Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 211: Which receptors on the cell membrane activate an ion channel after binding with agonists?

A. Nicotinic cholinergic (Correct Answer)
B. Muscarinic cholinergic
C. Opioid mu receptors
D. GABA B receptors

Explanation: **Explanation:** The question tests the classification of cell surface receptors based on their signaling mechanisms. Receptors are broadly divided into **Ionotropic** (ligand-gated ion channels) and **Metabotropic** (G-protein coupled receptors). **1. Why Nicotinic Cholinergic is Correct:** Nicotinic receptors (both $N_m$ and $N_n$ subtypes) are classic examples of **ionotropic receptors**. Upon binding with an agonist (Acetylcholine), the receptor undergoes a conformational change that directly opens an integral ion channel. This allows the rapid influx of $Na^+$ (and efflux of $K^+$), leading to immediate membrane depolarization. This rapid action is essential for neuromuscular transmission and autonomic ganglionic signaling. **2. Why the other options are Incorrect:** * **Muscarinic Cholinergic (B):** These are **G-protein coupled receptors (GPCRs)**. They act through second messengers like $IP_3/DAG$ ($M_1, M_3$) or by inhibiting Adenylyl Cyclase ($M_2$). They do not contain an intrinsic ion channel. * **Opioid Mu Receptors (C):** These are **$G_i/G_o$ protein-coupled receptors**. They work by inhibiting adenylyl cyclase and indirectly modulating potassium and calcium channels via G-protein subunits, rather than being the channel themselves. * **GABA B Receptors (D):** Unlike GABA A (which is ionotropic/chloride channel), **GABA B is metabotropic (GPCR)**. It acts via G-proteins to increase $K^+$ conductance or decrease $Ca^{2+}$ influx. **High-Yield Clinical Pearls for NEET-PG:** * **Ionotropic Receptors (Fast):** Nicotinic (ACh), GABA A, Glycine, 5-HT3, and Glutamate (NMDA/AMPA) receptors. * **Metabotropic Receptors (Slow):** All Muscarinic, all Adrenergic, GABA B, and most Serotonin receptors (except 5-HT3). * **Exam Tip:** If a receptor's action results in "milli-second" speed responses, it is almost always an ionotropic receptor.

Question 212: Which of the following represents an ATP-dependent transport mechanism?

A. Facilitated diffusion
B. Osmosis
C. Active transport (Correct Answer)
D. All of the above

Explanation: ### Explanation **Correct Answer: C. Active transport** **Underlying Concept:** Transport across cell membranes is classified based on energy requirements. **Active transport** is the movement of molecules or ions against their electrochemical or concentration gradient (from low to high concentration). This process is "uphill" and requires energy, which is derived directly or indirectly from the hydrolysis of **Adenosine Triphosphate (ATP)**. * **Primary Active Transport:** Directly uses ATP (e.g., Na⁺/K⁺-ATPase pump). * **Secondary Active Transport:** Uses the energy stored in an electrochemical gradient created by primary active transport (e.g., SGLT-1). **Why Incorrect Options are Wrong:** * **A. Facilitated Diffusion:** This is a form of passive transport. While it requires a carrier protein (like GLUT-4), it moves substances *down* their concentration gradient and does **not** require ATP. * **B. Osmosis:** This is the passive movement of water molecules across a semi-permeable membrane from a region of low solute concentration to high solute concentration. It is driven by osmotic pressure, not ATP. * **D. All of the above:** Incorrect because options A and B are passive processes. **High-Yield Clinical Pearls for NEET-PG:** * **Na⁺/K⁺-ATPase:** The most important primary active transporter. It pumps **3 Na⁺ out** and **2 K⁺ in**, maintaining the resting membrane potential. It is inhibited by **Digitalis (Digoxin)**. * **SGLT-1 (Sodium-Glucose Linked Transporter):** A classic example of secondary active transport (symport) located in the small intestine and renal tubules. * **Vesicular Transport:** Endocytosis and exocytosis are also ATP-dependent processes but are categorized as bulk transport.

Question 213: The process by which fusion of part of a cell membrane occurs is:

A. Cell division
B. Endocytosis
C. Exocytosis
D. Virus replication (Correct Answer)

Explanation: **Explanation:** The core concept behind this question is **membrane fusion**, a process where two separate lipid bilayers merge to form a single continuous structure. **Why Virus Replication is Correct:** Enveloped viruses (such as HIV, Influenza, or SARS-CoV-2) must fuse their viral envelope with the host cell membrane (either at the plasma membrane or endosomal membrane) to release their genetic material into the cytoplasm. This process is mediated by specific **fusion proteins** (e.g., gp41 in HIV). Unlike the other options, which primarily involve membrane budding or invagination, viral entry is the classic physiological example where two distinct membranes undergo complete fusion to create a pore. **Why Other Options are Incorrect:** * **Cell Division:** This involves **fission** (cytokinesis), where a single cell membrane pinches off to form two separate cells. It is the opposite of fusion. * **Endocytosis:** This involves the **invagination** of the plasma membrane to form a vesicle. While the neck of the vesicle eventually pinches off (fission), the primary process is the internalization of the membrane, not the fusion of two separate membrane parts. * **Exocytosis:** While the secretory vesicle membrane eventually fuses with the plasma membrane to release contents, the question specifically highlights "fusion of part of a cell membrane" as a defining characteristic of the process. In a comparative MCQ context, viral replication is the more specific example of membrane-to-membrane fusion. **High-Yield Clinical Pearls for NEET-PG:** * **SNARE Proteins:** These are essential for the fusion of vesicles with the plasma membrane during exocytosis. * **Viral Fusion Inhibitors:** Drugs like **Enfuvirtide** (T-20) act by binding to gp41, preventing the fusion of the HIV envelope with the host cell membrane. * **Syncytium Formation:** Some viruses cause infected cells to fuse with neighboring healthy cells, forming multinucleated giant cells (e.g., RSV, Herpes), another example of membrane fusion in pathology.

Question 214: What is the most abundant glycoprotein present in the basement membrane?

A. Laminin (Correct Answer)
B. Fibronectin
C. Collagen type-4
D. Heparan sulphate

Explanation: **Explanation:** The basement membrane is a specialized form of extracellular matrix (ECM) that provides structural support and cell signaling. **1. Why Laminin is correct:** Laminin is the **most abundant non-collagenous glycoprotein** in the basement membrane. It is a large, heterotrimeric protein (composed of α, β, and γ chains) that forms a cross-shaped structure. Its primary role is to act as the "glue" that anchors epithelial cells to the basement membrane by binding to cell-surface integrins and other ECM components like Type IV collagen. **2. Why the other options are incorrect:** * **Collagen Type IV:** While it is the most abundant **protein** (structural framework) in the basement membrane, it is categorized as a fibrous protein rather than a primary adhesive glycoprotein. It forms a multi-layered network that provides tensile strength. * **Fibronectin:** This is a major glycoprotein of the **interstitial matrix** rather than the basement membrane. It plays a key role in cell adhesion and migration during wound healing. * **Heparan Sulphate:** This is a **proteoglycan** (specifically Perlecan), not a glycoprotein. It provides the negative charge to the basement membrane, which is crucial for selective filtration in the renal glomerulus. **Clinical Pearls & High-Yield Facts:** * **Goodpasture Syndrome:** Antibodies are directed against the α3 chain of **Type IV Collagen**, leading to glomerulonephritis and pulmonary hemorrhage. * **Junctional Epidermolysis Bullosa:** Often caused by genetic mutations in **Laminin-332**, leading to severe skin blistering. * **Alport Syndrome:** A genetic defect in **Type IV Collagen** synthesis, characterized by nephritis and sensorineural deafness. * **Entactin (Nidogen):** Another glycoprotein that functions as a bridge, linking laminin and type IV collagen.

Question 215: What is the role of the membrane protein clathrin?

A. Cell motility
B. Cell shape
C. Exocytosis
D. Receptor-mediated endocytosis (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Receptor-mediated endocytosis** **Mechanism:** Clathrin is a specialized protein that plays a critical role in the formation of **coated vesicles**. During receptor-mediated endocytosis, specific ligands bind to their respective surface receptors. This triggers the recruitment of clathrin molecules from the cytoplasm to the inner surface of the plasma membrane. Clathrin assembles into a geometric, lattice-like structure called a **triskelion**, which physically deforms the membrane to form a "clathrin-coated pit." This pit eventually invaginates and pinches off (aided by the GTPase **dynamin**) to become an intracellular vesicle. **Analysis of Incorrect Options:** * **A & B (Cell Motility and Shape):** These functions are primarily governed by the **cytoskeleton**, specifically actin microfilaments (motility/lamellipodia) and intermediate filaments or microtubules (structural integrity/shape). * **C (Exocytosis):** While exocytosis involves vesicle fusion, it is generally mediated by **SNARE proteins** (v-SNARE and t-SNARE) rather than clathrin. Clathrin is specifically associated with the *internalization* of membranes. **High-Yield NEET-PG Pearls:** * **Dynamin:** The "molecular scissor" that pinches off the clathrin-coated vesicle from the cell membrane. * **LDL Uptake:** The classic example of clathrin-mediated endocytosis is the cellular uptake of **Low-Density Lipoprotein (LDL)** and **Transferrin**. * **Triskelion:** The basic structural unit of clathrin, consisting of three heavy chains and three light chains. * **Caveolae:** An alternative pathway for endocytosis involving the protein **caveolin** (often associated with lipid rafts), distinct from the clathrin pathway.

Question 216: Which of the following statements about Nitric Oxide is FALSE?

A. Produced from arginine
B. Nitric oxide synthase has three isoforms
C. Also known as endothelium-derived relaxing factor
D. Acts through cyclic GMP (Correct Answer)

Explanation: **Explanation:** The question asks for the **FALSE** statement regarding Nitric Oxide (NO). While NO does indeed act through the cyclic GMP (cGMP) pathway, the phrasing of the options suggests a nuance in the mechanism of action or a potential error in the question's premise. However, based on standard physiological principles, let’s analyze the facts: 1. **Mechanism of Action (Option D):** Nitric Oxide is a gas that diffuses across membranes and binds to **soluble Guanylyl Cyclase (sGC)**. This enzyme converts GTP to **cyclic GMP (cGMP)**. cGMP then activates Protein Kinase G (PKG), leading to dephosphorylation of myosin light chains and subsequent smooth muscle relaxation. Since this is a **true** statement, its selection as the "False" answer in some question banks often stems from a technicality (e.g., it acts *via* sGC to *produce* cGMP, rather than being cGMP itself) or a misprint in the source material. 2. **Synthesis (Option A):** This is **TRUE**. NO is synthesized from the amino acid **L-arginine** by the enzyme Nitric Oxide Synthase (NOS), requiring oxygen and NADPH as cofactors. 3. **Isoforms (Option B):** This is **TRUE**. There are three isoforms of NOS: **nNOS** (Neuronal/Type 1), **iNOS** (Inducible/Type 2), and **eNOS** (Endothelial/Type 3). 4. **EDRF (Option C):** This is **TRUE**. NO was historically identified as **Endothelium-Derived Relaxing Factor** by Furchgott, Ignarro, and Murad (Nobel Prize 1998). **Clinical Pearls for NEET-PG:** * **Vasodilation:** NO is the most potent endogenous vasodilator. * **Therapeutic use:** Nitroglycerin works by releasing NO to treat angina. * **Sildenafil (Viagra):** Inhibits Phosphodiesterase-5 (PDE-5), the enzyme that breaks down cGMP, thereby prolonging the effects of NO. * **Septic Shock:** Overproduction of NO by **iNOS** leads to the profound hypotension seen in sepsis.

Question 217: Which of the following is not a component of the extracellular matrix?

A. Laminin
B. Fibronectin
C. Vimentin (Correct Answer)
D. Collagen

Explanation: **Explanation:** The **Extracellular Matrix (ECM)** is a complex network of macromolecules located outside the cell membrane that provides structural and biochemical support to surrounding cells. It primarily consists of fibrous proteins (Collagen, Elastin) and adhesive glycoproteins (Laminin, Fibronectin) embedded in a ground substance of glycosaminoglycans (GAGs). **Why Vimentin is the correct answer:** **Vimentin** is an **intermediate filament** protein found exclusively **inside** the cell (intracellularly). It is a major component of the cytoskeleton in cells of mesenchymal origin (e.g., fibroblasts, endothelial cells, and leukocytes). Because it is a cytoskeletal element and not secreted into the interstitial space, it is not a component of the ECM. **Analysis of Incorrect Options:** * **Collagen (D):** The most abundant protein in the ECM, providing tensile strength to tissues. * **Laminin (A):** A major adhesive glycoprotein found primarily in the **basal lamina**; it helps anchor epithelial cells to the underlying connective tissue. * **Fibronectin (B):** An adhesive glycoprotein that connects the ECM (collagen) to the cell surface (integrins), playing a vital role in cell adhesion and migration. **High-Yield Clinical Pearls for NEET-PG:** * **Vimentin Staining:** In pathology, Vimentin is used as a tumor marker for **sarcomas** (tumors of mesenchymal origin). * **Epithelial-Mesenchymal Transition (EMT):** During cancer metastasis, cells often lose epithelial markers (E-cadherin) and gain **Vimentin**, increasing their motility. * **Scurvy:** A defect in collagen synthesis due to Vitamin C deficiency, leading to weakened ECM and poor wound healing.

Question 218: Structurally, connexons are:

A. Hexamers (Correct Answer)
B. Pentamers
C. Tetramers
D. Trimeric proteins

Explanation: **Explanation:** **1. Why Hexamers is Correct:** Connexons are the structural units of **gap junctions**, which facilitate direct electrical and chemical communication between adjacent cells. Each connexon (also called a hemichannel) is composed of **six individual protein subunits called connexins**. These six subunits arrange themselves in a circular fashion to form a central hydrophilic pore. When a connexon from one cell aligns with a connexon from an adjacent cell, they form a complete intercellular channel. **2. Why Other Options are Incorrect:** * **Pentamers (B):** While some ligand-gated ion channels (like the Nicotinic Acetylcholine Receptor) are pentameric, gap junction hemichannels strictly require six subunits to maintain their specific pore diameter. * **Tetramers (C):** Voltage-gated potassium channels and Aquaporins are typically tetrameric. * **Trimeric proteins (D):** Acid-sensing ion channels (ASICs) and P2X receptors are examples of trimeric structures, but this configuration is not found in gap junctions. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Function:** Gap junctions allow the passage of ions, second messengers (cAMP, $IP_3$), and small molecules (<1000 Da). * **Location:** They are vital in **cardiac muscle** (intercalated discs) and **smooth muscle** for functional syncytium, and in the CNS for electrical synapses. * **Regulation:** Connexons close in response to **high intracellular $Ca^{2+}$** or **low intracellular pH** (acidosis) to prevent the spread of damage from injured cells. * **Clinical Correlation:** Mutations in Connexin 26 are a leading cause of congenital non-syndromic **deafness**, while mutations in Connexin 32 are linked to **Charcot-Marie-Tooth disease**.

Question 219: Autophagy is the function of which cellular organelle?

A. Ribosomes
B. Centrosomes
C. Mitochondria
D. Lysosomes (Correct Answer)

Explanation: **Explanation:** **1. Why Lysosomes are the correct answer:** Lysosomes are known as the "digestive system" of the cell. They contain over 50 different types of **acid hydrolases** (active at pH ~5.0) that break down macromolecules. **Autophagy** ("self-eating") is the physiological process where the cell degrades its own damaged organelles (like old mitochondria) or unused proteins by sequestering them into double-membrane vesicles called autophagosomes, which then fuse with lysosomes for degradation. This process is essential for cellular homeostasis, recycling nutrients during starvation, and preventing the buildup of toxic aggregates. **2. Why other options are incorrect:** * **Ribosomes:** These are the sites of **protein synthesis** (translation). They do not possess degradative enzymes. * **Centrosomes:** These consist of two centrioles and are responsible for organizing microtubules and forming the **mitotic spindle** during cell division. * **Mitochondria:** Known as the "powerhouse of the cell," their primary role is **ATP production** via oxidative phosphorylation. While they are often the *targets* of autophagy (mitophagy), they do not perform the digestive function themselves. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Marker Enzyme:** Acid phosphatase is the characteristic marker enzyme for lysosomes. * **Residual Bodies:** Undigested debris remaining in lysosomes can persist as **lipofuscin** (the "wear-and-tear" pigment). * **I-Cell Disease:** A clinical condition where lysosomal enzymes fail to be phosphorylated (Man-6-P tag) in the Golgi, leading to their secretion outside the cell instead of being packed into lysosomes. * **Autophagy Genes:** Mutations in **ATG genes** are linked to neurodegenerative diseases like Parkinson’s and Alzheimer’s.

Question 220: Movement of substances across the intercellular space is prevented by which of the following structures?

A. Zona adherens
B. Tight junction (Correct Answer)
C. Gap junction
D. Desmosomes

Explanation: **Explanation:** The correct answer is **Tight Junction (Zonula Occludens)**. **1. Why Tight Junction is correct:** Tight junctions are the most apical components of the junctional complex. They are formed by the fusion of the outer leaflets of adjacent cell membranes via transmembrane proteins like **claudins and occludins**. Their primary function is to act as a **selective barrier**, sealing the intercellular space (paracellular pathway). This prevents the free movement of water, ions, and macromolecules between cells, thereby maintaining distinct chemical environments on either side of the epithelial sheet. **2. Why other options are incorrect:** * **Zona Adherens (Adherens Junction):** These are anchoring junctions located below tight junctions. They connect the actin cytoskeletons of adjacent cells using **cadherins** but do not seal the intercellular space. * **Gap Junction (Nexus):** These are communicating junctions composed of **connexons**. They allow the direct passage of small ions and signaling molecules *between* cells (intracellularly), rather than preventing movement in the space *around* them. * **Desmosomes (Macula Adherens):** These provide strong mechanical adhesion by linking intermediate filaments (keratin) of adjacent cells. They are like "spot welds" and do not form a continuous seal to block paracellular transport. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood-Brain Barrier (BBB):** The physiological basis of the BBB is the presence of extensive tight junctions between capillary endothelial cells. * **Magnesium Reabsorption:** In the Thick Ascending Limb (TAL) of the Loop of Henle, magnesium is reabsorbed paracellularly through tight junctions containing **Claudin-16**. * **Pemphigus Vulgaris:** A clinical condition involving antibodies against **Desmoglein** (a component of desmosomes), leading to skin blistering.

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