Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 41: Which of the following is a marker of the Golgi apparatus?

A. Galactosyl transferase (Correct Answer)
B. Glucose 6 phosphatase
C. Acid phosphatase
D. Pseudocatalase

Explanation: **Explanation:** The Golgi apparatus is the "packaging and processing center" of the cell. Its primary function involves the post-translational modification of proteins and lipids, specifically **glycosylation** (adding sugar moieties). **1. Why Galactosyl transferase is correct:** Galactosyl transferase is an enzyme located within the cisternae of the Golgi complex. It facilitates the transfer of galactose residues to proteins and lipids. Because it is highly localized to this organelle, it serves as the definitive **biochemical marker** for identifying the Golgi apparatus in subcellular fractionation studies. **2. Why the other options are incorrect:** * **Glucose 6 phosphatase:** This is the classic marker for the **Smooth Endoplasmic Reticulum (SER)**. It plays a critical role in gluconeogenesis and glycogenolysis by converting Glucose-6-phosphate to free glucose. * **Acid phosphatase:** This is the hallmark marker for **Lysosomes**. These organelles contain acid hydrolases that function optimally at a low pH to digest cellular debris. * **Pseudocatalase (and Catalase):** These are markers for **Peroxisomes** (microbodies), which are involved in the oxidation of very-long-chain fatty acids (VLCFA) and the detoxification of hydrogen peroxide. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Golgi Functions:** Proteolysis of precursors (e.g., proinsulin to insulin), sulfation, and sorting proteins to their final destinations. * **I-Cell Disease:** A clinical correlation where a deficiency in a Golgi enzyme (phosphotransferase) leads to the failure of tagging lysosomal enzymes with **Mannose-6-Phosphate**, causing them to be secreted extracellularly rather than sent to lysosomes. * **Other Markers to Remember:** * **Mitochondria:** ATP synthase / Cytochrome Oxidase. * **Nucleus:** DNA Polymerase. * **Plasma Membrane:** Na⁺-K⁺ ATPase / Adenylate cyclase.

Question 42: A single cell within a culture of freshly isolated cardiac muscle cells is injected with a fluorescent dye that cannot cross cell membranes. Within minutes, several adjacent cells become fluorescent. The most likely explanation for this observation is the presence of?

A. Gap junctions (Correct Answer)
B. IP3 receptors
C. Transverse tubules
D. Desmosomes

Explanation: **Explanation:** The observation of a membrane-impermeable dye spreading from one cardiac cell to its neighbors indicates the presence of **Gap Junctions**. These are specialized intercellular channels that allow the direct passage of ions and small water-soluble molecules (typically <1000 Daltons) between adjacent cells without entering the extracellular space. In cardiac muscle, gap junctions are located within the **intercalated discs**, facilitating electrical coupling and ensuring the heart functions as a **functional syncytium**. **Why other options are incorrect:** * **IP3 Receptors:** These are ligand-gated calcium channels located on the **Sarcoplasmic Reticulum (SR)** membrane. They mediate intracellular calcium release but do not provide a pathway for molecules to move between different cells. * **Transverse (T) Tubules:** These are deep invaginations of the sarcolemma that conduct action potentials into the cell interior to reach the SR. They communicate with the extracellular space, not with adjacent cells. * **Desmosomes (Macula Adherens):** Also found in intercalated discs, these provide **mechanical stability** by anchoring intermediate filaments (desmin) between cells. They prevent cells from pulling apart during contraction but do not allow for the exchange of cytoplasmic contents. **High-Yield NEET-PG Pearls:** * **Connexons:** Gap junctions are composed of six protein subunits called **connexins**. * **Velocity:** Gap junctions are most abundant in the **Purkinje fibers**, contributing to their high conduction velocity. * **Clinical Correlation:** Mutations in connexin genes (e.g., Connexin 43) are linked to arrhythmogenic disorders. * **Intercalated Disc Components:** Remember the triad—Gap junctions (electrical), Desmosomes (mechanical), and Fascia adherens (actin anchoring).

Question 43: Vesicles leaving the trans-Golgi carry on their surfaces a protein that targets them to the appropriate organelle. What is this protein?

A. t-SNARE
B. Coatomer
C. v-SNARE (Correct Answer)
D. Clathrin

Explanation: **Explanation:** The correct answer is **v-SNARE**. **Underlying Concept:** Vesicular transport is a highly regulated process ensuring that proteins reach their correct destination. When a vesicle buds off from the trans-Golgi network, it incorporates specific integral membrane proteins called **v-SNAREs** (vesicle-SNAREs) into its membrane. These act as "molecular addresses." When the vesicle reaches its target organelle, the v-SNARE binds specifically to a complementary **t-SNARE** (target-SNARE) on the destination membrane. This interaction forms a "SNARE complex," which provides the energy required for membrane fusion and cargo delivery. **Analysis of Incorrect Options:** * **t-SNARE:** These are located on the **target membrane** (e.g., plasma membrane or lysosome), not on the surface of the traveling vesicle. * **Coatomer (COP I/II):** These are protein complexes that help in the **formation/budding** of vesicles (COP II for anterograde and COP I for retrograde transport). They usually dissociate shortly after the vesicle is formed. * **Clathrin:** This is a coat protein involved in **endocytosis** and the formation of vesicles from the trans-Golgi destined for lysosomes. Like coatomers, clathrin helps shape the vesicle but does not serve as the primary targeting/docking signal. **High-Yield Clinical Pearls for NEET-PG:** * **Tetanus and Botulinum toxins:** These act by proteolytically cleaving SNARE proteins (like Synaptobrevin/v-SNARE or Syntaxin/t-SNARE), thereby inhibiting neurotransmitter release at the neuromuscular junction or spinal cord. * **Rab proteins:** These are small GTPases that help the initial "tethering" of the vesicle before the SNAREs lock together. * **Anterograde transport:** Mediated by **COP II** (Golgi → ER). * **Retrograde transport:** Mediated by **COP I** (ER → Golgi).

Question 44: Which of the following best describes exocytosis?

A. Extrusion of cell-bound vesicles (Correct Answer)
B. Inclusion of liquid particles
C. Inclusion of solid particles
D. All of the above

Explanation: **Explanation:** **Exocytosis** is the process of bulk transport where a cell exports materials into the extracellular fluid. This occurs when intracellular membrane-bound vesicles (containing proteins, hormones, or neurotransmitters) migrate to the cell membrane, fuse with it, and release their contents into the external environment. This is an **energy-dependent (active)** process often triggered by an increase in intracellular calcium levels. **Analysis of Options:** * **Option A (Correct):** Exocytosis literally means "out of the cell." It involves the **extrusion** of materials via vesicles. Examples include the release of insulin from pancreatic beta cells or neurotransmitters at the synaptic cleft. * **Option B (Incorrect):** The inclusion (taking in) of liquid particles is called **Pinocytosis** ("cell drinking"), which is a form of endocytosis. * **Option C (Incorrect):** The inclusion of solid particles (like bacteria or cell debris) is called **Phagocytosis** ("cell eating"), another form of endocytosis. * **Option D (Incorrect):** Since B and C describe endocytosis (internalization), they are functionally opposite to exocytosis. **High-Yield NEET-PG Pearls:** 1. **SNARE Proteins:** These are critical for exocytosis. **v-SNAREs** (on the vesicle) and **t-SNAREs** (on the target membrane) interact to facilitate docking and fusion. 2. **Clinical Correlation:** **Tetanus and Botulinum toxins** act by cleaving SNARE proteins, thereby inhibiting the exocytosis of neurotransmitters (GABA/Glycine and Acetylcholine, respectively). 3. **Transcytosis:** A combination of endocytosis and exocytosis used to move substances across an entire cell (e.g., IgA secretion).

Question 45: IP-3 facilitates the entry of which of the following ions into the cytoplasm?

A. Na+
B. K+
C. Ca2+ (Correct Answer)
D. Mg2+

Explanation: **Explanation:** The correct answer is **C. Ca2+**. **Mechanism of Action:** Inositol triphosphate (IP3) is a key second messenger in the **G-protein coupled receptor (GPCR) signaling pathway**, specifically the Gq pathway. When a ligand binds to a Gq-coupled receptor, the enzyme **Phospholipase C (PLC)** is activated. PLC cleaves the membrane phospholipid PIP2 into two fragments: **Diacylglycerol (DAG)** and **IP3**. IP3 is water-soluble and diffuses into the cytoplasm, where it binds to specific **IP3 receptors** located on the membrane of the **Endoplasmic Reticulum (ER)** (or Sarcoplasmic Reticulum in muscle). These receptors act as ligand-gated calcium channels. Upon binding, they open and allow Ca2+ to flow down its concentration gradient from the ER lumen into the cytoplasm, leading to various cellular responses such as muscle contraction, secretion, or enzyme activation. **Why other options are incorrect:** * **A (Na+) & B (K+):** These ions are primarily regulated by voltage-gated channels or ion pumps (like Na+/K+ ATPase) and are involved in membrane potential and action potentials, not the IP3 signaling pathway. * **D (Mg2+):** Magnesium acts as a cofactor for many enzymes (especially those using ATP) but is not mobilized from intracellular stores by the IP3 pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Gq-coupled receptors (Mnemonic: HAV 1 M&M):** **H**1, **A**lpha-1, **V**1, **M**1, and **M**3 receptors all utilize the IP3/DAG pathway. * **DAG's Role:** While IP3 increases intracellular Ca2+, DAG remains in the membrane to activate **Protein Kinase C (PKC)**. * **Termination:** The IP3 signal is terminated by dephosphorylation into IP2 or phosphorylation into IP4.

Question 46: Across the cell membrane of a skeletal muscle cell in the resting state with a non-electrogenic pump (1 Na+ : 1K+), which of the following statements is true?

A. Both sodium and potassium are in electrochemical equilibrium.
B. The rate of sodium influx is equal to the rate of potassium efflux. (Correct Answer)
C. The rate of sodium influx is much less than the rate of potassium efflux.
D. The rate of sodium influx is much greater than the rate of potassium efflux.

Explanation: ### Explanation **Concept: The Steady State of Resting Membrane Potential (RMP)** In a resting skeletal muscle cell, the membrane potential is stable (not changing). For the RMP to remain constant, there must be no net change in the total charge inside the cell. While the membrane is significantly more permeable to $K^+$ than to $Na^+$, the driving force for $Na^+$ (moving into the cell) is much larger than the driving force for $K^+$ (moving out of the cell). In this specific scenario involving a **non-electrogenic pump (1:1 ratio)**, the pump moves one $Na^+$ out for every one $K^+$ it moves in. To maintain a steady state where ionic concentrations do not change over time: * The amount of $Na^+$ leaking **in** must be exactly countered by the pump moving $Na^+$ **out**. * The amount of $K^+$ leaking **out** must be exactly countered by the pump moving $K^+$ **in**. * Since the pump moves $Na^+$ and $K^+$ at a **1:1 rate**, the passive leak rates (influx of $Na^+$ and efflux of $K^+$) must also be **equal** to maintain equilibrium. **Analysis of Options:** * **Option A is incorrect:** Neither ion is in electrochemical equilibrium at RMP. If they were, their respective equilibrium potentials ($E_{Na} \approx +65mV, E_K \approx -90mV$) would equal the RMP ($\approx -80$ to $-90mV$). * **Options C and D are incorrect:** If influx and efflux were unequal, the intracellular concentrations would shift, and the membrane potential would continuously change, violating the definition of a "resting state." **High-Yield NEET-PG Pearls:** 1. **Normal Na+/K+ ATPase:** In physiological conditions, the pump is **electrogenic (3 $Na^+$ out : 2 $K^+$ in)**. In that case, the $Na^+$ influx is actually $1.5 \times$ the $K^+$ efflux to maintain steady state. 2. **Gibbs-Donnan Effect:** Describes the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly due to impermeable proteins. 3. **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP by considering the permeability and concentration gradients of all major ions ($Na^+, K^+, Cl^-$).

Question 47: All of the following are true about gap junctions except:

A. Permits passage of molecules up to 1000 Da molecular weight.
B. Opening of gap junctions does not depend on transmembrane voltage. (Correct Answer)
C. Halothane closes gap junctions reversibly.
D. A gap junction is made up of connexons, each made of six identical connexins.

Explanation: ### Explanation **Gap junctions** are specialized intercellular connections that allow direct electrical and chemical communication between adjacent cells. **Why Option B is the Correct Answer (The False Statement):** Contrary to the statement, gap junction conductance is **voltage-dependent**. The permeability of these channels is regulated by both the **transmembrane voltage** (potential difference between the inside and outside of the cell) and the **transjunctional voltage** (potential difference between the two coupled cells). This gating mechanism ensures that communication can be modulated based on the electrical state of the tissue. **Analysis of Other Options:** * **Option A:** Gap junctions act as molecular sieves. They typically permit the passage of water, ions (Na⁺, K⁺, Ca²⁺), and small signaling molecules (cAMP, IP3) with a molecular weight cutoff of approximately **1000 Da**. * **Option C:** Gap junctions are sensitive to chemical environments. General anesthetics like **halothane** and long-chain alcohols (e.g., heptanol) are known to **reversibly close** gap junctions, disrupting cellular coupling. * **Option D:** The structural unit of a gap junction is the **connexon** (hemichannel). Each connexon is a hexamer composed of **six protein subunits called connexins**. When connexons from two neighboring cells align, they form a continuous aqueous pore. **High-Yield NEET-PG Pearls:** * **Location:** Most abundant in **cardiac muscle** (intercalated discs) and **smooth muscle**, facilitating "functional syncytium." They are notably **absent in skeletal muscle**. * **Regulation:** Gap junctions **close** in response to **high intracellular Ca²⁺** and **low intracellular pH (acidosis)**. This serves as a protective mechanism to decouple damaged cells from healthy ones. * **Clinical Correlation:** Mutations in connexin genes are linked to specific pathologies, such as **Connexin 26** mutations causing congenital deafness and **Connexin 32** mutations linked to X-linked Charcot-Marie-Tooth disease.

Question 48: Which of the following is NOT a component of the basement membrane?

A. Tenascin
B. Laminin
C. Enatactin (Nidogen)
D. Rhodopsin (Correct Answer)

Explanation: ### Explanation The **Basement Membrane (BM)** is a specialized form of extracellular matrix that provides structural support to tissues and acts as a selective barrier. It is primarily composed of four major glycoproteins: **Type IV collagen, Laminin, Entactin (Nidogen), and Heparan sulfate proteoglycans (Perlecan).** **Why Rhodopsin is the Correct Answer:** **Rhodopsin** is a biological pigment found in the rod cells of the retina. It is a G-protein-coupled receptor (GPCR) responsible for **phototransduction** (vision in low light). It is a transmembrane protein, not a structural component of the extracellular basement membrane. **Analysis of Incorrect Options:** * **Laminin:** This is the most abundant non-collagenous glycoprotein in the basement membrane. It is crucial for cell adhesion and anchoring the epithelium to the BM. * **Entactin (Nidogen):** This is a rod-like molecule that functions as a "molecular bridge." It binds both Laminin and Type IV collagen, stabilizing the basement membrane structure. * **Tenascin:** This is an extracellular matrix glycoprotein involved in cell adhesion. While more prominent during embryogenesis and wound healing, it is a recognized component of the basal lamina/basement membrane complex in various tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Goodpasture Syndrome:** Characterized by autoantibodies against the **α-3 chain of Type IV collagen**, leading to glomerulonephritis and pulmonary hemorrhage. * **Alport Syndrome:** A genetic defect in **Type IV collagen** synthesis, resulting in "split basement membrane" (basket-weave appearance), leading to nephritis and sensorineural deafness. * **Laminin-5 Deficiency:** Associated with **Junctional Epidermolysis Bullosa**, a severe skin blistering disease.

Question 49: Which of the following statements is NOT true about microtubules?

A. Exhibit dynamic instability
B. Possess polarity
C. Are charged
D. Do not require GTP (Correct Answer)

Explanation: ### Explanation **Microtubules** are hollow, cylindrical structures composed of **$\alpha$- and $\beta$-tubulin dimers**. They are essential components of the cytoskeleton, providing structural support and acting as tracks for intracellular transport. **Why Option D is the Correct Answer (The "Not True" Statement):** Microtubule assembly is a **GTP-dependent process**. Each tubulin dimer binds to two molecules of GTP. While the GTP bound to $\alpha$-tubulin is structural, the GTP bound to $\beta$-tubulin is hydrolyzed to GDP shortly after polymerization. This hydrolysis weakens the binding affinity between dimers; therefore, a "GTP cap" is required at the growing end to maintain stability. Without GTP, microtubules cannot polymerize and will undergo rapid depolymerization (catastrophe). **Analysis of Incorrect Options:** * **A. Exhibit dynamic instability:** This is a hallmark of microtubules. They exist in a constant state of switching between phases of slow growth and rapid shrinkage, allowing the cell to remodel its cytoskeleton quickly. * **B. Possess polarity:** Microtubules are polar structures. They have a **plus (+) end** (fast-growing, extending toward the cell periphery) and a **minus (-) end** (usually anchored in the Microtubule Organizing Center or Centrosome). * **C. Are charged:** Tubulin proteins contain a high proportion of acidic amino acids (like glutamate), giving microtubules a **net negative surface charge**. This charge is vital for interacting with motor proteins and ions. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Motors:** **Kinesin** moves cargo toward the (+) end (anterograde), while **Dynein** moves cargo toward the (-) end (retrograde). * **Drugs targeting Microtubules:** * *Inhibit Polymerization:* Colchicine, Griseofulvin, Vincristine/Vinblastine. * *Inhibit Depolymerization (Stabilizers):* Paclitaxel (Taxanes). * **Structure:** They form the core of **cilia and flagella** in a 9+2 arrangement (axoneme).

Question 50: All of the following statements about phagocytosis are true, except:

A. Amoeba and other unicellular protozoa make their living out of this process.
B. Used to ingest particles < 0.5 microns in size. (Correct Answer)
C. Used to ingest particles > 0.5 microns in size.
D. Digestion occurs within phagolysosomes.

Explanation: **Explanation:** **1. Why Option B is the Correct Answer (The "Except" Statement):** Phagocytosis, often referred to as "cell eating," is a specific form of endocytosis used for the ingestion of **large particulate matter**. By definition, phagocytosis involves the engulfment of particles **greater than 0.5 µm** in diameter (such as bacteria, dead tissue cells, or mineral particles). Ingestion of particles **smaller than 0.5 µm** occurs via **pinocytosis** ("cell drinking") or receptor-mediated endocytosis. Therefore, statement B is factually incorrect regarding phagocytosis. **2. Analysis of Other Options:** * **Option A:** In unicellular organisms like Amoeba, phagocytosis is the primary mechanism for acquiring nutrients (feeding), making it essential for their survival. * **Option C:** This is the defining characteristic of phagocytosis. It requires the reorganization of the actin cytoskeleton to form pseudopodia that wrap around large targets. * **Option D:** Once a particle is internalized in a **phagosome**, it fuses with a lysosome containing hydrolytic enzymes and reactive oxygen species. This combined structure is the **phagolysosome**, where enzymatic digestion occurs. **3. NEET-PG High-Yield Pearls:** * **Professional Phagocytes:** In humans, the primary phagocytic cells are **Neutrophils** and **Macrophages** (Monocytes). * **Mechanism:** It is an **active, ATP-dependent process** triggered by the binding of ligands (like opsonins) to specific receptors on the phagocyte surface. * **Opsonization:** The process of coating a particle with antibodies (IgG) or complement (C3b) to enhance phagocytosis is a frequent exam topic. * **Clinical Correlation:** Defects in phagocytosis are seen in **Chediak-Higashi syndrome** (microtubule defect) and **Chronic Granulomatous Disease** (NADPH oxidase deficiency).

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