General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 411: Which of the following is involved in the transport of large molecules from the cytoplasm to the cell nucleus?

A. Clathrin (Correct Answer)
B. Caveolin
C. Flotillin
D. Importins

Explanation: **Explanation:** The correct answer is **Clathrin**. **1. Why Clathrin is Correct:** Clathrin is a protein that plays a critical role in **receptor-mediated endocytosis** and the formation of coated vesicles. While it is primarily known for transporting molecules from the plasma membrane to the interior of the cell, it is also essential for the **retrograde transport** of large molecules and vesicles from the cytoplasm/trans-Golgi network toward the nuclear periphery and other organelles. Clathrin-coated pits facilitate the internalization of large ligands (like LDL or iron-transferrin) and their subsequent intracellular trafficking. **2. Why Other Options are Incorrect:** * **Caveolin:** These are integral membrane proteins found in **caveolae** (flask-shaped invaginations of the plasma membrane). They are involved in transcytosis and signal transduction but are not the primary mediators for transport toward the nucleus. * **Flotillin:** These are marker proteins for **lipid rafts**. They assist in non-clathrin-mediated endocytosis and help organize signaling microdomains on the cell membrane. * **Importins:** While the name suggests nuclear transport, Importins are **karyopherins** that transport proteins *through* the nuclear pore complex (NPC) into the nucleus. However, in the context of general vesicular trafficking of "large molecules" from the cytoplasm (often involving membrane-bound vesicles), Clathrin is the structural protein responsible for the initial budding and transport mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **Clathrin Structure:** It forms a **triskelion** shape (three-legged structure) which assembles into a polyhedral lattice to deform the membrane. * **Dynamin:** This GTPase is the "molecular scissor" required to pinch off clathrin-coated vesicles from the membrane. * **Familial Hypercholesterolemia:** Often caused by mutations in the LDL receptor's cytoplasmic tail, preventing it from associating with clathrin-coated pits, leading to high plasma cholesterol.

Question 412: Glucose is transported in the presence of Na+ ions in the intestine. This is a type of transport?

A. Secondary active transport (Correct Answer)
B. Primary active transport
C. Facilitated diffusion
D. Simple diffusion

Explanation: **Explanation:** Glucose transport in the intestine occurs via **Secondary Active Transport**, specifically through the **SGLT-1 (Sodium-Glucose Linked Transporter)**. 1. **Why it is Secondary Active Transport:** This process does not use ATP directly. Instead, it relies on the electrochemical gradient created by the **Na+/K+ ATPase pump** (Primary Active Transport) on the basolateral membrane. This pump keeps intracellular Na+ levels low. Glucose then "hitchhikes" with Na+ as it moves down its concentration gradient into the cell. Because glucose is moving *against* its concentration gradient using energy derived from an existing ion gradient, it is classified as secondary active transport (specifically, **Symport/Co-transport**). **Analysis of Incorrect Options:** * **Primary Active Transport:** This involves the direct hydrolysis of ATP to move substances (e.g., Na+/K+ ATPase, Ca2+ ATPase). Glucose transport does not hydrolyze ATP directly. * **Facilitated Diffusion:** This is a passive process using a carrier protein without energy. While glucose *leaves* the intestinal cell into the blood via facilitated diffusion (using **GLUT-2**), its entry from the lumen is active. * **Simple Diffusion:** This involves molecules moving freely across the lipid bilayer (e.g., O2, CO2). Glucose is large and polar, requiring specific transporter proteins. **NEET-PG High-Yield Pearls:** * **SGLT-1:** Located in the small intestine and late proximal tubule of the kidney. * **SGLT-2:** Located in the early proximal tubule; inhibited by **Gliflozins** (e.g., Dapagliflozin) to treat Diabetes Mellitus. * **Oral Rehydration Therapy (ORS):** Based on the principle of Na+-Glucose co-transport; the presence of glucose enhances Na+ (and subsequently water) absorption.

Question 413: The fluidity of the plasma membrane is increased by

A. Arachidonic acid (Correct Answer)
B. Cholesterol
C. Palmitic acid
D. Stearic acid

Explanation: **Explanation:** The fluidity of the plasma membrane is primarily determined by the **degree of saturation** of the fatty acid tails in the phospholipid bilayer and the presence of cholesterol. **1. Why Arachidonic Acid is Correct:** Arachidonic acid is a **polyunsaturated fatty acid (PUFA)** containing 20 carbons and **four double bonds**. In the plasma membrane, double bonds in fatty acid chains create "kinks" or bends. These kinks prevent the phospholipid molecules from packing tightly together, increasing the lateral movement of lipids and proteins, thereby **increasing membrane fluidity**. **2. Why the Other Options are Incorrect:** * **Palmitic Acid (C16) and Stearic Acid (C18):** These are **saturated fatty acids** (no double bonds). Their straight-chain structure allows them to pack tightly together via van der Waals forces, which stabilizes the membrane and **decreases fluidity** (making it more rigid). * **Cholesterol:** At physiological temperatures, cholesterol acts as a "fluidity buffer." It inserts itself between the fatty acid tails, restricting their movement and **decreasing fluidity**. (Note: At very low temperatures, it prevents the membrane from freezing, but in the context of standard human physiology, it generally stabilizes/stiffens the membrane). **High-Yield Clinical Pearls for NEET-PG:** * **Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972); emphasizes that the membrane is a dynamic, fluid structure. * **Temperature Effect:** Fluidity increases with higher temperatures and decreases with lower temperatures. * **Chain Length:** Shorter fatty acid chains increase fluidity because they have less surface area to undergo van der Waals interactions. * **Ratio:** A high **Unsaturated:Saturated** fatty acid ratio is a hallmark of high membrane fluidity.

Question 414: Electrical synapse is closely associated with which of the following structures?

A. Neuromuscular junction
B. Tight junction
C. Gap junction (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Why Gap Junction is the Correct Answer:** Electrical synapses are specialized connections between neurons that allow for the direct, passive flow of electric current from one cell to another. This communication is mediated by **Gap Junctions**. These junctions consist of paired channels called **connexons** (each made of six connexin proteins) that bridge the narrow gap between pre- and post-synaptic membranes. Unlike chemical synapses, electrical synapses are characterized by **minimal synaptic delay**, bidirectional flow, and high reliability, making them ideal for synchronizing the activity of groups of neurons (e.g., in the hypothalamus or cardiac muscle). **Analysis of Incorrect Options:** * **A. Neuromuscular Junction (NMJ):** This is a classic example of a **chemical synapse**. It relies on the release of the neurotransmitter Acetylcholine (ACh) into the synaptic cleft to trigger an action potential in the muscle fiber. * **B. Tight Junction (Zonula Occludens):** These are occluding junctions that seal the space between epithelial cells to prevent the leakage of molecules (e.g., in the Blood-Brain Barrier). They do not facilitate electrical communication. **High-Yield Facts for NEET-PG:** * **Speed:** Electrical synapses are faster than chemical synapses because they bypass the steps of neurotransmitter release and receptor binding. * **Location:** In humans, they are found in the retina, cerebral cortex, and cardiac muscle (intercalated discs). * **Connexin 36:** This is the most common protein forming gap junctions in the central nervous system. * **Directionality:** While most are bidirectional, some electrical synapses are "rectifying," allowing current to flow in only one direction.

Question 415: What is the primary contributor to plasma osmolarity?

A. Urea
B. Glucose
C. Sodium (Correct Answer)
D. Albumin

Explanation: **Explanation:** The correct answer is **Sodium (C)**. Plasma osmolarity is the measure of the concentration of solutes in the blood plasma. Under normal physiological conditions, sodium is the most abundant extracellular cation. Because it is accompanied by anions (primarily chloride and bicarbonate) to maintain electrical neutrality, sodium and its associated anions account for approximately **90-95% of the total plasma osmolarity**. The standard formula to estimate plasma osmolarity is: $2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$ This formula highlights that sodium is doubled in the calculation, reflecting its dominant role. **Analysis of Incorrect Options:** * **Urea (A):** While urea contributes to total osmolarity, it is a "permeable solute" that moves freely across cell membranes. Therefore, it does not contribute to *effective* osmolarity (tonicity). * **Glucose (B):** Glucose contributes only about 5–10 mOsm/L in a healthy individual. It becomes a significant contributor only in pathological states like Diabetes Mellitus (e.g., HHS). * **Albumin (D):** Albumin is the primary contributor to **oncotic pressure** (colloid osmotic pressure), which is essential for fluid exchange at the capillary level. However, its molar concentration is very low, making its contribution to total plasma osmolarity negligible (less than 1 mOsm/L). **Clinical Pearls for NEET-PG:** * **Normal Plasma Osmolarity:** 280–295 mOsm/L. * **Osmolar Gap:** The difference between measured and calculated osmolarity. A gap >10 mOsm/L suggests the presence of unmeasured toxins (e.g., Ethanol, Methanol, Ethylene glycol). * **Tonicity:** Sodium is the primary determinant of tonicity, which dictates the movement of water across the blood-brain barrier.

Question 416: Which ion is required for exocytosis?

A. Potassium
B. Sodium
C. Calcium (Correct Answer)
D. Magnesium

Explanation: **Explanation:** **Why Calcium is Correct:** Exocytosis is the process by which a cell transports secretory products out of the cytoplasm via membrane-bound vesicles. **Calcium ($Ca^{2+}$)** is the essential trigger for this process. When an action potential reaches a nerve terminal or a secretory cell, voltage-gated calcium channels open, leading to an influx of $Ca^{2+}$. This increase in intracellular calcium concentration acts as a signal for **SNARE proteins** (Synaptobrevin, Syntaxin, and SNAP-25) to facilitate the docking and fusion of the vesicle with the plasma membrane, releasing its contents into the extracellular space. **Why Incorrect Options are Wrong:** * **Potassium ($K^+$):** Primarily responsible for maintaining the resting membrane potential and the repolarization phase of the action potential. * **Sodium ($Na^+$):** Primarily responsible for the depolarization phase of the action potential but does not directly trigger the fusion of secretory vesicles. * **Magnesium ($Mg^{2+}$):** Often acts as a calcium antagonist. High levels of magnesium can actually inhibit exocytosis (e.g., inhibiting acetylcholine release at the neuromuscular junction) by competing with calcium for entry into the presynaptic terminal. **High-Yield NEET-PG Pearls:** * **Synaptotagmin:** This is the specific protein on the vesicle membrane that acts as the **calcium sensor** to trigger fusion. * **Toxins:** Botulinum and Tetanus toxins work by cleaving SNARE proteins, thereby preventing $Ca^{2+}$-induced exocytosis of neurotransmitters. * **Lambert-Eaton Syndrome:** Antibodies against voltage-gated calcium channels prevent $Ca^{2+}$ influx, leading to impaired exocytosis of Acetylcholine and muscle weakness.

Question 417: Which cellular component maintains the skeletal framework of a cell?

A. Nucleus
B. Microtubules (Correct Answer)
C. Ribosomes
D. Mitochondria

Explanation: **Explanation:** The correct answer is **B. Microtubules**. The structural framework of a cell is maintained by the **cytoskeleton**, a complex network of protein filaments. Microtubules are the largest components of this system (approx. 25 nm in diameter). They are hollow tubes made of alpha and beta-tubulin dimers. Beyond providing structural support and maintaining cell shape, they are essential for intracellular transport (acting as "tracks" for motor proteins like dynein and kinesin), forming the mitotic spindle during cell division, and powering the movement of cilia and flagella. **Why other options are incorrect:** * **A. Nucleus:** This is the control center of the cell containing genetic material (DNA). While it is the largest organelle, it does not provide the structural scaffolding for the entire cytoplasm. * **C. Ribosomes:** These are the sites of protein synthesis (translation). They are small ribonucleoprotein complexes and have no structural role in maintaining cell shape. * **D. Mitochondria:** Known as the "powerhouse of the cell," they are responsible for ATP production via oxidative phosphorylation. **High-Yield NEET-PG Pearls:** * **Cytoskeleton Components:** Consists of Microtubules (thickest), Intermediate Filaments (provide tensile strength), and Microfilaments/Actin (thinnest; involved in muscle contraction and cell movement). * **Clinical Correlation:** Drugs like **Colchicine**, **Vincristine**, and **Paclitaxel** act by inhibiting microtubule polymerization or depolymerization, making them vital in treating gout and cancer. * **Kartagener Syndrome:** Caused by a defect in dynein arms within microtubules, leading to immobile cilia, bronchiectasis, and situs inversus.

Question 418: Residual bodies are related to which of the following cellular organelles?

A. Mitochondria
B. Nucleus
C. Golgi apparatus
D. Lysosomes (Correct Answer)

Explanation: **Explanation:** The correct answer is **Lysosomes**. **Why Lysosomes?** Lysosomes are the "digestive system" of the cell, containing hydrolytic enzymes that break down macromolecules, damaged organelles (autophagy), and foreign substances (heterophagy). After the enzymatic digestion of these materials within a secondary lysosome (phagolysosome), some indigestible substances remain. The vesicle containing these undigested residues is termed a **Residual Body** (or tertiary lysosome). In many cells, these are eliminated via exocytosis, but in long-lived cells like neurons and cardiac muscles, they persist as **lipofuscin granules** (the "wear-and-tear" pigment). **Why other options are incorrect:** * **Mitochondria:** These are the "powerhouses" of the cell, primarily involved in ATP production via oxidative phosphorylation and apoptosis regulation. They do not form residual bodies. * **Nucleus:** This is the genetic control center containing DNA. While it undergoes changes during cell death (pyknosis, karyorrhexis), it is not involved in the lysosomal digestive pathway. * **Golgi Apparatus:** This organelle is responsible for packaging, modifying, and sorting proteins. While it is involved in the formation of *primary* lysosomes, it does not contain the undigested waste that characterizes residual bodies. **High-Yield NEET-PG Pearls:** * **Lipofuscin:** A brown-yellow pigment found in residual bodies; it is a hallmark of aging and free radical damage. * **Primary Lysosome:** A newly formed vesicle from the Golgi containing inactive enzymes. * **Secondary Lysosome:** Formed by the fusion of a primary lysosome with a phagosome/endosome; this is where active digestion occurs. * **Autophagy:** The process by which lysosomes digest the cell's own components, a key survival mechanism during starvation.

Question 419: Which of the following best describes the action of the Na-K ATPase pump?

A. 3 Na+ ions are transported out of the cell for every 2 K+ ions transported into the cell. (Correct Answer)
B. 3 Na+ ions are transported into the cell for every 2 K+ ions transported out of the cell.
C. 2 Na+ ions are transported out of the cell for every 3 K+ ions transported into the cell.
D. 2 Na+ ions are transported into the cell for every 3 K+ ions transported out of the cell.

Explanation: The **Na⁺-K⁺ ATPase pump** (Sodium-Potassium Pump) is a classic example of **Primary Active Transport**. It utilizes energy derived from the hydrolysis of ATP to move ions against their respective electrochemical gradients. ### Why Option A is Correct The pump functions as an **electrogenic pump**, meaning it creates a net charge imbalance across the membrane. In each cycle, it binds 3 Na⁺ ions from the intracellular fluid and 2 K⁺ ions from the extracellular fluid. By pumping **3 Na⁺ OUT** and **2 K⁺ IN**, it results in a net loss of one positive charge from the cell interior, contributing to the negative Resting Membrane Potential (RMP). ### Why Other Options are Incorrect * **Options B & D:** These suggest Na⁺ moves into the cell. In reality, Na⁺ concentration is already high outside; moving it inward would be passive (diffusion), not active transport. * **Option C:** This reverses the stoichiometry. Pumping more K⁺ in than Na⁺ out would make the cell interior more positive, which contradicts physiological reality. ### High-Yield NEET-PG Pearls * **Inhibitor:** The pump is specifically inhibited by **Cardiac Glycosides** (e.g., **Ouabain** and **Digoxin**). Digoxin binds to the extracellular alpha-subunit. * **Subunits:** It is a heterodimer consisting of an **Alpha subunit** (catalytic, contains ATP and ion binding sites) and a **Beta subunit** (essential for membrane trafficking). * **Energy Consumption:** This pump accounts for approximately **24% to 33%** of the total basal energy expenditure in most cells (up to 70% in neurons). * **Functions:** It maintains cell volume (preventing swelling), maintains the Na⁺ gradient for secondary active transport (e.g., SGLT), and contributes to the RMP.

Question 420: Which of the following is not a component of the dystrophin-glycoprotein complex?

A. Perlecan (Correct Answer)
B. Dystrophin
C. Dystroglycan
D. Sarcoglycan

Explanation: **Explanation:** The **Dystrophin-Glycoprotein Complex (DGC)** is a vital multisubunit complex that bridges the inner cytoskeleton (actin) of a muscle fiber to the surrounding extracellular matrix (laminin). This linkage provides structural stability to the sarcolemma during muscle contraction and relaxation. * **Why Perlecan is the correct answer:** Perlecan is a large heparan sulfate proteoglycan found in the **extracellular matrix (basal lamina)**. While it interacts with components of the muscle basement membrane, it is **not** an integral component of the DGC itself. * **Why other options are incorrect:** * **Dystrophin:** An intracellular protein that links the F-actin cytoskeleton to the β-dystroglycan. * **Dystroglycans (α and β):** These subunits span the membrane; α-dystroglycan binds to laminin-2 in the matrix, while β-dystroglycan binds to dystrophin. * **Sarcoglycans (α, β, γ, δ):** A subcomplex of transmembrane proteins that stabilize the DGC. **High-Yield Clinical Pearls for NEET-PG:** 1. **Duchenne Muscular Dystrophy (DMD):** Caused by an **absence** of dystrophin (X-linked recessive). It is the most common and severe form. 2. **Becker Muscular Dystrophy (BMD):** Caused by **abnormal/reduced** dystrophin; presents with a milder phenotype. 3. **Limb-Girdle Muscular Dystrophy (LGMD):** Often associated with mutations in the **sarcoglycan** complex. 4. **Gowers' Sign:** A classic clinical finding in DMD where the child uses their hands to "climb up" their own thighs to stand up due to proximal muscle weakness.

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