsible for pumping blood, not controlling blood vessel diameter.
• Skeletal muscle: Attached to bones and responsible for voluntary movement, not involved in blood vessel regulation.

Isosmotic volume contraction refers to a loss of fluid from the body that is isotonic (same osmolarity as plasma), leading to a decrease in extracellular fluid volume without a change in osmolarity. This occurs in conditions where both water and electrolytes are lost in equal proportions.

• Diarrhea: Results in the loss of isotonic fluid from the gastrointestinal tract, causing isosmotic volume contraction.
• Increased antidiuretic hormone (ADH) secretion: Leads to water retention and hyponatremia, not isosmotic volume contraction.
• Increased absorption of salts from the intestine: Does not typically cause volume contraction.
• Excessive blood loss: Causes hypovolemia but is not necessarily isosmotic, as it involves loss of both plasma and cells.

The question is about the mechanism of hyperpolarization during an action potential, a key concept in physiology, particularly in neurophysiology. Hyperpolarization refers to a change in the membrane potential of a cell to a more negative value than the resting membrane potential. Let’s analyze each option to determine which one correctly explains the cause of hyperpolarization.

Correct Answer: Prolonged opening of voltage-gated K⁺ channels

Why is this correct?
– During an action potential, voltage-gated K⁺ channels open in response to membrane depolarization. These channels open more slowly than voltage-gated Na⁺ channels and remain open for a longer duration.
– As K⁺ ions move out of the cell due to the electrochemical gradient, the membrane potential becomes more negative, leading to hyperpolarization.
– This prolonged efflux of K⁺ ions drives the membrane potential closer to the equilibrium potential for K⁺ (approximately -90 mV), which is more negative than the resting membrane potential (approximately -70 mV).

Incorrect Options:

Prolonged opening of voltage-gated sodium channels:
– Voltage-gated Na⁺ channels are responsible for the rapid depolarization phase of the action potential. They open quickly in response to membrane depolarization and then inactivate shortly afterward.
– Why it’s wrong: Prolonged opening of Na⁺ channels would cause sustained depolarization, not hyperpolarization. Additionally, Na⁺ channels do not remain open long enough to contribute to hyperpolarization.

Closure of Cl⁻ channels:
– Chloride ions (Cl⁻) typically contribute to the resting membrane potential. Closure of Cl⁻ channels would reduce the influx of Cl⁻ ions, which could slightly depolarize the membrane.
– Why it’s wrong: Closure of Cl⁻ channels does not directly cause hyperpolarization. In fact, it would have the opposite effect by reducing the negative charge inside the cell.

Sodium leaky channels:
– Sodium leak channels allow a small, constant influx of Na⁺ ions into the cell, contributing to the resting membrane potential.
– Why it’s wrong:Sodium leak channels do not play a significant role in hyperpolarization. Their activity is more relevant to maintaining the resting membrane potential.

Prolonged closure of K⁺ ion channels:
– If K⁺ channels were to remain closed for an extended period, K⁺ ions would not efflux from the cell, and the membrane potential would not become more negative.
– Why it’s wrong:Prolonged closure of K⁺ channels would prevent hyperpolarization, as the efflux of K⁺ ions is necessary to drive the membrane potential toward more negative values.

Gap junctions form tube-like channels between adjacent cells, allowing for direct communication and the passage of ions, nutrients, and signaling molecules.

• In cardiomyocytes (heart muscle cells), gap junctions are essential for electrical coupling and synchronized contraction.
• They are formed by connexins, which create connexon channels, allowing ions like Ca²⁺ and Na⁺ to pass freely between cells.
• This enables the rapid spread of action potentials, ensuring coordinated contraction of the heart.

Why the Other Options Are Wrong:

1. Occluding junction (Tight Junction) – (Incorrect, Because It Acts as a Barrier, Not a Channel)

   • Tight junctions create a seal between cells to prevent leakage (e.g., in the intestines and blood-brain barrier).
   • They do not form tube-like channels or allow electrical communication.

2. Desmosome – (Incorrect, Because It Provides Mechanical Strength, Not Communication)

   • Desmosomes anchor adjacent cells together, providing mechanical strength (e.g., in skin and cardiac muscle).
   • They do not form direct channels for communication like gap junctions.

3. Hemidesmosome – (Incorrect, Because It Attaches Cells to the Basement Membrane, Not to Each Other)

   • Hemidesmosomes attach cells to the extracellular matrix (like the basement membrane), not to adjacent cells.
   • They are not involved in intercellular communication.

4. Tight junction – (Incorrect, Because It Seals Cells Rather Than Connecting Them for Communication)

   • Tight junctions seal adjacent cells, preventing fluid leakage (e.g., in epithelial barriers).
   • They do not facilitate ion passage or electrical conduction.

The single cell is the simplest unit of a living system. Cells are the basic structural and functional units of all living organisms. They can carry out all the processes necessary for life, such as metabolism, reproduction, and response to stimuli. Unicellular organisms, like bacteria and protozoa, consist of a single cell, while multicellular organisms are composed of many cells organized into tissues, organs, and systems.

Why the other options are wrong:

1. Multiple cells: While multiple cells make up tissues and organs, they are not the simplest unit. A single cell is simpler and more fundamental.

2. Organs: Organs are composed of multiple tissues and are more complex than a single cell.

3. Bones: Bones are organs made up of multiple cell types and tissues, making them more complex than a single cell.

4. Tissues: Tissues are groups of similar cells working together, making them more complex than a single cell.

Depolarization occurs when the membrane potential becomes less negative (more positive) due to a change in membrane permeability that allows positive ions (mostly Na⁺ or Ca²⁺) to enter the cell.

• In neurons and skeletal muscle cells, depolarization is mainly due to the opening of voltage-gated Na⁺ channels, allowing Na⁺ influx.
• In cardiac pacemaker cells, depolarization is initiated by Ca²⁺ influx through voltage-gated Ca²⁺ channels.

The key trigger for depolarization is a change in membrane permeability, leading to the inward movement of positively charged ions.

Why the Other Options Are Incorrect

• Opening of slow K⁺ channels:
  Potassium efflux through slow K⁺ channels occurs during repolarization, not depolarization.

• Rapid movement of Ca²⁺ out of cells:
  Calcium exit from the cell would make the inside more negative, which does not contribute to depolarization. Instead, Ca²⁺ influx can initiate depolarization in cardiac pacemaker cells.

• Closing of Na⁺ channels:
  Closing Na⁺ channels would stop depolarization. Instead, opening of Na⁺ channels is responsible for initiating depolarization in neurons and muscles.

• Rapid movement of K⁺ out of cells:
  This occurs during repolarization, as K⁺ efflux restores the resting membrane potential after depolarization.

This range represents the normal physiological concentration of sodium ions in the extracellular fluid (including blood plasma). Maintaining sodium within this narrow range is crucial for proper fluid balance, nerve impulse transmission, muscle contraction, and other essential bodily functions.

Why other options are incorrect:

• 12-40 mEq/L: This range is far too low for blood sodium levels and would be indicative of a serious electrolyte imbalance (hyponatremia).

• 30-70 mEq/L: This range is also too low and would be incompatible with life.

• 1.3-4.5 mEq/L: This is the normal range for potassium (K+) in the blood, not sodium.

• 20-50 mEq/L: Again, far too low for sodium and would signify a severe electrolyte deficiency.

Explanation:

The blood vessels in the skin are primarily controlled by the autonomic nervous system, which regulates involuntary functions such as blood flow, heart rate, and digestion. More specifically, the sympathetic branch of the autonomic nervous system plays a key role in regulating the blood vessels in the skin. Let’s break down each option:

Autonomic sympathetic fibers:

The blood vessels in the skin are primarily controlled by the sympathetic fibers of the autonomic nervous system. These fibers release norepinephrine, which causes vasoconstriction (narrowing of blood vessels) to reduce blood flow, especially during cold conditions. In some situations, like increased temperature, sympathetic activation can also lead to vasodilation (widening of blood vessels) to help cool the body. Therefore, this is the correct answer.

Autonomic parasympathetic fibers:

The parasympathetic fibers of the autonomic nervous system typically have a role in regulating functions like digestion, slowing the heart rate, and promoting rest and relaxation. However, they do not play a significant role in regulating blood flow to the skin. Therefore, this is not the correct answer.

Somatic sympathetic fibers:

There is no specific category of somatic sympathetic fibers. The term “somatic” refers to voluntary control of body movements via the somatic nervous system, which controls skeletal muscles. Sympathetic fibers are part of the autonomic nervous system and are not classified under the somatic system. Therefore, this is not the correct answer.

Somatic sensory nerves:

Somatic sensory nerves transmit sensory information (such as pain, temperature, touch) from the skin to the brain. They are not responsible for regulating blood vessel tone or blood flow in the skin. Therefore, this is not the correct answer.

Somatic motor nerves:

Somatic motor nerves control voluntary muscle movements, but they do not regulate blood vessel tone or blood flow. The regulation of blood vessels is handled by the autonomic nervous system, specifically sympathetic fibers. Therefore, this is not the correct answer.

Extracellular fluid (ECF) includes blood plasma and interstitial fluid (the fluid surrounding cells). Sodium ions (Na⁺) are the most abundant cation in the ECF and play a crucial role in maintaining fluid balance, nerve impulse transmission, and muscle contraction. They are the primary determinant of ECF volume.   

Why the other options are incorrect:

• HCO₃⁻ (Bicarbonate): Bicarbonate is a major anion (negatively charged ion) in the ECF and is important for buffering pH. While it’s crucial for ECF function, it’s not the major cation.

• K⁺ (Potassium): Potassium is the major cation of intracellular fluid (ICF), the fluid inside cells. While potassium is present in the ECF, its concentration is much lower than sodium’s. The difference in Na⁺ and K⁺ concentrations across cell membranes is essential for many physiological processes.   

• Cl⁻ (Chloride): Chloride is the major anion in the ECF, balancing the positive charge of sodium. It’s an important electrolyte, but not the major cation.   

• Mg²⁺ (Magnesium): Magnesium is the second most abundant cation in intracellular fluid. While it’s present in the ECF, its concentration is much lower than sodium’s and it is not the major cation of the ECF.

Osmosis is the movement of water molecules across a semi-permeable membrane from an area of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration). This process continues until equilibrium is reached, where the water concentration becomes balanced on both sides of the membrane.

Key Characteristics of Osmosis:

• Passive process: Does not require energy (ATP).
• Semi-permeable membrane: Allows water to pass, but restricts larger solutes.
• Driven by concentration gradient: Water moves to dilute the higher solute concentration.

Real-life example:

• Absorption of water in the intestines.
• Reabsorption of water in kidney tubules.

Why the other options are incorrect:

• Facilitated diffusion: The passive movement of solutes (like glucose or ions) across a membrane through transport proteins, not water movement.
• Endocytosis: An active process where the cell engulfs large molecules or particles by forming vesicles, not water movement.
• Active transport: Movement of solutes against their concentration gradient, requiring energy (ATP) — water movement in osmosis is always passive.
• Diffusion: The passive movement of solutes from an area of high concentration to low concentration, but without a membrane specificity — not restricted to water.

Correct Answer: Potassium (K⁺)

Explanation:
The resting membrane potential of a cell is primarily determined by the movement of ions across the cell membrane. Potassium (K⁺) is the key ion responsible for maintaining the negative charge inside the cell. Here’s why:

1. Resting Membrane Potential: At rest, the cell membrane is more permeable to K⁺ than to other ions due to the presence of leaky potassium channels.
2. Potassium Gradient: There is a high concentration of K⁺ inside the cell and a low concentration outside. Due to the concentration gradient, K⁺ tends to diffuse out of the cell.
3. Effect on Membrane Potential: As K⁺ leaves the cell, it takes positive charges with it, making the inside of the cell more negative relative to the outside. This contributes to the resting membrane potential of approximately -70 mV.

Thus, increased permeability to K⁺ makes the inside of the cell more negative.

Why the Other Options Are Wrong:

1. Chloride (Cl⁻):
   • Chloride is a negatively charged ion. If the membrane becomes more permeable to Cl⁻, it would either enter the cell or leave depending on the electrochemical gradient. However, its movement does not directly contribute to making the inside of the cell more negative. In fact, Cl⁻ influx could make the cell less negative (more depolarized).
2. Sodium (Na⁺):
   • Sodium is a positively charged ion. Increased permeability to Na⁺ would cause it to rush into the cell due to its concentration gradient (high outside, low inside). This influx of positive charges would make the inside of the cell less negative (more depolarized), not more negative.
3. Calcium (Ca²⁺):
   • Calcium is also a positively charged ion. Like Na⁺, increased permeability to Ca²⁺ would cause it to enter the cell, making the inside less negative (more depolarized).
4. Hydrogen (H⁺):
   • Hydrogen ions are positively charged. Increased permeability to H⁺ would cause it to move into the cell, making the inside less negative (more depolarized).

Intracellular Ca²⁺ release from the sarcoplasmic reticulum (SR) is crucial for muscle contraction, and it is primarily regulated by IP₃ (Inositol 1,4,5-trisphosphate) in smooth muscle and certain signaling pathways.

How IP₃ Mediates Ca²⁺ Release from the Sarcoplasmic Reticulum?

✅ IP₃ is a second messenger produced via the phospholipase C (PLC) pathway.
✅ Mechanism:

• A hormone or neurotransmitter binds to a Gq protein-coupled receptor (Gq-GPCR).
• Phospholipase C (PLC) is activated, which cleaves PIP₂ (phosphatidylinositol 4,5-bisphosphate) into:
  • IP₃ → Diffuses into the cytoplasm and binds to IP₃ receptors on the SR, causing Ca²⁺ release.
  • DAG (diacylglycerol) → Activates protein kinase C (PKC).
    ✅ The released Ca²⁺ triggers muscle contraction in smooth muscle and other signaling functions in different tissues.

Why not the other options?

• Ca²⁺ ions → Ca²⁺ itself does not act as a second messenger; instead, it is regulated by second messengers like IP₃.
• cGMP → Mainly involved in smooth muscle relaxation (e.g., nitric oxide pathway in vasodilation).
• cAMP → Regulates cardiac muscle contraction by activating protein kinase A (PKA) but does not directly release Ca²⁺ from the SR.
• DAG → Works alongside IP₃ but activates PKC, not Ca²⁺ release.

Thus, the correct answer is “IP₃,” as it directly triggers the release of Ca²⁺ from the sarcoplasmic reticulum, leading to muscle contraction.

The question asks which structure, when activated, leads to the generation of an action potential. An action potential is a rapid rise and fall in membrane potential that allows neurons to transmit electrical signals. To answer this, we need to understand the roles of the different ion channels and pumps listed in the options. Let’s analyze each option to determine why the correct answer is Voltage-gated Na channel and why the others are incorrect.

Correct Answer: Voltage-gated Na channel
– Why it’s correct: The voltage-gated Na channel** is critical for the generation of an action potential. When the membrane potential reaches the threshold (around -55 mV), these channels open, allowing an influx of sodium ions (Na⁺) into the cell. This rapid influx of Na⁺ causes depolarization, which is the rising phase of the action potential. Without the activation of voltage-gated Na channels, the rapid depolarization required for an action potential cannot occur.

Incorrect Answers:

Voltage-gated K channel
– Why it’s wrong: Voltage-gated K channels are involved in the repolarization and hyperpolarization phases of the action potential, not its generation. These channels open later in the action potential, allowing potassium ions (K⁺) to leave the cell, which restores the negative membrane potential. They do not initiate the action potential.

Sodium leaky channel
– Why it’s wrong: Sodium leaky channels allow a small, constant influx of Na⁺ into the cell, contributing to the resting membrane potential. However, they do not play a direct role in the rapid depolarization required for the generation of an action potential. Their activity is not voltage-dependent and is not sufficient to trigger an action potential.

Na/K ATPase pump
– Why it’s wrong: The Na/K ATPase pump maintains the resting membrane potential by actively transporting 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, using ATP. While it is essential for maintaining ion gradients, it does not directly participate in the rapid depolarization or generation of an action potential.

Tandem domain channels
– Why it’s wrong: Tandem domain channels (e.g., TPCs or TRP channels) are involved in various cellular processes, such as calcium signaling and sensory transduction, but they are not directly responsible for the generation of action potentials in neurons. Their activation does not lead to the rapid depolarization required for an action potential.

The question is about the Na⁺/K⁺ pump, also known as the sodium-potassium ATPase. This pump is a critical membrane protein that maintains the electrochemical gradient across the cell membrane, which is essential for many cellular processes, including nerve impulse transmission and muscle contraction. Let’s analyze each option to determine the correct action of the Na⁺/K⁺ pump.

Correct Answer: 3Na⁺ out, 2K⁺ in

Why is this correct?
– The Na⁺/K⁺ pump actively transports **3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell for every ATP molecule hydrolyzed.
– This process requires energy (ATP) and helps maintain the resting membrane potential of the cell, which is crucial for electrical signaling in neurons and muscle cells.
– The pump creates a high concentration of Na⁺ outside the cell and a high concentration of K⁺ inside the cell, which is essential for many physiological processes.

Incorrect Options:

2Na⁺ out, 3K⁺ in:
– This option reverses the correct stoichiometry of the Na⁺/K⁺ pump.
– Why it’s wrong: The pump transports 3 Na⁺ out, not 2, and 2 K⁺ in, not 3.

2Na⁺ in, 3K⁺ out:
– This option incorrectly describes the direction of ion transport.
– Why it’s wrong: The pump moves Na⁺ out of the cell and K⁺ into the cell, not the other way around.

2Na⁺ in, 3K⁺ in:
– This option suggests that both Na⁺ and K⁺ are transported into the cell, which is incorrect.
– Why it’s wrong: The pump moves Na⁺ out of the cell and K⁺ into the cell.

3Na⁺ in, 2K⁺ out:
– This option reverses the direction of ion transport.
– Why it’s wrong:The pump moves Na⁺ out of the cell and K⁺ into the cell, not Na⁺ in and K⁺ out.

The G2 phase is the pre-mitotic phase, not the post-mitotic phase. It occurs after the S phase (DNA synthesis) and before mitosis (M phase). During the G2 phase, the cell prepares for division by synthesizing proteins and organelles and ensuring DNA replication is complete and accurate. The post-mitotic phase refers to the period after mitosis, which is part of the G1 phase or G0 phase in non-dividing cells.Why the other options are wrong:

1. Centrosome replicates in the S phase of the cell cycle:
This is correct. The centrosome, which organizes the mitotic spindle, replicates during the S phase.

2. Terminally differentiated cells are said to be permanently in G0 phase:
This is correct. Terminally differentiated cells, such as neurons and muscle cells, exit the cell cycle and remain in the G0 phase indefinitely.

3. In G1 phase cell volume increases:
This is correct. During the G1 phase, the cell grows in size and synthesizes proteins and organelles to prepare for DNA replication.

4. Rapidly dividing cells have shorter interphase:
This is correct. Rapidly dividing cells spend less time in interphase (G1, S, and G2 phases) and more time in mitosis (M phase).

The growth of hyaline cartilage, like the development of bones and other tissues, is influenced by several hormones, particularly those secreted by the anterior pituitary gland. The most important hormone involved in the growth of hyaline cartilage is somatotropin, also known as growth hormone (GH).

Let’s break down the options:

1. Gonadotropin:

Gonadotropins (such as FSH and LH) are hormones that primarily regulate the function of the gonads (ovaries and testes). They are not involved in the growth of hyaline cartilage. Therefore, this is not the correct answer.

2. Oxytocin:

Oxytocin is a hormone produced in the hypothalamus and released by the posterior pituitary. It plays a key role in childbirth and lactation but is not directly involved in cartilage growth. Therefore, this is not the correct answer.

3. Thyrotropin:

Thyrotropin, also known as TSH (Thyroid-Stimulating Hormone), stimulates the thyroid gland to release thyroid hormones that regulate metabolism. While thyroid hormones do affect growth and development, thyrotropin itself is not the main hormone responsible for the growth of hyaline cartilage. Therefore, this is not the correct answer.

4. Somatotropin (Growth hormone): (Correct)

Somatotropin, or growth hormone (GH), is produced by the anterior pituitary gland and plays a crucial role in stimulating growth, particularly by influencing the growth of bones, cartilage, and other tissues. It stimulates chondrocytes (cartilage cells) to produce the matrix of cartilage and also promotes overall growth and development. Therefore, this is the correct answer.

5. Androgen:

Androgens are male sex hormones, like testosterone, that influence the development of male secondary sexual characteristics and also have some effect on bone and muscle growth. However, they are not primarily responsible for the growth of hyaline cartilage. Therefore, this is not the correct answer.

The question asks for the percentage of total body weight that is water in a 70 kg man. Water makes up a significant proportion of the human body, but the exact percentage can vary based on factors like age, sex, and body composition (muscle mass vs fat).

Breakdown of the Answer Choices:

1. 75%:

   • Explanation: 75% water would be an unusually high proportion for an average adult. While newborns may have water content approaching this level, adults typically have a lower percentage of water.
   • Why it’s incorrect: This is too high for an adult male, especially at 70 kg.

2. 50%:

   • Explanation: 50% is low for the total water content in the body of a healthy adult man. It may apply to individuals with low muscle mass or high fat percentages, but it’s generally not the average for most adults.
   • Why it’s incorrect: While possible for some people, this is generally lower than the typical range for a healthy adult male.

3. 60% (Correct Answer):

   • Explanation: 60% is the approximate average percentage of body weight that is water in a typical adult male. This value can vary based on factors like age, sex, and body fat percentage. For a healthy adult man, water generally accounts for about 60% of total body weight.
   • Why it’s correct: 60% is the most typical estimate for an adult male’s total body water percentage.

4. 30%:

   • Explanation: 30% is too low to be considered a normal percentage of body water for a healthy adult. This could apply to individuals with extreme dehydration or very high body fat percentages.
   • Why it’s incorrect: This is far lower than the typical range for a healthy adult male.

5. 42%:

   • Explanation: 42% could be accurate for an individual with low muscle mass or high body fat, but it is not the average for a typical adult male.
   • Why it’s incorrect: It’s on the low end of the normal range but still lower than the typical 60%.

Among the given options, ventricular tachycardia (VT) is the most lethal dysrhythmia because it can rapidly deteriorate into ventricular fibrillation (VF), leading to sudden cardiac arrest and death if not treated immediately.

Why is Ventricular Tachycardia the Most Lethal?

• Origin: VT arises from the ventricles and is characterized by rapid, abnormal electrical activity that disrupts normal ventricular contraction.
• Effect on Cardiac Output: The ventricles contract so fast that they do not have enough time to fill with blood, leading to severely reduced cardiac output. This can cause hypotension, shock, and unconsciousness.
• Progression to VF: Sustained VT (lasting >30 seconds) can quickly progress to ventricular fibrillation (VF), a chaotic rhythm with no effective cardiac output, ultimately leading to death if untreated.
• Emergency Treatment: VT requires immediate intervention, including defibrillation, antiarrhythmic drugs (e.g., amiodarone, lidocaine), or cardioversion, depending on whether the patient is stable or unstable.

Why the Other Options Are Incorrect:

1. Atrial Fibrillation (AF):

• Common but not immediately lethal.
• Chaotic atrial impulses lead to irregular ventricular response, but the ventricles still contract and pump blood.
• Risk: Increases the risk of stroke due to blood stasis and clot formation in the atria. However, it is not immediately life-threatening.

2. Sick Sinus Syndrome (SSS):

• A dysfunction of the SA node, leading to episodes of bradycardia (slow heart rate), tachycardia, or sinus pauses.
• Can cause syncope (fainting) and fatigue but is not typically lethal unless it leads to asystole (complete heart stoppage).

3. Second-Degree Heart Block:

• A partial conduction delay between the atria and ventricles.
• It can cause bradycardia but is not immediately fatal, unless it progresses to complete heart block.

4. Complete Heart Block (Third-Degree AV Block):

• A complete failure of electrical conduction from the atria to the ventricles.
• The ventricles beat at their own slower rate (escape rhythm), leading to severe bradycardia and reduced cardiac output.
• While serious, it is not as acutely lethal as ventricular tachycardia because the ventricles still generate some rhythm. A pacemaker is the definitive treatment.

The question asks about the source of energy used in secondary active transport. Secondary active transport is a mechanism that moves molecules across a cell membrane using the energy stored in an electrochemical gradient, rather than directly using ATP. Let’s analyze each option to determine why the correct answer is Movement due to high gradient of Na+ ions outside and why the others are incorrect.

Correct Answer: Movement due to high gradient of Na+ ions outside

– Why it’s correct: Secondary active transport relies on the electrochemical gradient of Na+ ions across the cell membrane. This gradient is established by the Na+/K+ ATPase pump, which uses ATP to pump Na+ out of the cell and K+ into the cell, creating a high concentration of Na+ outside the cell. In secondary active transport, the movement of Na+ down its concentration gradient into the cell provides the energy to transport another molecule (e.g., glucose or amino acids) against its concentration gradient. This process is called co-transport (symport) or counter-transport (antiport).

Incorrect Answers:

Breakdown of glycogen 
– Why it’s wrong: The breakdown of glycogen (glycogenolysis) produces glucose, which can be used for energy production. However, this process is not directly involved in secondary active transport, which relies on the Na+ gradient, not glucose metabolism.

ATP
– Why it’s wrong: ATP is the direct energy source for primary active transport, such as the Na+/K+ ATPase pump. In secondary active transport, ATP is not used directly; instead, the energy comes from the Na+ gradient established by primary active transport.

ADP
– Why it’s wrong: ADP is a byproduct of ATP hydrolysis and is not a source of energy for transport processes. It is involved in energy metabolism but does not play a role in secondary active transport.

Movement of glucose/amino acid 
– Why it’s wrong: The movement of glucose or amino acids is the result of secondary active transport, not the energy source. These molecules are transported into the cell using the energy provided by the Na+ gradient.

Let’s break down different types of membrane transport to understand why antiport is the right answer:

• Antiport (Correct Option):

  • Antiporters are coupled transporters that move two different molecules or ions across the membrane in opposite directions.
  • One molecule moves into the cell, while the other moves out of the cell.
  • This movement often uses secondary active transport, where one molecule moves down its concentration gradient, providing the energy to move the other against its gradient.
  • Example:
    • Sodium-calcium exchanger (NCX): Na⁺ enters the cell while Ca²⁺ exits.
    • Sodium-hydrogen exchanger (NHE): Na⁺ enters while H⁺ leaves, helping regulate intracellular pH.

• Symport (Incorrect Option):

  • Symporters move two different molecules in the same direction across the membrane.
  • This also often uses secondary active transport, where one molecule moves down its gradient, driving the movement of the other against its gradient.
  • Example:
    • Sodium-glucose cotransporter (SGLT): Both Na⁺ and glucose move into the cell.

• Cotransport (Incorrect Option):

  • Cotransport is a general term for any coupled transport of two different molecules across the membrane — it can include symport or antiport.
  • It describes the mechanism but doesn’t specify the direction of the molecules.

• Uniport (Incorrect Option):

  • A uniporter transports a single type of molecule across the membrane, typically down its concentration gradient.
  • No energy input is needed in this facilitated diffusion process.
  • Example:
    • Glucose transporter (GLUT): Transports glucose into cells based on concentration gradient.

• Biport (Incorrect Option):

  • “Biport” is not a recognized term in physiology or biochemistry for membrane transport.

The parasympathetic nervous system (PNS) primarily uses acetylcholine (ACh) as its neurotransmitter at both preganglionic and postganglionic nerve terminals:

• Preganglionic neurons release acetylcholine (ACh) at nicotinic receptors (Nn) in the autonomic ganglia.
• Postganglionic neurons release acetylcholine (ACh) at muscarinic receptors (M1–M5) on target organs such as the heart, lungs, and digestive system.

This system is responsible for the “rest and digest” functions, including slowing heart rate, increasing digestion, and constricting pupils.

Why the Other Options Are Wrong:

1. Epinephrine and Acetylcholine → ❌ Wrong
   • Epinephrine is not released by parasympathetic neurons.
   • The adrenal medulla releases epinephrine, but it is part of the sympathetic nervous system.
2. Acetylcholine and Epinephrine → ❌ Wrong
   • Epinephrine is not released by parasympathetic neurons.
   • Epinephrine is primarily released by the adrenal medulla in response to sympathetic activation.
3. Acetylcholine and Norepinephrine → ❌ Wrong
   • Norepinephrine is the main neurotransmitter of the sympathetic postganglionic neurons, not the parasympathetic system.
   • In contrast, the parasympathetic system only uses acetylcholine.
4. Norepinephrine and Acetylcholine → ❌ Wrong
   • Norepinephrine is not released at parasympathetic preganglionic terminals.
   • Instead, preganglionic neurons release acetylcholine, which binds to nicotinic receptors (Nn).

Why is this correct?
Sertoli cells, also known as sustentacular cells, play a crucial role in supporting and nurturing developing sperm cells during spermatogenesis. One of their key functions is to isolate gamete cells from the bloodstream and other external influences. This is achieved through the formation of the blood-testis barrier, which is created by tight junctions between adjacent Sertoli cells. This barrier:
1. Protects developing sperm cells from the immune system, as they express unique antigens that could be recognized as foreign.
2. Maintains a specialized microenvironment for spermatogenesis by regulating the passage of nutrients, hormones, and other molecules.
3. Prevents toxins or harmful substances in the blood from affecting the developing sperm cells.

Thus, Sertoli cells are essential for creating a safe and controlled environment for spermatogenesis.

—

Incorrect Answers

Replicate type A spermatogonia
– Type A spermatogonia are stem cells that undergo mitosis to produce more spermatogonia or differentiate into type B spermatogonia. This replication process is intrinsic to the spermatogonia themselves and is not mediated by Sertoli cells.
– Why it’s wrong: Sertoli cells support and nurture developing sperm cells but do not directly participate in the replication of spermatogonia.

 Attach to basal lamina
– While Sertoli cells are anchored to the basal lamina of the seminiferous tubules, this is not their primary role in spermatogenesis. Their attachment is a structural feature, not a functional one.
– Why it’s wrong: This describes a structural relationship rather than a functional role in spermatogenesis.

Providing a population of type B spermatogonia
– Type B spermatogonia are produced by the differentiation of type A spermatogonia, not by Sertoli cells. Sertoli cells support the development of these cells but do not generate them.
– Why it’s wrong: The production of type B spermatogonia is a function of spermatogonial stem cells, not Sertoli cells.

4 Form the junctional complex 
– While Sertoli cells do form tight junctions with each other to create the blood-testis barrier, this is a means to an end (isolating gamete cells) rather than their primary role.
– Why it’s wrong: This is a mechanism by which Sertoli cells isolate gamete cells, but it is not their overarching role in spermatogenesis.

Peroxisomes: These organelles contain enzymes, most notably catalase, that break down various substances, including alcohol.

They play a crucial role in detoxifying harmful substances in the liver. They specifically handle the initial step in alcohol detoxification.

The resting membrane potential refers to the difference in charge across the membrane of a cell when it is not actively transmitting a signal. This potential is due to the distribution of ions (such as sodium and potassium) across the membrane and is maintained by ion channels and pumps, particularly the sodium-potassium pump.

In this question, we’re specifically asked about the resting membrane potential of a typical mammalian nerve fiber.

Breakdown of the Answer Choices:

1. -95 mV:

   • Explanation: A membrane potential of -95 mV is quite negative and typically found in some muscle fibers or specialized cells but is not the typical resting membrane potential for a nerve fiber.
   • Why it’s incorrect: This is not the standard value for the resting membrane potential of typical mammalian nerve fibers.

2. -105 mV:

   • Explanation: A membrane potential of -105 mV is too negative for a typical mammalian nerve fiber’s resting membrane potential. It is lower than the expected value for most cells.
   • Why it’s incorrect: This value is not commonly seen in typical mammalian nerve fibers.

3. -80 mV:

   • Explanation: A resting membrane potential of -80 mV is somewhat closer to the expected value, but it’s still slightly more negative than the typical resting membrane potential of a nerve fiber.
   • Why it’s incorrect: While -80 mV is close, it is still a bit more negative than the typical value for a nerve fiber.

4. -70 mV (Correct Answer):

   • Explanation: The resting membrane potential of a typical mammalian nerve fiber is approximately -70 mV. This is because the inside of the cell is more negatively charged relative to the outside due to the distribution of ions, mainly potassium (K⁺) inside the cell and sodium (Na⁺) outside the cell. This potential is maintained by the sodium-potassium pump, which actively pumps Na⁺ out and K⁺ in, and leak channels that allow for the passive movement of ions.
   • Why it’s correct: The standard resting membrane potential for a typical mammalian nerve fiber is about -70 mV. This is a general value for neurons, and it reflects the typical ionic gradients across the membrane.

5. -50 mV:

   • Explanation: A resting membrane potential of -50 mV is more positive than the typical value for most mammalian nerve fibers, which are usually in the -60 to -70 mV range.
   • Why it’s incorrect: -50 mV is too positive for the resting membrane potential of a typical mammalian nerve fiber.

The extracellular fluid (ECF) contains high sodium (Na⁺) and low potassium (K⁺) concentrations, while the intracellular fluid (ICF) contains the opposite (high K⁺, low Na⁺). This balance is crucial for nerve signaling, muscle contraction, and maintaining resting membrane potential.

Normal ECF Concentrations:

• Sodium (Na⁺): ~135-145 mEq/L
  • Most abundant extracellular cation.
  • Plays a key role in osmotic balance, nerve function, and blood pressure regulation.
• Potassium (K⁺): ~3.5-5.0 mEq/L
  • Major intracellular cation (higher inside cells, low outside).
  • Critical for maintaining resting membrane potential and action potentials.

The given option Na⁺ = 142 mEq/L and K⁺ = 4.2 mEq/L falls within the normal physiological range, making it the correct answer.

Why the Other Options Are Wrong:

1. Na⁺ = 145 mEq/L and K⁺ = 4.9 mEq/L – (Sodium on the higher end, potassium slightly elevated but still normal)

   • While 145 mEq/L Na⁺ is at the upper limit of normal, and 4.9 mEq/L K⁺ is also slightly high, the best choice for “normal values” is 142/4.2.

2. Na⁺ = 138 mEq/L and K⁺ = 4.6 mEq/L – (Sodium on the lower end, potassium slightly above normal)

   • 138 mEq/L Na⁺ is on the lower end of normal, and 4.6 mEq/L K⁺ is slightly above the preferred normal value.

3. Na⁺ = 130 mEq/L and K⁺ = 5.6 mEq/L – (Indicates Hyponatremia and Hyperkalemia, Abnormal Values)

   • 130 mEq/L Na⁺ suggests hyponatremia, which can cause confusion, seizures, and coma.
   • 5.6 mEq/L K⁺ indicates hyperkalemia, which can cause cardiac arrhythmias.

4. Na⁺ = 140 mEq/L and K⁺ = 4.0 mEq/L – (Very Close to Normal, But Slightly Less Accurate than 142/4.2 mEq/L)

   • This is almost correct, but 142/4.2 is more precise based on standard physiological values.

The alpha subunit of the Na+/K+ ATPase (sodium-potassium pump) is the catalytic subunit. It possesses the ATPase activity, meaning it hydrolyzes ATP (adenosine triphosphate) to ADP (adenosine diphosphate) and inorganic phosphate. This hydrolysis provides the energy required for the pump to transport sodium ions out of the cell and potassium ions into the cell against their concentration gradients.   

Why other options are incorrect:

• Beta: The beta subunit is a glycoprotein essential for the proper assembly and trafficking of the alpha subunit to the plasma membrane. It does not have ATPase activity.   

• Gamma, Delta, Sigma: These are not subunits of the Na+/K+ pump.

Sodium (Na⁺) concentration in the extracellular fluid (ECF):

• Sodium is the main cation of the ECF, and its concentration in the plasma and interstitial fluid is typically around 135–145 mEq/L.
• The most commonly cited average value is around 142 mEq/L.
• This high sodium concentration is essential for maintaining osmotic balance, fluid volume, and nerve and muscle function.

2. Why the other options are incorrect:

• 14 mmol/L and 14 mEq/L:

  • These values are much lower than the normal sodium concentration in the ECF.
  • Intracellular sodium concentration is closer to 14 mEq/L, not extracellular.

• 1.42 mEq/L:

  • This is far below the physiological range for sodium in the ECF.
  • Such a low sodium level would lead to severe hyponatremia, which is incompatible with life.

• 18.9 mmol/L:

  • This value is also too low for ECF sodium.
  • It’s closer to concentrations seen in certain intracellular compartments, but not the ECF.

• The alpha (α) subunit is the catalytic and functional core of the Na⁺/K⁺ ATPase pump.
  1. ATPase activity site — to hydrolyze ATP and release energy.
  2. Binding sites for Na⁺ and K⁺ ions.
  3. Phosphorylation sites — to mediate conformational changes required for ion transport.
     

     ❌ Why are the Other Options Incorrect?

     1. Beta (β):

        • Glycoprotein that helps in proper folding, stabilization, and membrane localization of the alpha subunit.
        • Does NOT have ATPase activity or direct ion binding.
        • ❌ Incorrect.

     2. Gamma (γ):

        • Part of the FXYD protein family, modulates pump function under specific conditions (e.g., in kidneys or heart).
        • Does NOT possess ATPase activity.
        • ❌ Incorrect.

     3. Sigma (σ) & Delta (δ):

        • Not recognized subunits of the Na⁺/K⁺ ATPase pump.
        • May exist as parts of other protein complexes but do not relate to this pump.
        • ❌ Incorrect.

The alpha subunit of the Na+/K+ ATPase (sodium-potassium pump) is the catalytic subunit. It contains the binding sites for both sodium (Na+) and potassium (K+), as well as the ATP binding site and the phosphorylation site. This subunit is responsible for the actual transport of the ions across the cell membrane.   

Why other options are incorrect:

• Beta: The beta subunit is a glycoprotein that is essential for proper assembly and trafficking of the alpha subunit to the cell membrane. It does not directly bind sodium or potassium.   

• Gamma, Delta, Sigma: These are not subunits of the Na+/K+ pump.

A symport is a type of secondary active transport where two or more substances move in the same direction across a membrane, driven by an electrochemical gradient.

• The Na-K-2Cl carrier protein is an example of a symport co-transporter. It is found in the thick ascending limb of the loop of Henle in the kidney and plays a crucial role in the reabsorption of sodium, potassium, and chloride.
• It simultaneously transports one sodium (Na⁺), one potassium (K⁺), and two chloride (Cl⁻) ions into the cell in the same direction.

Why the Other Options Are Wrong:

1. Sodium-hydrogen counter transport (Na⁺/H⁺ exchanger) – (Antiport, Not Symport)

   • This transporter exchanges sodium ions (Na⁺ moving in) with hydrogen ions (H⁺ moving out), meaning they move in opposite directions.
   • This is an example of antiport (counter-transport), not symport.

2. Sodium-calcium counter transport (Na⁺/Ca²⁺ exchanger) – (Antiport, Not Symport)

   • This exchanger moves Na⁺ into the cell and Ca²⁺ out of the cell (or vice versa), which is in opposite directions.
   • Since the movement is opposite, this is antiport, not symport.

3. Hydrogen pump (H⁺ ATPase) – (Primary Active Transport, Not Co-transport)

   • The hydrogen pump actively pumps H⁺ ions out of the cell using ATP, making it a primary active transport mechanism, not a co-transporter (symport or antiport).

4. Sodium-potassium pump (Na⁺/K⁺ ATPase) – (Primary Active Transport, Not Co-transport)

   • The Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in, using ATP.
   • This is an example of primary active transport, not a symport.

Histamine is a key mediator of allergic reactions and inflammation. It is stored in the basophilic granules of mast cells, which are a type of immune cell found in connective tissues throughout the body. When mast cells are activated (e.g., by allergens or tissue injury), they release histamine, leading to vasodilation, increased vascular permeability, and other inflammatory responses.

Why the other options are wrong:
1. Platelets: Platelets are involved in blood clotting and wound healing. They do not store or release histamine.

2. Adipocytes: Adipocytes are fat cells that store energy in the form of lipids. They are not involved in histamine release.

3. Fibroblasts: Fibroblasts are cells that produce collagen and other extracellular matrix components. They play a role in tissue repair but do not release histamine.

4. Plasma cells: Plasma cells are derived from B lymphocytes and produce antibodies. They are part of the adaptive immune system and do not release histamine.

The normal osmolarity of body fluids typically ranges between 280-295 mOsm/L, with an average value of ~280 mOsm/L. This measurement reflects the concentration of solutes (electrolytes, glucose, and urea) in body fluids.

• Plasma osmolarity is primarily determined by sodium (Na⁺) concentration, as well as glucose and urea.
• It is crucial for maintaining fluid balance between intracellular and extracellular compartments through osmosis.
• The body regulates osmolarity via mechanisms such as antidiuretic hormone (ADH) release and renal function.

Why the Other Options Are Wrong:

1. 300 mOsm/L – (Incorrect, Because the Normal Range Is 280-295 mOsm/L, and 300 Is Slightly Above Normal)

   • Some textbooks round up to 300 mOsm/L, but the more precise average value is 280 mOsm/L.
   • Values above 295 mOsm/L indicate hyperosmolarity, often seen in dehydration, diabetes insipidus, or hypernatremia.

2. 260 mOsm/L – (Incorrect, Because This Is Below the Normal Range, Suggesting Hypo-osmolarity)

   • Values below 280 mOsm/L suggest hypo-osmolarity, which occurs in conditions like hyponatremia (low sodium levels) or overhydration.

3. 250 mOsm/L – (Incorrect, Because It Falls Well Below Normal Plasma Osmolarity and Can Indicate Severe Hyponatremia)

   • This low osmolarity could cause cellular swelling due to excess water shifting into cells, potentially leading to neurological symptoms.

4. None of these – (Incorrect, Because 280 mOsm/L Is the Correct Answer)

   • The normal osmolarity of the body is clearly defined, so this option is incorrect.

The question asks about the response of the autonomic nervous system (ANS) to a stressful situation, such as a nervous student taking a viva exam. The autonomic nervous system has two main divisions: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). In stressful situations, the sympathetic nervous system dominates, preparing the body for a “fight or flight” response. Let’s analyze each option to determine why the correct answer is right and why the others are wrong.

Correct Answer: Increased heart rate

– Why it’s correct:The sympathetic nervous system increases heart rate (tachycardia) to pump more blood to muscles and vital organs, preparing the body for action. This is a classic response to stress, anxiety, or fear.
– Key point: Increased heart rate is a hallmark of sympathetic activation during stress.

Incorrect Answers:

Bronchoconstriction
– Why it’s wrong: The sympathetic nervous system causes bronchodilation, not bronchoconstriction. Bronchodilation allows for increased airflow to the lungs, ensuring adequate oxygen supply during stress. Bronchoconstriction is associated with the parasympathetic nervous system or conditions like asthma.
– Key point:Sympathetic activation leads to bronchodilation, not constriction.

Vasoconstriction
– Why it’s wrong:While vasoconstriction does occur in certain areas (e.g., the skin and digestive system) during sympathetic activation, it is not a universal response. In muscles and vital organs, vasodilation occurs to increase blood flow. The question does not specify a particular tissue, so this option is too narrow and potentially misleading.
– Key point:Vasoconstriction is context-dependent and not a primary systemic response to stress.

Constriction of pupil
– Why it’s wrong: The sympathetic nervous system causes pupil dilation (mydriasis), not constriction. Pupil dilation allows more light to enter the eye, improving vision in stressful situations. Constriction of the pupil (miosis) is a parasympathetic response.
– Key point:Sympathetic activation leads to pupil dilation, not constriction.

Increased gastrointestinal mobility
– Why it’s wrong: The sympathetic nervous system decreases gastrointestinal motility as part of the “fight or flight” response. Digestive processes are not a priority during stress, so blood flow and energy are redirected to muscles and the brain. Increased gastrointestinal mobility is a parasympathetic response.
– Key point: Sympathetic activation inhibits gastrointestinal activity.

Explanation:

The diffusion potential refers to the potential difference across a cell membrane that results from the movement of ions due to diffusion. This is also known as the equilibrium potential for an ion and can be described by the Nernst equation, which calculates the potential of a membrane when only one ion is considered. However, in a real biological membrane, several ions contribute to the overall membrane potential. Here’s a breakdown of the options:

Na+ (Sodium ions)

Sodium ions play a significant role in generating the resting membrane potential and action potentials, especially in excitable cells like neurons and muscle cells. However, sodium alone does not solely determine the overall diffusion potential since other ions also contribute. Therefore, this is not the correct answer.

Several different ions:

The overall diffusion potential or resting membrane potential is influenced by the movement of several ions, including potassium (K+), sodium (Na+), chloride (Cl-), and others. The most significant contributors are K+ and Na+, with potassium typically having the greatest influence due to its higher permeability across the membrane at rest. This is the correct answer, as multiple ions contribute to the membrane potential.

K+ (Potassium ions):

While K+ ions are the most significant in determining the resting membrane potential, they do not account for the full diffusion potential alone. Potassium ions contribute predominantly because of their high permeability across the membrane, but other ions like sodium and chloride also play a role in membrane potential. Therefore, this is not the complete answer.

Cl- (Chloride ions):

Chloride ions do contribute to the membrane potential, but their contribution is less significant than that of potassium and sodium ions. Chloride is typically in equilibrium across the membrane, and its effect is more passive. It does not solely determine the diffusion potential. Therefore, this is not the correct answer.

HCO3- (Bicarbonate ions):

Bicarbonate ions also have a minor effect on the cell’s membrane potential, particularly in regulating pH balance and contributing to the acid-base homeostasis. However, their movement is not primarily responsible for the diffusion potential of the cell membrane. Therefore, this is not the correct answer.

Pinocytosis is a form of endocytosis, often called “cell drinking,” where the cell engulfs extracellular fluid and dissolved molecules.

When large molecules bind to specific receptors on the plasma membrane, these receptors are commonly clustered in specialized areas called coated pits. These pits are invaginated regions of the membrane that are coated on their cytoplasmic side with a protein called clathrin.

The coated pit is the initial site where receptor-mediated endocytosis begins. Once the molecule binds to its receptor within this pit, the membrane invaginates and pinches off to form a coated vesicle, which will later shed its clathrin coat and fuse with endosomes for further processing.

Thus, coated pits are essential for the selective uptake of molecules in processes like pinocytosis and receptor-mediated endocytosis.

❌ Why Other Options Are Incorrect:

❌ Clathrin:

• Clathrin is a protein that lines the coated pit and helps it form, but the site itself where receptors and molecules gather is called a coated pit, not “clathrin.”
• So while clathrin is associated with the coated pit, it is not the site.

❌ Myosin:

• Myosin is a motor protein associated with actin filaments and is mainly involved in muscle contraction and intracellular transport, not receptor-mediated endocytosis.

❌ Actin:

• Actin filaments are part of the cytoskeleton and may assist in the movement of vesicles or membrane changes, but the initial site for receptor-mediated uptake is the coated pit, not actin itself.

❌ Filaments:

• A vague term, but generally refers to structural proteins like actin or intermediate filaments. Again, these may support the process but are not the site where receptors gather for endocytosis.

Edema is the abnormal accumulation of fluid in the interstitial space (the space between cells). It happens when there’s a disruption in the balance of fluid movement between the capillaries and the interstitial space.

One of the most common causes of edema is increased capillary hydrostatic pressure — the pressure exerted by blood inside the capillaries. Let’s break down how this works:

• Capillary hydrostatic pressure: This pushes fluid out of the capillaries into the interstitial space.
• Increased capillary pressure means that more fluid is forced out, leading to fluid buildup and swelling (edema).

Causes of increased capillary pressure:

• Congestive heart failure (CHF): Blood backs up, increasing venous pressure and capillary pressure.
• Deep vein thrombosis (DVT): Blocked veins lead to increased pressure downstream.
• Renal failure: Fluid retention increases blood volume and pressure.

Why the other options are wrong:

• Thickened endothelium:

  • This would restrict fluid movement, not increase it.
  • It’s not a common cause of edema.

• Decreased capillary pressure:

  • Would lead to reduced fluid movement out of the capillaries — the opposite of edema.

• Fever:

  • Fever can cause vasodilation, but it doesn’t directly cause edema unless it’s part of an inflammatory response.

• Decreased water consumption:

  • Would lead to dehydration, not fluid accumulation.
  • Edema is caused by fluid retention, not fluid deficit.

0.9% NaCl (normal saline) is an isotonic solution, meaning it has the same osmolarity as plasma (~300 mOsm/L). When infused intravenously, it primarily expands the extracellular fluid (ECF) compartment without causing significant water movement between compartments.

Effects of 0.9% NaCl Infusion:

✅ Increase in ECF volume → Since NaCl does not freely cross cell membranes, the infused fluid stays in the ECF (plasma + interstitial fluid).
✅ No significant change in ICF volume → Because it is isotonic, there is no osmotic gradient to drive water into or out of cells.
✅ No major effect on plasma osmolarity → Since the infused solution has the same osmolarity as plasma, it does not disrupt osmotic balance.

Why not the other options?

• Decrease in ECF → Incorrect; infusion increases ECF volume.
• Increase in ICF → Incorrect; since 0.9% NaCl is isotonic, it does not shift water into cells.
• ECF will remain the same → Incorrect; ECF volume increases due to the additional fluid.
• Decrease in ICF → Incorrect; ICF remains unchanged because there is no osmotic gradient.

Thus, the correct answer is “Increase in ECF,” as the infused isotonic saline expands the extracellular fluid compartment without affecting ICF.

Prothrombin is a plasma protein produced by the liver and plays a crucial role in blood clotting. In a normal and healthy endothelium, prothrombin remains inactive and does not promote clotting unless activated by clotting factors. When an injury occurs, the coagulation cascade is triggered, leading to the conversion of prothrombin into thrombin by the enzyme prothrombinase (factor Xa). Thrombin then helps convert fibrinogen into fibrin, forming a stable blood clot.

The inactive state of prothrombin in healthy endothelium prevents unnecessary clot formation, maintaining normal blood flow.

Why the other options are incorrect:

• Fibrin:
  • Incorrect — Fibrin is the active end product of the coagulation cascade, forming mesh-like structures that stabilize blood clots. It is not inactive in a healthy endothelium — it only appears after thrombin activation.
• Thrombin:
  • Incorrect — Thrombin is the active enzyme that converts fibrinogen to fibrin, promoting clot formation. It is not inactive — it’s only produced when clotting is needed.
• Platelets:
  • Incorrect — Platelets are cell fragments responsible for plugging injured vessels and releasing clotting factors, but they are not inactive — they’re constantly present, ready to act when needed.
• Albumin:
  • Incorrect — Albumin is a plasma protein involved in maintaining oncotic pressure and transporting substances — it plays no direct role in clotting.

• Amoeboid movement is a type of cellular locomotion seen in amoeba and certain human cells (like neutrophils and macrophages).
• It involves the extension of pseudopodia (false feet) — temporary projections of the cell membrane filled with cytoplasm.
• This movement is used for crawling, engulfing particles, and moving toward or away from stimuli.

Chemotaxis:

• Chemotaxis is the directed movement of cells in response to chemical signals in the environment.
• Cells move toward attractant chemicals (like nutrients or cytokines) or away from repellent chemicals.
• Neutrophils, for example, move toward sites of infection in response to chemotactic factors like cytokines.

Why the Other Options Are Incorrect:

• Engine:

  • Amoeboid movement is biological, not mechanical — it doesn’t involve engines.
  • The cytoskeleton, particularly actin filaments, powers the flow of cytoplasm.

• Phototropism:

  • Refers to growth in response to light, seen in plants, not amoeboid movement.

• Bones:

  • Bones provide structure and movement for multicellular organisms through muscles and joints.
  • Amoebas and single-celled organisms have no bones.

• Geotaxis:

  • Refers to movement in response to gravity, seen in some insects and larger organisms, not amoeboid cells.

The Na+/K+ ATPase (sodium-potassium pump) is electrogenic because it contributes to the electrical potential across the cell membrane. By pumping 3 positive charges (Na+) out and only 2 positive charges (K+) in, it creates a net outward movement of positive charge, making the inside of the cell more negative relative to the outside. This contributes to the resting membrane potential.   

Why other options are incorrect:

• Photogenic: Photogenic refers to something that produces light. The Na+/K+ pump does not produce light.

• Mechanogenic: Mechanogenic refers to something that produces mechanical work or movement. While the Na+/K+ pump does involve movement of ions, the term “mechanogenic” isn’t typically used to describe it.

• Hydrogenic: Hydrogenic relates to hydrogen. While ATP hydrolysis is involved, the pump’s primary characteristic is its generation of an electrical potential, not its involvement with hydrogen.

• Chemogenic: Chemogenic refers to the production of chemical change. While the pump involves a chemical reaction (ATP hydrolysis), the key characteristic being described is its electrical effect.

This statement is false because endocrine glands, like other cells, can contain intracellular vacuoles. These vacuoles may store or transport substances within the cell. The other statements are true for endocrine glands:
1. Their secretions are called hormones: Endocrine glands secrete hormones, which are chemical messengers that regulate various physiological processes.
2. They have ductless glands: Endocrine glands release their secretions directly into the bloodstream, not through ducts.
3. Pour their secretions directly into blood: This is a defining characteristic of endocrine glands.
4. Their secretions are regulated by feedback loops: Hormone secretion is tightly regulated by feedback mechanisms to maintain homeostasis.

Why the other options are correct:
1. Their secretions are called hormones: This is true. Hormones are the primary secretions of endocrine glands.

2. They have ductless glands: This is true. Endocrine glands lack ducts and release hormones directly into the bloodstream.

3. Pour their secretions directly into blood: This is true. Endocrine glands are distinguished by their ability to secrete hormones into the blood.

The answer is Burst.

Here’s why:

• Red blood cells have a salt concentration of about 0.9%. A 0.3% NaCl solution is hypotonic to the red blood cells, meaning it has a lower solute concentration.  

• Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. In this case, water will move into the red blood cells because the solution has a lower concentration of solute (salt) than the inside of the cells.  

• As water rushes into the cells, they swell up like balloons. Eventually, they will burst (lyse) because their cell membrane cannot withstand the increased internal pressure.  

In summary: Placing red blood cells in a hypotonic solution like 0.3% NaCl causes them to swell and burst due to the influx of water.

Potassium (K⁺) is the most abundant intracellular ion. Inside the cell, potassium concentration is high, while sodium (Na⁺) concentration is low—a balance maintained by the Na⁺/K⁺ ATPase pump. This pump actively transports 3 Na⁺ ions out and 2 K⁺ ions in, ensuring high intracellular K⁺ levels.

The functions of intracellular potassium include:

• Maintaining resting membrane potential
• Regulating cell volume
• Involvement in protein synthesis and enzymatic reactions

Why the Other Options Are Incorrect

• Magnesium (Mg²⁺):
  Magnesium is an important intracellular ion involved in ATP activation and enzyme function, but it is not as abundant as potassium.

• Calcium (Ca²⁺):
  Intracellular calcium levels are very low under normal conditions because most calcium is stored in the endoplasmic reticulum (ER) and mitochondria. When needed, calcium is released as a secondary messenger.

• Sodium (Na⁺):
  Sodium is the most abundant extracellular ion, not intracellular. The Na⁺/K⁺ pump actively keeps Na⁺ outside the cell.

• Chloride (Cl⁻):
  Chloride is mainly found in the extracellular environment. It plays a role in maintaining osmotic balance and electrical neutrality but is not the dominant intracellular ion.

The diffusion potential of the cell membrane is calculated using the Goldman-Hodgkin-Katz (GHK) equation, which takes into account the contributions of multiple ions(e.g., Na⁺, K⁺, Cl⁻) that can permeate the membrane. The GHK equation considers the concentration gradients and relative permeabilities of these ions to determine the membrane potential. This is more accurate than the Nernst equation, which calculates the equilibrium potential for a single ion.

Why the other options are wrong:1. Cl⁻ only: The diffusion potential is not determined by chloride ions alone. While Cl⁻ contributes to the membrane potential, it is not the only ion involved.2. Ca²⁺ only:Calcium ions (Ca²⁺) have a very low permeability in most cells and do not significantly contribute to the resting membrane potential.3. Na⁺ only:Sodium ions (Na⁺) contribute to the membrane potential, but the diffusion potential is not calculated using Na⁺ alone.4. K⁺ only: Potassium ions (K⁺) are the primary contributors to the resting membrane potential, but the diffusion potential is not calculated using K⁺ alone. The GHK equation requires multiple ions for an accurate calculation.

Secondary active transport uses the energy from one ion’s movement down its concentration gradient to drive the movement of another ion against its gradient. It does not directly use ATP but relies on the electrochemical gradient created by primary active transport.

The Na⁺/K⁺ pump (Na⁺/K⁺ ATPase), however, is an example of primary active transport, meaning it directly uses ATP to move ions against their gradients. It pumps:

• 3 Na⁺ out of the cell (against its gradient).
• 2 K⁺ into the cell (against its gradient).

Since Na⁺ and K⁺ transport is ATP-dependent, it does not fall under secondary active transport.

Why the Other Options Are Incorrect

• Na⁺ and glucose:
  Transported via SGLT (sodium-glucose linked transporter), where Na⁺ moves down its gradient, pulling glucose against its gradient → Secondary active transport.

• Na⁺ and H⁺:
  The Na⁺/H⁺ exchanger (antiporter) uses the Na⁺ gradient to remove H⁺ ions, helping regulate pH → Secondary active transport.

• Na⁺ and Ca²⁺:
  The Na⁺/Ca²⁺ exchanger removes Ca²⁺ from the cell using Na⁺ influx → Secondary active transport.

• None of these:
  Incorrect because Na⁺/K⁺ transport requires ATP (primary active transport), unlike the other options.

Pseudo-unipolar neurons are a type of sensory neuron characterized by having a single, short process that branches into two parts: one part extends toward the periphery (from a receptor), and the other part extends toward the spinal cord. These neurons are called pseudo-unipolar because, although they appear to have a single axon, they actually originate from a bipolar structure during development.

Let’s break down the options:

Conveys message to pons:

Pseudo-unipolar neurons are involved in sensory functions, particularly transmitting sensory information from the periphery to the spinal cord or the brainstem. However, their primary function is not specifically to convey messages to the pons, but to the spinal cord or other sensory areas. Therefore, this is not the correct answer.

Stimulates effectors:

Pseudo-unipolar neurons are sensory neurons, meaning they do not directly stimulate effectors (such as muscles or glands). Instead, they transmit sensory information toward the central nervous system (CNS). Motor neurons, not sensory neurons, stimulate effectors. Therefore, this is not the correct answer.

 Conveys message to spinal cord:

The primary function of pseudo-unipolar neurons is to convey sensory information from the periphery (such as the skin, muscles, or organs) to the spinal cord or brainstem. This is typically involved in transmitting sensory input like touch, pain, and temperature. Therefore, option 3 is the correct answer.

Acts as a receptor for external stimulation:

Although pseudo-unipolar neurons are involved in transmitting sensory information, they are not directly receptors. Receptors are specialized structures (like sensory organs or receptors at the ends of sensory neurons) that detect external stimuli. The pseudo-unipolar neurons carry the information from these receptors to the CNS. Therefore, this is not the correct answer.

None of these:

This option would be correct if none of the above options described the function of pseudo-unipolar neurons, but option 3 accurately describes their function. Therefore, this is not the correct answer.

• The human body is approximately 60% water by weight in a healthy adult male.
• In females, it’s slightly lower (about 50-55%) due to higher fat content (fat has less water).
• In infants, the water percentage is higher, around 70-75%.
• With aging, the water content decreases due to loss of lean body mass.

Water Distribution:

• Intracellular fluid (ICF): ~ 40% of body weight (or two-thirds of total body water).
• Extracellular fluid (ECF): ~ 20% of body weight (or one-third of total body water), which includes:
  • Interstitial fluid: ~ 15% of body weight.
  • Plasma (intravascular fluid): ~ 5% of body weight.

Why the Other Options Are Incorrect:

• 70%:

  • Seen in newborns and infants, not adults.

• 35%:

  • Too low to represent total body water — closer to the intracellular fluid proportion.

• 85%:

  • Unrealistic value — even in infants, body water doesn’t reach this level.

• 40%:

  • Represents the intracellular fluid compartment, not total body water.

Correct Answer: Chloride (Cl⁻)
Chloride (Cl⁻) is the most abundant anion in extracellular fluid (ECF), including plasma and interstitial fluid. Here’s why:
– Concentration: ~100-110 mEq/L in ECF (compared to ~24 mEq/L for HCO₃⁻).
– Roles:
– Maintains electroneutrality (balances Na⁺, the major cation).
– Critical for osmotic pressure (with Na⁺, regulates fluid balance).
– Involved in acid-base balance (chloride shift in RBCs). Proteins:

Why its incorrect: Proteins are primarily anions in the body, but they are mostly found in intracellular fluid and are present in relatively small amounts in the extracellular fluid. They are not the major anion in extracellular fluid, although they do contribute to colloid osmotic pressure.

HCO3 (Bicarbonate):

Why its incorrect: While bicarbonate (HCO3) is an important buffering system in the blood and is present in significant amounts in extracellular fluid, it is not the major anion. The concentration of bicarbonate in extracellular fluid is much lower than that of chloride, making chloride (Cl) the primary extracellular anion.

SO4 (Sulfate):

Why its incorrect: Sulfate (SO4) is an anion that is found in smaller amounts in extracellular fluid. It is not as abundant as chloride and is involved in various metabolic processes but does not serve as the major anion in extracellular fluid.

HCl (Hydrochloric acid):

Why its incorrect: Hydrochloric acid (HCl) is not a significant anion in extracellular fluid. HCl is found in the gastric environment as a strong acid, primarily in the stomach, and not in the general extracellular space. The chloride ion (Cl) in HCl is the relevant anion in the context of extracellular fluid, but HCl itself is not the major anion.

During an action potential, neurons undergo several phases: depolarization, repolarization, and hyperpolarization.

1. Depolarization: Voltage-gated Na⁺ channels open, allowing Na⁺ to rush into the cell, making the inside more positive.
2. Repolarization: Voltage-gated K⁺ channels open, allowing K⁺ to leave the cell, restoring a negative charge inside.
3. Hyperpolarization: After repolarization, the voltage-gated K⁺ channels stay open longer than necessary, causing excess K⁺ to leave the neuron. This lowers the membrane potential beyond the resting potential (more negative than -70mV), resulting in hyperpolarization.

Hyperpolarization ensures that the neuron briefly becomes less excitable, preventing immediate re-firing and enforcing the refractory period.

Why the Other Options Are Wrong:

1. Sodium Leaky Channels ❌

   • These contribute to the resting membrane potential but do not cause hyperpolarization during an action potential.
   • Hyperpolarization is caused by excess K⁺ leaving the cell, not by Na⁺ movement.

2. Closure of Cl⁻ Channels ❌

   • Cl⁻ channels are mostly relevant in inhibitory neurotransmission, where Cl⁻ entry makes the neuron more negative.
   • Their closure would actually make the membrane less negative, which does not cause hyperpolarization.

3. Prolonged Opening of Voltage-Gated Sodium Channels ❌

   • If Na⁺ channels remained open, depolarization would continue, making the membrane less negative rather than hyperpolarized.

4. Prolonged Closure of K⁺ Ion Channels ❌

   • If K⁺ channels remained closed, repolarization would be delayed, but hyperpolarization would not occur.

The resting membrane potential (RMP) of a cell is determined primarily by the movement of ions across the cell membrane. Typically, the inside of the cell is negative relative to the outside, with a resting potential of about -70 mV in neurons. The key ion responsible for making the inside of the cell more negative is potassium (K⁺) due to its efflux (movement out of the cell).

How Increased Permeability to K⁺ Makes the Inside More Negative:

1. At rest, K⁺ is more concentrated inside the cell due to the action of the Na⁺/K⁺ ATPase pump, which actively transports 3 Na⁺ out and 2 K⁺ in.
2. The cell membrane is highly permeable to K⁺ due to leak channels, allowing K⁺ to move out of the cell down its concentration gradient.
3. As positively charged K⁺ ions leave, the inside of the cell becomes more negative relative to the outside.
4. If the membrane permeability to K⁺ further increases, more K⁺ will leave, causing hyperpolarization (making the cell even more negative).

Why the Other Options Are Incorrect:

• Chloride (Cl⁻):

  • Cl⁻ is a negatively charged ion, and increasing its permeability would make the inside of the cell more negative.
  • However, in most cells, Cl⁻ movement is passive and follows Na⁺, so it does not primarily set the resting potential like K⁺ does.

• Hydrogen (H⁺):

  • H⁺ ions play a major role in pH regulation, but they do not significantly affect membrane potential under normal physiological conditions.

• Calcium (Ca²⁺):

  • Ca²⁺ is much higher outside the cell than inside.
  • If permeability to Ca²⁺ increased, Ca²⁺ would enter the cell, making it more positive, not negative.

• Sodium (Na⁺):

  • At rest, Na⁺ is more concentrated outside the cell.
  • Increased Na⁺ permeability would allow Na⁺ to enter, making the cell more positive (depolarization), not negative.

The fundamental difference between prokaryotic and eukaryotic cells lies in the presence of membrane-bound organelles. Let’s break it down:

• Eukaryotic cells:

  • Have membrane-bound organelles like the nucleus, endoplasmic reticulum (ER), Golgi apparatus, mitochondria, and lysosomes.
  • Their DNA is enclosed in a nucleus.
  • More complex and larger (10–100 μm) compared to prokaryotic cells.

• Prokaryotic cells:

  • Lack membrane-bound organelles — no nucleus, no ER, no mitochondria, etc.
  • Their DNA floats freely in the cytoplasm in a region called the nucleoid.
  • Smaller and simpler (0.2–5 μm) than eukaryotic cells.

Why the other options are wrong:

• Shape of cell:

  • Both prokaryotic and eukaryotic cells come in a variety of shapes.
  • Shape depends on cell type and function, not the cell’s classification (e.g., cocci, bacilli, spirilla in prokaryotes; cuboidal, columnar, squamous in eukaryotes).

• Presence of multiple nuclei:

  • Not a general feature of either cell type.
  • Some eukaryotic cells can have multiple nuclei (like skeletal muscle cells), but most have a single nucleus.
  • Prokaryotes never have nuclei, so this isn’t a distinguishing feature.

• Presence of ribosomes:

  • Both prokaryotic and eukaryotic cells have ribosomes for protein synthesis.
  • The difference lies in their size:
    • Prokaryotic ribosomes: 70S (smaller)
    • Eukaryotic ribosomes: 80S (larger)

• Presence of vacuoles:

  • Both cell types can have vacuoles, though they’re more prominent in eukaryotic plant cells.
  • Vacuoles in prokaryotic cells tend to be smaller and less specialized

Endoplasmic reticulum dysfunction

Explanation:

Protein aggregation is a pathological process in which proteins misfold and accumulate in the cell, often forming insoluble clumps that can disrupt cellular function. Let’s break down each option to determine which one causes this issue.

Endoplasmic Reticulum (ER) Dysfunction:

The endoplasmic reticulum (ER) is crucial for the synthesis and proper folding of proteins. When the ER is stressed or dysfunctional, it can lead to misfolded proteins. This is often associated with the unfolded protein response (UPR), a mechanism that attempts to refold misfolded proteins or remove them. However, if the stress continues or becomes chronic, it can lead to protein aggregation. Such aggregates are often seen in neurodegenerative diseases (e.g., Alzheimer’s, Parkinson’s). Therefore, ER dysfunction is directly associated with protein aggregation, making option 1 the correct answer.

Ribosomal Dysfunction:

Ribosomes are responsible for protein synthesis, not protein folding. While ribosomal dysfunction can lead to impaired protein synthesis or the production of defective proteins, it is not primarily involved in the aggregation of proteins. Protein aggregation is more closely linked to the misfolding of proteins rather than their synthesis. Therefore, ribosomal dysfunction is not the main cause of protein aggregation.

Cell Wall Dysfunction:

Cell walls are found in plants, fungi, and bacteria, and they provide structural support. In human cells, there is no cell wall. The cell membrane and the cytoskeleton are involved in structural integrity. Cell wall dysfunction would not be directly related to protein aggregation in human cells, as the mechanism of aggregation is related to intracellular processes, particularly protein folding and quality control systems. Thus, this option is incorrect.

Nucleic Acid Dysfunction:

Nucleic acids (DNA and RNA) are involved in genetic information storage and protein synthesis. While defects in nucleic acid function can lead to mutations or problems in protein synthesis, nucleic acid dysfunction does not directly cause protein aggregation. Protein aggregation is primarily a result of issues in the folding or processing of proteins, which is largely dependent on the ER and other cellular mechanisms.

Golgi Dysfunction:

The Golgi apparatus is involved in protein modification, sorting, and packaging. While dysfunction in the Golgi could affect protein processing or transport, it does not directly cause protein aggregation. The Golgi’s main role is in modifying and sorting proteins, and dysfunction could lead to improperly processed proteins, but it is not the primary cause of protein aggregation.

The sodium-potassium (Na⁺/K⁺) pump is a classic example of active transport because it requires ATP to move ions against their concentration gradients. This pump:

• Moves 3 Na⁺ out of the cell and 2 K⁺ in, maintaining the electrochemical gradient.
• Requires ATP hydrolysis because sodium is transported against its gradient (from low to high concentration outside the cell), and potassium is transported against its gradient (from low to high concentration inside the cell).

This pump is essential for maintaining resting membrane potential, regulating cell volume, and enabling nerve impulses.

Why the Other Options Are Incorrect

• Bulk transport:
  This involves endocytosis and exocytosis, which transport large molecules or particles, not individual ions.

• Osmosis:
  Osmosis is the passive movement of water across a membrane due to a concentration gradient, not an ion transport mechanism.

• Facilitated diffusion:
  Facilitated diffusion moves substances down their concentration gradient via carrier proteins without ATP, whereas the Na⁺/K⁺ pump works against the gradient and requires energy.

• Diffusion:
  Simple diffusion moves molecules from high to low concentration without energy, whereas the Na⁺/K⁺ pump moves ions against their gradients using ATP.

The Goldman–Hodgkin–Katz (GHK) equation is used to calculate the diffusion potential across a membrane when the membrane is permeable to multiple ions. This equation takes into account:
1. The permeability of the membrane to each ion.
2. The concentration gradient of each ion across the membrane.
3. The valence of each ion.

The GHK equation is a more generalized form of the Nernst equation and is particularly useful for biological membranes, which are often permeable to several ions simultaneously (e.g., Na⁺, K⁺, and Cl⁻). It provides a more accurate prediction of the membrane potential under these conditions.

Incorrect Answers

Diffusion equation 
– The diffusion equation describes the movement of particles from an area of high concentration to an area of low concentration over time. However, it does not account for the electrical potential generated by ion movement across a membrane.
– Why it’s wrong: The diffusion equation is not used to calculate membrane potential.

Nernst equation- The Nernst equation calculates the equilibrium potential for a single ion across a membrane. It assumes the membrane is permeable to only one ion at a time, which is not the case for most biological membranes.
– Why it’s wrong: The Nernst equation is limited to single-ion systems and cannot account for multiple permeabilities.

Threshold equation 
– The threshold equation is not a standard term in membrane potential calculations. It may refer to the threshold potential required to trigger an action potential in excitable cells, but it is not used to calculate diffusion potential.
– Why it’s wrong: This is not a recognized equation for calculating membrane potential.

Transmission equation 
– There is no standard “transmission equation” in the context of membrane potentials. This term is not relevant to the calculation of diffusion potential.
– Why it’s wrong: This is not a recognized equation in membrane physiology.

The question asks about the cause of hyperpolarization during an action potential. Hyperpolarization is a phase where the membrane potential becomes more negative than the resting membrane potential. The correct answer is prolonged opening of voltage-gated K+ channels. Let’s break down why this is the correct answer and why the other options are incorrect.

Correct Option: Prolonged Opening of Voltage-Gated K+ Channels
– During an action potential, voltage-gated K+ channels open in response to depolarization. These channels open more slowly than voltage-gated Na+ channels and remain open for a longer duration.
– As K+ ions move out of the cell due to the electrochemical gradient, the membrane potential becomes more negative, leading to hyperpolarization.
– This phase is also known as the after-hyperpolarization or undershoot phase of the action potential.

Why the Other Options Are Incorrect:

Prolonged opening of voltage-gated sodium channels:
Voltage-gated Na+ channels are responsible for the rapid depolarization phase of the action potential. Their prolonged opening would cause sustained depolarization, not hyperpolarization.

Closure of Cl- channels:
– Chloride ions (Cl-) typically contribute to the resting membrane potential. Closure of Cl- channels would reduce the influx of negative ions, potentially causing depolarization rather than hyperpolarization.

Sodium leaky channels:
– Sodium leaky channels allow a small, constant influx of Na+ ions, which helps maintain the resting membrane potential. They do not play a significant role in hyperpolarization.

Prolonged closure of K+ ion channels:
– Closure of K+ channels would prevent K+ ions from leaving the cell, reducing the outward flow of positive charges. This would make the membrane potential less negative, leading to depolarization rather than hyperpolarization.

The question asks about the complex of proteins and filaments that span the entire cell and are responsible for giving the cell its shape. Let’s break down each option to understand why the correct answer is right and why the others are wrong.

Correct Answer: Cytoskeleton

– Why it’s correct: The cytoskeleton is a network of protein filaments and tubules that extends throughout the cytoplasm of the cell. It provides structural support, maintains cell shape, enables cell movement, and facilitates intracellular transport. The cytoskeleton is composed of three main types of filaments: microfilaments (actin), intermediate filaments, and microtubules. These components work together to give the cell its shape and mechanical resistance to deformation.
– Key point:The cytoskeleton is directly responsible for maintaining cell shape and structure, which aligns perfectly with the question.

—

Incorrect Answers:

Plasma proteins
– Why it’s wrong:Plasma proteins are primarily found in the blood plasma, not inside cells. They play roles in maintaining osmotic pressure, transporting substances, and immune responses, but they are not involved in maintaining cell shape or structure.
– Key point:Plasma proteins are extracellular and do not form a structural network within the cell.

Cytosol– Why it’s wrong: The cytosol is the liquid component of the cytoplasm, in which organelles and other cellular components are suspended. While it is part of the intracellular environment, it does not provide structural support or shape to the cell.
– Key point: The cytosol is a fluid medium, not a structural framework.

Centrioles
– Why it’s wrong: Centrioles are cylindrical structures involved in organizing microtubules during cell division (mitosis and meiosis) and forming the basal bodies of cilia and flagella. While they are part of the cytoskeletal system, they are not the primary structure responsible for maintaining cell shape.
– Key point: Centrioles have a specific role in cell division and motility but do not span the entire cell or determine its shape.

Connective tissue
– Why it’s wrong: Connective tissue is a type of tissue found outside cells, providing support and structure to organs and the body as a whole. It is not a cellular component and does not contribute to the shape of individual cells.
– Key point: Connective tissue is extracellular and operates at the tissue level, not the cellular level.

Let’s break down the distribution of total body fluid (TBF) and see how we arrive at this fraction:

1. Total Body Fluid (TBF) makes up about 60% of body weight in an average adult.

   • For a 70 kg person, total body fluid is around 42 liters.

2. TBF is divided into two main compartments:

   • Intracellular fluid (ICF):
     • Fluid inside the cells
     • Makes up 2/3 of total body fluid (~28 liters)
   • Extracellular fluid (ECF):
     • Fluid outside the cells
     • Makes up 1/3 of total body fluid (~14 liters)

3. ECF is further divided into:

   • Interstitial fluid:
     • Fluid surrounding the cells in tissues
     • Makes up 3/4 of the ECF (~10.5 liters)
   • Plasma:
     • Fluid within the blood vessels, but outside the blood cells
     • Makes up 1/4 of the ECF (~3.5 liters)

4. Plasma as a fraction of total body fluid:

   • Since plasma is 1/4 of the ECF, and the ECF is 1/3 of total body fluid, the fraction of plasma in relation to total body fluid is:

     Plasma=14×13=112Plasma=41​×31​=121​

   So, plasma is 1/12 of the total body fluid.

Why the other options are wrong:

• 1/2 of total body fluid: This would imply that plasma makes up half of all body fluid, which is incorrect — most fluid is inside the cells (ICF).
• 2/3 of total body fluid: This fraction applies to the intracellular fluid, not the plasma.
• 1/10 of total body fluid: Plasma is actually slightly more than 1/10 — 1/12 is more accurate.
• 1/4 of total body fluid: Plasma is 1/4 of the extracellular fluid, not total body fluid.

Why is this correct?
The cell membrane, also known as the plasma membrane, is a phospholipid bilayer with embedded proteins. When visualized under an electron microscope, the membrane appears as a trilaminar structure (two dark lines separated by a light region), often referred to as the “unit membrane.” The total thickness of the cell membrane, including the phospholipid bilayer and associated proteins, typically ranges from 7.5 to 10 nanometers (nm). This measurement accounts for:
1. The hydrophobic core of the lipid bilayer (about 3–4 nm).
2. The polar head groups of the phospholipids on either side (about 2–3 nm each).
3. The associated proteins that may extend beyond the lipid bilayer.

Thus, the total thickness falls within the 7.5–10 nm range.

Incorrect Answers

1. 5–11 nm
– While this range includes the correct thickness (7.5–10 nm), it is too broad and includes values that are not typical for the cell membrane.
– Why it’s wrong: The lower limit (5 nm) is too small, and the upper limit (11 nm) is slightly larger than the accepted range.

2. 4–10 nm
– This range is also too broad and includes values that are not consistent with the typical thickness of the cell membrane.
– Why it’s wrong: The lower limit (4 nm) is too small, as the phospholipid bilayer alone is about 5–6 nm thick.

3. 2.5–4 nm
– This range is far too small to represent the total thickness of the cell membrane. It might correspond to the thickness of a single lipid monolayer or a very thin membrane structure, but not the entire plasma membrane.
– Why it’s wrong: The values are much smaller than the actual thickness of the cell membrane.

4. 3–5 nm
– This range is also too small and does not account for the full thickness of the phospholipid bilayer and associated proteins.
– Why it’s wrong: The values are below the minimum thickness of the cell membrane.