Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 221: Which of the following does not belong to the molecular motor family?

A. Kinesin
B. Dynein
C. Myosin
D. Actin (Correct Answer)

Explanation: **Explanation:** Molecular motors are specialized proteins that utilize the chemical energy of **ATP hydrolysis** to generate mechanical work and movement along cytoskeletal tracks. **Why Actin is the correct answer:** Actin is a **structural protein** that forms microfilaments. It serves as the **track** or "railway" upon which molecular motors (specifically Myosin) move. While actin is essential for motility and muscle contraction, it lacks intrinsic ATPase activity to function as a motor itself. Therefore, it is a component of the cytoskeleton, not a molecular motor. **Analysis of incorrect options:** * **Kinesin:** An ATPase motor protein that typically moves toward the **plus-end** (periphery) of microtubules. It is responsible for **anterograde axonal transport**. * **Dynein:** An ATPase motor protein that moves toward the **minus-end** (centrosome) of microtubules. It mediates **retrograde axonal transport** and the beating of cilia/flagella. * **Myosin:** A large family of motor proteins (e.g., Myosin II in muscle) that move along **actin filaments**. They convert ATP into mechanical force for muscle contraction and vesicle transport. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Dynein arms**, leading to immobile cilia, bronchiectasis, and situs inversus. * **Axonal Transport:** Remember **"K"**inesin for **"K"**arrying away from the cell body (Anterograde) and **D**ynein for **D**riving back to the cell body (Retrograde). * **Microtubules** are the tracks for Kinesin and Dynein; **Microfilaments (Actin)** are the tracks for Myosin.

Question 222: What is the correct sequence of vesicle transport?

A. ER to Trans-Golgi Network to Lysosome
B. ER to Cis-Golgi Network to Trans-Golgi Network to Cell Membrane (Correct Answer)
C. ER to Lysosome to Trans-Golgi Network to Cis-Golgi Network
D. Cis-Golgi Network to ER to Trans-Golgi Network to Cell Membrane

Explanation: ### Explanation **Concept Overview** The correct sequence follows the **Secretory Pathway** (Anterograde transport). Proteins are synthesized in the Rough Endoplasmic Reticulum (ER) and must undergo post-translational modification and sorting before reaching their destination. **Why Option B is Correct** 1. **ER:** The starting point where translation and initial folding occur. 2. **Cis-Golgi Network (CGN):** The "receiving" face of the Golgi, located closest to the ER. Vesicles fuse here first. 3. **Trans-Golgi Network (TGN):** The "shipping" face. After passing through the medial cisternae for glycosylation, proteins reach the TGN for sorting into secretory vesicles. 4. **Cell Membrane:** Vesicles bud off the TGN and undergo exocytosis at the plasma membrane. **Analysis of Incorrect Options** * **Option A:** While the ER to TGN to Lysosome pathway exists (for acid hydrolases), it skips the essential **Cis-Golgi** step required for processing. * **Option C:** This suggests a retrograde or illogical flow; lysosomes are terminal degradative organelles, not intermediaries for Golgi transport. * **Option D:** This incorrectly places the Cis-Golgi before the ER. Transport from Golgi back to ER is "Retrograde" transport, but the primary secretory flow always begins at the ER. **High-Yield NEET-PG Pearls** * **COPII:** Mediates **Anterograde** transport (ER $\rightarrow$ Golgi). *Mnemonic: "Two (II) steps forward."* * **COPI:** Mediates **Retrograde** transport (Golgi $\rightarrow$ ER). *Mnemonic: "One (I) step back."* * **Clathrin:** Involved in transport from the TGN to lysosomes and endocytosis from the cell membrane. * **KDEL Sequence:** A retrieval signal on ER-resident proteins that ensures they are sent back to the ER via COPI vesicles if they accidentally escape to the Golgi. * **I-Cell Disease:** A clinical correlation where a defect in phosphotransferase prevents the tagging of enzymes with Mannose-6-Phosphate, leading to protein mistrafficking.

Question 223: What is a marker of the lysosome?

A. Acid Phosphatase (Correct Answer)
B. Lactate dehydrogenase
C. Oxidase
D. Na K ATPase

Explanation: **Explanation:** **Correct Answer: A. Acid Phosphatase** Lysosomes are membrane-bound organelles containing hydrolytic enzymes responsible for intracellular digestion. The hallmark of a lysosome is its acidic internal environment (pH ~4.5–5.0), maintained by a proton pump (V-type ATPase). **Acid phosphatase** is the primary marker enzyme for lysosomes because it is specifically localized within these organelles and requires an acidic pH for its catalytic activity. It is used histochemically to identify lysosomes in various tissues. **Analysis of Incorrect Options:** * **B. Lactate dehydrogenase (LDH):** This is a key enzyme in anaerobic glycolysis and is a classic marker for the **Cytosol**. Its presence in extracellular fluid often indicates cell membrane damage or necrosis. * **C. Oxidase:** Enzymes like Urate oxidase and Catalase are characteristic markers for **Peroxisomes** (microbodies), which are involved in long-chain fatty acid oxidation and hydrogen peroxide metabolism. * **D. Na-K ATPase:** This is the primary active transport pump located on the **Plasma Membrane**. It is the standard marker used to identify cell membrane fractions during cell fractionation. **High-Yield Clinical Pearls for NEET-PG:** * **Other Lysosomal Markers:** Cathepsins, Glucuronidase, and LAMP-1 (Lysosome-associated membrane protein 1). * **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 rather than being transported to the lysosome. * **Mitochondrial Markers:** ATP synthase (Inner membrane) and Monoamine oxidase (Outer membrane). * **Golgi Marker:** Galactosyltransferase.

Question 224: Which of the following is a function of the Golgi apparatus?

A. Modification of proteins (Correct Answer)
B. mRNA synthesis
C. Protein storage
D. tRNA synthesis

Explanation: **Explanation:** The **Golgi apparatus** acts as the "post office" or "packaging center" of the cell. Its primary role is the post-translational modification, sorting, and packaging of proteins received from the Rough Endoplasmic Reticulum (RER). **1. Why Option A is Correct:** Once proteins are synthesized in the RER, they are transported to the Golgi complex. Here, they undergo critical **modifications**, such as **glycosylation** (adding carbohydrate chains), sulfation, and phosphorylation. These modifications are essential for the protein's functional maturity and for "tagging" them to their final destinations (e.g., lysosomes, cell membrane, or secretion). **2. Why Other Options are Incorrect:** * **Options B & D (mRNA and tRNA synthesis):** These are processes of **transcription**, which occur exclusively in the **nucleus** (via RNA Polymerase II and III, respectively). The Golgi has no role in nucleic acid synthesis. * **Option C (Protein storage):** While the Golgi packages proteins into vesicles, it is not a storage organelle. Long-term protein storage is not a primary cellular function, though the RER and secretory granules hold them temporarily before transport. **High-Yield Clinical Pearls for NEET-PG:** * **I-Cell Disease:** A lysosomal storage disorder caused by a failure of the Golgi to add **Mannose-6-Phosphate** tags to enzymes. This results in enzymes being secreted extracellularly rather than being sent to lysosomes. * **Polarity:** The Golgi has a **Cis-face** (entry point near the RER) and a **Trans-face** (exit point where vesicles bud off). * **Specific Marker:** **Thiamine Pyrophosphatase (TPP)** is a histochemical marker for the Golgi apparatus.

Question 225: All of the following act as cell adhesion molecules, EXCEPT:

A. Integrin
B. Selectin
C. Cadherin
D. Lecithin (Correct Answer)

Explanation: **Explanation:** Cell Adhesion Molecules (CAMs) are specialized transmembrane proteins located on the cell surface that facilitate cell-to-cell and cell-to-matrix interactions. They are essential for tissue integrity, inflammation, and wound healing. **Why Lecithin is the correct answer:** **Lecithin (Phosphatidylcholine)** is a **phospholipid**, not a protein. It is a major structural component of cell membranes and acts as a surfactant in the lungs. It does not function as an adhesion molecule. In the context of biochemistry, it is also a source of choline for acetylcholine synthesis. **Analysis of other options:** * **Integrins (Option A):** These are heterodimeric receptors that primarily mediate **cell-matrix adhesion** (e.g., binding to fibronectin or collagen). They also play a crucial role in "inside-out" signaling and the firm adhesion phase of leukocyte extravasation. * **Selectins (Option B):** These are carbohydrate-binding lectins responsible for the initial **"rolling"** phase of leukocytes along the vascular endothelium during inflammation. (e.g., P-selectin, E-selectin). * **Cadherins (Option C):** These are **calcium-dependent** homophilic adhesion molecules. They are vital for maintaining tissue architecture (e.g., E-cadherin in epithelial desmosomes). Loss of E-cadherin is a hallmark of Epithelial-Mesenchymal Transition (EMT) in cancer metastasis. **High-Yield Clinical Pearls for NEET-PG:** * **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a defect in **Integrins** (CD18). Presents with delayed umbilical cord separation and recurrent infections without pus. * **LAD Type 2:** Caused by a defect in **Sialyl-Lewis X** (ligand for Selectins). * **Pemphigus Vulgaris:** An autoimmune disease where antibodies target **Desmoglein** (a type of Cadherin), leading to skin blistering.

Question 226: Which of the following plays a central role in the mitotic spindle formation in cellular division?

A. Ubiquitin
B. Gamma-Tubulin (Correct Answer)
C. Laminin
D. Keratin

Explanation: **Explanation:** **Correct Answer: B. Gamma-Tubulin** The mitotic spindle is composed of microtubules, which are polymers of alpha and beta-tubulin. However, the **nucleation** (initiation) of these microtubules occurs at the centrosome (Microtubule Organizing Center - MTOC). **Gamma-tubulin** is a specialized tubulin isoform located within the pericentriolar material of the centrosome. It forms the **$\gamma$-tubulin ring complex ($\gamma$TuRC)**, which acts as a circular template or "scaffold" upon which alpha and beta-tubulin dimers assemble. Without gamma-tubulin, the rapid assembly of the mitotic spindle required for chromosome segregation during mitosis would not occur. **Incorrect Options:** * **A. Ubiquitin:** A small regulatory protein that tags damaged or unneeded proteins for degradation via the proteasome (Ubiquitin-Proteasome Pathway). It is not a structural component of the spindle. * **C. Laminin:** A major glycoprotein component of the **extracellular matrix (ECM)** and basal lamina. It mediates cell adhesion and migration, not intracellular division. (Note: Do not confuse with *Lamins*, which form the nuclear scaffold). * **D. Keratin:** A type of **intermediate filament** found primarily in epithelial cells, providing mechanical strength to tissues. It does not participate in the dynamic formation of the mitotic spindle. **High-Yield Clinical Pearls for NEET-PG:** * **Microtubule Inhibitors:** Drugs like **Vincristine/Vinblastine** (inhibit polymerization) and **Paclitaxel** (inhibit depolymerization) target the mitotic spindle to arrest the cell cycle in the M-phase, making them potent anti-cancer agents. * **Structure:** Microtubules follow a **13-protofilament** arrangement. * **Molecular Motors:** **Dynein** (retrograde transport, moves toward the '-' end) and **Kinesin** (anterograde transport, moves toward the '+' end) are the motor proteins that move along these microtubules.

Question 227: Which of the following best describes the ion transport mechanism of the Na+–K+ ATPase pump?

A. 3 Na+ ions out / 2 K+ ions in (Correct Answer)
B. 3 Na+ ions in / 2 K+ ions out
C. 2 Na+ ions out / 3 K+ ions in
D. 2 Na+ ions in / 3 K+ ions out

Explanation: **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. **1. 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, the enzyme binds **3 Na+ ions** from the intracellular fluid and expels them **out** of the cell, while simultaneously binding **2 K+ ions** from the extracellular fluid and transporting them **into** the cell. This 3:2 ratio results in a net loss of one positive charge from the cell, contributing to the negative Resting Membrane Potential (RMP). **2. Why Other Options are Incorrect:** * **Options B & D:** These suggest Na+ moving *in* and K+ moving *out*. This describes the passive movement of ions through leak channels during depolarization/repolarization, not the active transport required to maintain gradients. * **Option C:** This reverses the stoichiometry. The pump must move more sodium out than potassium in to maintain osmotic balance and the electrical gradient. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitors:** The pump is specifically inhibited by **Cardiac Glycosides** (e.g., **Digoxin** and Ouabain). Digoxin binds to the extracellular K+ binding site, increasing intracellular Na+, which subsequently slows the Na+/Ca2+ exchanger, leading to increased intracellular Ca2+ and increased cardiac contractility. * **Energy Consumption:** Approximately 1/3rd of a cell’s total energy (and up to 70% in neurons) is dedicated to this pump. * **Structure:** It is a P-type ATPase consisting of $\alpha$ and $\beta$ subunits; the **$\alpha$ subunit** is the catalytic site where ATP and ions bind.

Question 228: Which of the following organelles is the major site for anaerobic metabolism?

A. Centrioles
B. Mitochondria
C. Golgi apparatus
D. Cytoplasm (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Cytoplasm** **1. Why Cytoplasm is Correct:** Anaerobic metabolism, primarily represented by **glycolysis**, occurs entirely within the **cytoplasm** (cytosol) of the cell. During this process, one molecule of glucose is broken down into two molecules of pyruvate (or lactate in the absence of oxygen). This pathway does not require oxygen or specialized membrane-bound organelles to generate ATP, making the cytoplasm the primary site for anaerobic energy production. **2. Why Other Options are Incorrect:** * **B. Mitochondria:** This is the site of **aerobic metabolism**. The Krebs cycle (TCA cycle) and the Electron Transport Chain (ETC) occur here, requiring oxygen to produce a high yield of ATP. * **C. Golgi apparatus:** This organelle is involved in the **post-translational modification**, sorting, and packaging of proteins and lipids. It does not play a direct role in energy metabolism. * **A. Centrioles:** These are cylindrical structures involved in **cell division** (spindle fiber formation) and are not involved in metabolic pathways. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **RBC Metabolism:** Mature Red Blood Cells lack mitochondria; therefore, they rely **exclusively on anaerobic glycolysis** in the cytoplasm for their energy needs. * **End Product:** In anaerobic conditions (or in cells lacking mitochondria), pyruvate is converted to **lactate** by the enzyme *Lactate Dehydrogenase (LDH)*. * **ATP Yield:** Anaerobic metabolism is inefficient, yielding only **2 ATP** per glucose molecule, compared to the ~30-32 ATP produced via aerobic respiration in the mitochondria. * **Warburg Effect:** Cancer cells often prefer anaerobic glycolysis even in the presence of oxygen (aerobic glycolysis) to support rapid growth.

Question 229: Which of these organelles functions as the digestive system of the cell?

A. Mitochondria
B. Rough endoplasmic reticulum
C. Golgi apparatus
D. Lysosomes (Correct Answer)

Explanation: **Explanation:** **Lysosomes** are known as the "digestive system" or "suicide bags" of the cell. They are membrane-bound organelles containing over 50 different types of **acid hydrolases** (e.g., proteases, nucleases, glycosidases). These enzymes function optimally at an acidic pH (~5.0), maintained by V-type ATPase proton pumps in the lysosomal membrane. They degrade macromolecules derived from endocytosis (extracellular material), autophagy (internal damaged organelles), and phagocytosis. **Why other options are incorrect:** * **Mitochondria:** Known as the "powerhouse of the cell," they are the primary site for ATP production via oxidative phosphorylation and play a key role in apoptosis (releasing Cytochrome C). * **Rough Endoplasmic Reticulum (RER):** Studded with ribosomes, its primary function is the synthesis and post-translational modification of proteins destined for secretion or membrane integration. * **Golgi Apparatus:** Acts as the "post office" or "shipping center," responsible for sorting, packaging, and modifying proteins and lipids received from the ER. **High-Yield Clinical Pearls for NEET-PG:** * **Lysosomal Storage Diseases (LSDs):** Result from a deficiency of specific lysosomal enzymes, leading to the accumulation of undigested substrates (e.g., **Gaucher’s disease**—Glucocerebrosidase deficiency; **Tay-Sachs disease**—Hexosaminidase A deficiency). * **I-Cell Disease:** A protein-targeting defect where lysosomal enzymes lack the **Mannose-6-Phosphate** tag, causing them to be secreted extracellularly rather than delivered to lysosomes. * **Marker Enzyme:** Acid phosphatase is the classic biochemical marker for lysosomes.

Question 230: Which event will NOT occur during the depolarization phase?

A. Na+ channels open
B. Ca+ channels open
C. K+ channels open (Correct Answer)
D. None of the above

Explanation: ### Explanation The correct answer is **C. K+ channels open**. #### 1. Underlying Medical Concept Depolarization is the phase of an action potential where the membrane potential becomes more positive (less negative). In a typical nerve or muscle cell, this is driven by the **influx of cations** (positive ions) into the cell. * **K+ channels** are primarily responsible for the **repolarization** phase. When K+ channels open, K+ ions exit the cell (efflux) along their concentration gradient, making the interior of the cell more negative again. Therefore, K+ channel opening is an event that follows depolarization, rather than occurring during it. #### 2. Analysis of Incorrect Options * **A. Na+ channels open:** This is the hallmark of the depolarization phase in neurons and skeletal muscle. Rapid opening of voltage-gated Na+ channels leads to a massive Na+ influx, causing the rapid upstroke of the action potential. * **B. Ca+ channels open:** In specific tissues, such as cardiac pacemaker cells (SA node) and smooth muscle, depolarization is mediated by the opening of voltage-gated Ca²+ channels. Even in ventricular myocytes, Ca²+ channels open during the late phase of depolarization to contribute to the plateau phase. #### 3. High-Yield Clinical Pearls for NEET-PG * **Threshold Potential:** The specific membrane potential (usually -55mV in nerves) at which voltage-gated Na+ channels open to trigger depolarization. * **Tetrodotoxin (TTX):** A potent neurotoxin (from Pufferfish) that blocks voltage-gated Na+ channels, preventing depolarization. * **Hyperkalemia:** Increases resting membrane potential (making it less negative), which initially makes cells more excitable but eventually leads to inactivation of Na+ channels, causing paralysis or arrhythmias. * **Absolute Refractory Period:** Occurs during depolarization and early repolarization when Na+ channels are either already open or in an inactivated state.

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