Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 191: Various cells respond differentially to a second messenger (such as increased cAMP) because they have different what?

A. Receptors
B. Enzymatic composition (Correct Answer)
C. Nuclei
D. Membrane lipids

Explanation: ### Explanation The correct answer is **B. Enzymatic composition**. **1. Why Enzymatic Composition is Correct:** The concept of "differential response" refers to how a single second messenger (like cAMP) can trigger vastly different physiological effects in different tissues. While cAMP is a universal signal, its primary function is to activate **Protein Kinase A (PKA)**. The specific outcome depends on the **intracellular enzymatic machinery** available for PKA to phosphorylate. * **Example:** In liver cells, cAMP activates enzymes leading to glycogenolysis (breakdown of glycogen). In cardiac myocytes, cAMP activates enzymes that increase calcium influx, leading to increased contractility (inotropy). The "message" is the same, but the "machinery" (enzymes) differs. **2. Why Other Options are Incorrect:** * **A. Receptors:** Receptors determine *if* a cell responds to a primary messenger (hormone/ligand). However, the question asks about the response *to* a second messenger that has already been generated inside the cell. * **C. Nuclei:** While gene expression differs between cells, the immediate metabolic or physiological response to second messengers is mediated by cytoplasmic enzymes, not the structural characteristics of the nucleus. * **D. Membrane lipids:** These provide the structural matrix of the cell and serve as precursors for some signals (like IP3/DAG), but they do not dictate the specific functional response to cAMP. **3. High-Yield Clinical Pearls for NEET-PG:** * **Signal Amplification:** A single ligand-receptor interaction can produce thousands of cAMP molecules, which in turn activate numerous enzymes—this is known as the **biological cascade**. * **Phosphodiesterases (PDE):** These enzymes terminate cAMP signaling. Drugs like **Theophylline** and **Sildenafil** work by inhibiting specific PDEs, thereby prolonging the effect of the second messenger. * **G-Protein Specificity:** Remember that **Gs** stimulates Adenylyl Cyclase (↑ cAMP), while **Gi** inhibits it. This is a frequent target for questions on autonomic pharmacology.

Question 192: All of the following statements regarding the Golgi apparatus are true, EXCEPT:

A. The cis face is the receiving end.
B. The trans face is the secretory end.
C. It is a nonpolarized structure. (Correct Answer)
D. It is situated near the nucleus.

Explanation: **Explanation:** The Golgi apparatus is a highly organized organelle essential for the modification, sorting, and packaging of proteins and lipids. **Why Option C is the correct answer:** The Golgi apparatus is a **highly polarized** structure, both morphologically and functionally. It possesses two distinct faces with different biochemical compositions and enzymatic activities. Stating that it is "nonpolarized" is factually incorrect, making it the right choice for an "EXCEPT" question. **Analysis of other options:** * **Option A (Cis face is the receiving end):** This is true. The *cis* face (forming face) is convex and oriented toward the endoplasmic reticulum (ER). It receives transport vesicles containing newly synthesized proteins from the ER. * **Option B (Trans face is the secretory end):** This is true. The *trans* face (maturing face) is concave and oriented toward the plasma membrane. It acts as the exit portal where proteins are sorted into secretory vesicles for their final destinations. * **Option D (Situated near the nucleus):** This is true. In most animal cells, the Golgi complex is localized in the perinuclear region, often held in place by the microtubule network near the centrosome. **High-Yield NEET-PG Pearls:** * **Function:** The Golgi is the primary site for **glycosylation** (addition of sugar moieties), sulfation, and phosphorylation of proteins. * **I-Cell Disease:** A clinical correlation where a deficiency in the enzyme *phosphotransferase* prevents the tagging of lysosomal enzymes with **Mannose-6-Phosphate** in the Golgi. This leads to enzymes being secreted extracellularly rather than sent to lysosomes. * **Silver Staining:** The Golgi was first identified by Camillo Golgi using the "black reaction" (silver nitrate stain).

Question 193: Which of the following is NOT true about cell division?

A. Produces a haploid number of chromosomes (Correct Answer)
B. Produces the same number of chromosomes
C. Produces two cells
D. None of the above

Explanation: **Explanation:** The question asks for the statement that is **NOT true** regarding general cell division (Mitosis). **1. Why Option A is the Correct Answer (The False Statement):** In the context of general somatic cell division (Mitosis), the process results in two daughter cells that are genetically identical to the parent cell. Therefore, it produces a **diploid (2n)** number of chromosomes, not a haploid (n) number. Haploid cells are only produced during **Meiosis**, a specialized form of reductive division occurring in germ cells (sperm and ova). **2. Analysis of Incorrect Options:** * **Option B (Produces the same number of chromosomes):** This is a true statement for Mitosis. It is an equational division where the chromosome count remains constant (46 chromosomes in humans). * **Option C (Produces two cells):** This is a true statement. One parent cell divides to form two daughter cells during the M-phase of the cell cycle. * **Option D (None of the above):** This is incorrect because Option A is clearly a false statement regarding standard cell division. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mitosis vs. Meiosis:** Mitosis occurs in somatic cells (growth/repair); Meiosis occurs in gonads (gametogenesis). * **Cell Cycle Phases:** The longest phase is **Interphase** (G1, S, G2). DNA replication occurs specifically in the **S-phase**. * **Colchicine:** A high-yield pharmacological correlate; it inhibits mitosis by interfering with microtubule formation (spindle poisons), arresting cells in **Metaphase**. * **Karyotyping:** Usually performed during Metaphase because chromosomes are most condensed and visible.

Question 194: What is true about the Na+-K+ pump?

A. It involves ATPase activity. (Correct Answer)
B. It can move Na+ both into and out of the cell.
C. It is electrically neutral.
D. It pumps out one Na+ for one K+.

Explanation: The **Na⁺-K⁺ ATPase pump** is a classic example of **Primary Active Transport**, essential for maintaining cellular resting membrane potential and volume. ### **Explanation of the Correct Option** **A. It involves ATPase activity:** The pump is an enzyme (Na⁺-K⁺ ATPase) located on the plasma membrane. It requires energy to move ions against their concentration gradients. It catalyzes the hydrolysis of **ATP into ADP and inorganic phosphate**, using the released energy to drive the conformational changes necessary for ion transport. ### **Why Other Options are Incorrect** * **B. It can move Na⁺ both into and out of the cell:** This is incorrect. The pump is unidirectional; it **only** moves Na⁺ from the intracellular fluid to the extracellular fluid (against its gradient). * **C. It is electrically neutral:** This is incorrect. The pump is **electrogenic**. Because it moves 3 positive charges out and only 2 in, it creates a net deficit of positive ions inside the cell, contributing to the negativity of the Resting Membrane Potential (RMP). * **D. It pumps out one Na⁺ for one K⁺:** The stoichiometry is fixed at **3 Na⁺ ions out** and **2 K⁺ ions in** for every molecule of ATP hydrolyzed. ### **High-Yield NEET-PG Pearls** * **Inhibitor:** The pump is specifically inhibited by **Cardiac Glycosides** (e.g., **Ouabain** and **Digoxin**), which bind to the extracellular alpha subunit. * **Structure:** It is a heterodimer consisting of an **alpha subunit** (catalytic site, ion binding, ATP binding) and a **beta subunit** (essential for membrane localization). * **Function:** It accounts for nearly **25-30%** of the total energy expenditure in a resting cell (up to 70% in neurons). * **Insulin & Epinephrine:** Both hormones stimulate the pump, shifting K⁺ into cells (used clinically to treat hyperkalemia).

Question 195: Which statement is TRUE regarding the Na+/K+ pump?

A. Potassium is pumped against its electrochemical gradient.
B. Two potassium ions are exchanged for five sodium ions. (Correct Answer)
C. Hypercalcemia causes arrest of the Na+/K+ pump.
D. An increase in intracellular sodium increases the action potential.

Explanation: **Explanation:** The **Na+/K+ ATPase (Sodium-Potassium Pump)** is a primary active transporter found in the membranes of almost all animal cells. It plays a critical role in maintaining resting membrane potential and cell volume. **1. Why Option B is Correct:** The pump operates by transporting **3 Na⁺ ions out** of the cell and **2 K⁺ ions into** the cell for every molecule of ATP hydrolyzed. This stoichiometry (3:2 ratio) makes the pump **electrogenic**, as it creates a net deficit of positive charges inside the cell, contributing to the negative resting membrane potential. **2. Analysis of Incorrect Options:** * **Option A:** Both ions are pumped against their concentration gradients. However, while Na⁺ is pumped against its electrochemical gradient, K⁺ is pumped against its chemical gradient but *along* the electrical gradient (since the cell interior is negative). Therefore, the statement is partially incomplete compared to the specific stoichiometry of Option B. * **Option C:** Hypercalcemia does not arrest the pump. However, **Digitalis (Cardiac glycosides)** like Digoxin specifically inhibit the pump by binding to the extracellular alpha-subunit. * **Option D:** An increase in intracellular sodium actually *decreases* the concentration gradient for Na⁺ influx, which would typically decrease the amplitude of the action potential, not increase it. **High-Yield NEET-PG Pearls:** * **Structure:** It is a P-type ATPase consisting of $\alpha$ (catalytic), $\beta$, and $\gamma$ subunits. * **Inhibitors:** Ouabain and Digoxin. * **Stimulators:** Insulin, Aldosterone, and Beta-adrenergic agonists (all shift K⁺ into cells, potentially causing hypokalemia). * **Energy Consumption:** It accounts for approximately 30-40% of the total energy expenditure in a resting cell (up to 70% in neurons).

Question 196: Which of the following is NOT an example of a cellular adhesion molecule?

A. Integrin
B. Cadherin
C. Selectin
D. None of the above (Correct Answer)

Explanation: ### Explanation The correct answer is **None of the above** because all three options listed (Integrins, Cadherins, and Selectins) are major families of **Cell Adhesion Molecules (CAMs)**. CAMs are transmembrane proteins that facilitate cell-to-cell or cell-to-extracellular matrix (ECM) interactions, which are vital for tissue integrity, signal transduction, and leukocyte trafficking. **Analysis of Options:** * **Integrins (Option A):** These are primarily involved in **cell-matrix** interactions. They link the intracellular cytoskeleton (actin) to ECM proteins like fibronectin and laminin. They play a crucial role in "inside-out" and "outside-in" signaling. * **Cadherins (Option B):** These are **calcium-dependent** glycoproteins responsible for **cell-cell** adhesion. They are the primary components of adherens junctions and desmosomes (e.g., E-cadherin in epithelial tissues). * **Selectins (Option C):** These are carbohydrate-binding lectins involved in the **initial rolling** phase of leukocyte extravasation. They mediate weak, transient interactions between leukocytes and vascular endothelium. **High-Yield NEET-PG Pearls:** 1. **Calcium Dependency:** Cadherins and Selectins are **Calcium-dependent**, whereas the Immunoglobulin (Ig) superfamily and most Integrins are not. 2. **Leukocyte Migration Sequence:** * **Rolling:** Mediated by **Selectins** (P and E-selectin). * **Tight Adhesion/Activation:** Mediated by **Integrins** (LFA-1, VLA-4) binding to ICAM-1/VCAM-1. * **Diapedesis:** Mediated by **PECAM-1** (CD31). 3. **Clinical Correlation:** A deficiency in Integrins (specifically CD18) leads to **Leukocyte Adhesion Deficiency (LAD) Type 1**, characterized by delayed umbilical cord separation and recurrent bacterial infections without pus formation.

Question 197: Mutations in connexin can cause which of the following diseases?

A. Charcot Marie Tooth disease (Correct Answer)
B. Huntington's chorea
C. Friedreich's ataxia
D. None of the above

Explanation: **Explanation:** **Connexins** are the structural protein subunits that form **gap junctions**, which are specialized intercellular channels allowing the passage of ions and small molecules between adjacent cells. Six connexins assemble to form a hemichannel called a **connexon**. **1. Why Option A is Correct:** **Charcot-Marie-Tooth (CMT) disease**, specifically the X-linked form (**CMTX1**), is caused by mutations in the **GJB1 gene**, which encodes the protein **Connexin 32 (Cx32)**. In the peripheral nervous system, Cx32 is expressed in Schwann cells and is located in the Schmidt-Lanterman incisures and paranodal regions. Mutations disrupt the gap junction-mediated communication between the layers of the myelin sheath, leading to progressive demyelination and peripheral neuropathy. **2. Why Incorrect Options are Wrong:** * **Huntington’s Chorea (B):** This is an autosomal dominant neurodegenerative disorder caused by a **CAG trinucleotide repeat expansion** in the *HTT* gene on chromosome 4, leading to the accumulation of the huntingtin protein. * **Friedreich’s Ataxia (C):** This is an autosomal recessive condition caused by a **GAA trinucleotide repeat expansion** in the *FXN* gene, which leads to a deficiency of the mitochondrial protein **frataxin**. **High-Yield Clinical Pearls for NEET-PG:** * **Connexin 26 (Cx26):** Mutations are the most common cause of non-syndromic **congenital sensorineural deafness**. * **Connexin 46 & 50:** Mutations are associated with **congenital cataracts** (expressed in the lens). * **Connexin 43:** The primary connexin in the **ventricular myocardium**; alterations are linked to cardiac arrhythmias. * **Gap Junctions** are the only type of cell junction that allows direct metabolic and electrical coupling.

Question 198: Protein synthesis occurs in:

A. Smooth endoplasmic reticulum
B. Rough endoplasmic reticulum (Correct Answer)
C. Golgi bodies
D. Nucleus

Explanation: **Explanation:** The **Rough Endoplasmic Reticulum (RER)** is the primary site for the synthesis of proteins destined for secretion, incorporation into the plasma membrane, or storage within lysosomes. Its "rough" appearance under an electron microscope is due to the presence of **ribosomes** attached to its cytosolic surface. These ribosomes translate mRNA into polypeptide chains, which are then translocated into the RER lumen for folding and post-translational modifications. **Analysis of Options:** * **Smooth Endoplasmic Reticulum (SER):** Lacks ribosomes. Its primary functions include lipid and steroid synthesis, detoxification of drugs (especially in hepatocytes), and calcium storage (as sarcoplasmic reticulum in muscles). * **Golgi Bodies:** These act as the "post office" of the cell. They do not synthesize proteins but are responsible for modifying (e.g., glycosylation), sorting, and packaging proteins received from the RER into vesicles. * **Nucleus:** This is the site of **transcription** (DNA to mRNA) and DNA replication, but not translation (protein synthesis). **High-Yield NEET-PG Pearls:** * **Nissl Bodies:** Found in neurons, these are large clusters of RER and free ribosomes; they are highly active in protein synthesis. * **Free Ribosomes:** Proteins intended for use *within* the cytosol (e.g., hemoglobin, glycolysis enzymes) are synthesized on free ribosomes, not the RER. * **Signal Hypothesis:** Proteins destined for the RER have a specific "signal sequence" that directs the ribosome to dock on the RER membrane via a Signal Recognition Particle (SRP).

Question 199: The electrical potential difference necessary for a single ion to be at equilibrium across a membrane is best described by which equation?

A. Goldman equation
B. Van't Hoff equation
C. Fick's law
D. Nernst equation (Correct Answer)

Explanation: ### Explanation **1. Why the Nernst Equation is Correct:** The **Nernst equation** is used to calculate the **equilibrium potential** (also called the Nernst potential) for a **single ion**. It represents the electrical potential difference across a membrane that exactly balances the concentration gradient of that specific ion, resulting in no net movement of the ion into or out of the cell. * **Formula:** $E = \frac{61}{z} \times \log \frac{[Ion]_{outside}}{[Ion]_{inside}}$ (at body temperature). * **Key Concept:** It assumes the membrane is permeable to only one ion at a time. **2. Why the Other Options are Incorrect:** * **A. Goldman Equation:** Unlike the Nernst equation, the Goldman-Hodgkin-Katz (GHK) equation calculates the **Resting Membrane Potential (RMP)** by considering the concentrations and **permeabilities of multiple ions** (primarily $Na^+$, $K^+$, and $Cl^-$) simultaneously. * **B. Van't Hoff Equation:** This equation is used to calculate **osmotic pressure** based on the concentration of solutes in a solution. It is unrelated to electrical potentials. * **C. Fick's Law:** This describes the **rate of diffusion** of a gas or solute across a membrane. It states that the flux is proportional to the concentration gradient and surface area but does not calculate electrical equilibrium. **3. High-Yield Clinical Pearls for NEET-PG:** * **RMP of a Neuron:** Typically **-70 mV**, which is closest to the equilibrium potential of $K^+$ (-94 mV) because the resting membrane is most permeable to Potassium. * **Equilibrium Potentials to Remember:** * $K^+$: -94 mV * $Na^+$: +61 mV * $Cl^-$: -70 to -90 mV * $Ca^{2+}$: +132 mV * **The Sodium-Potassium Pump ($Na^+/K^+$ ATPase):** It is **electrogenic**, contributing about -4 mV to the RMP by pumping 3 $Na^+$ out for every 2 $K^+$ in.

Question 200: Which of the following is NOT a major function of the endoplasmic reticulum?

A. Synthesis of lipids
B. Synthesis of proteins
C. Providing enzymes that control glycogen breakdown
D. Secretion of proteins synthesized in the cell (Correct Answer)

Explanation: The **Endoplasmic Reticulum (ER)** is a multifunctional organelle, but its primary roles are limited to synthesis, processing, and storage. ### Why Option D is the Correct Answer While the ER is responsible for the **synthesis** and **folding** of proteins (via the Rough ER), it does **not** perform the actual **secretion** of these proteins. Once proteins are synthesized in the ER, they are packaged into transport vesicles and sent to the **Golgi Apparatus**. The Golgi is the "post office" of the cell; it modifies, sorts, and packages these proteins into secretory vesicles for exocytosis. Therefore, secretion is a function of the Golgi Apparatus, not the ER. ### Analysis of Incorrect Options * **A. Synthesis of lipids:** This is a major function of the **Smooth ER (SER)**. It is the site for the synthesis of phospholipids, cholesterol, and steroid hormones (e.g., testosterone, estrogen). * **B. Synthesis of proteins:** This is the primary function of the **Rough ER (RER)**, which is studded with ribosomes. It synthesizes proteins destined for membranes, lysosomes, or secretion. * **C. Providing enzymes for glycogen breakdown:** The **Smooth ER** contains **Glucose-6-phosphatase**, a key enzyme in glycogenolysis (the breakdown of glycogen to glucose), particularly in liver cells. ### High-Yield NEET-PG Pearls * **Sarcoplasmic Reticulum:** A specialized form of SER in muscle cells that stores and releases **Calcium ions ($Ca^{2+}$)** for contraction. * **Detoxification:** The SER in hepatocytes contains the **Cytochrome P450** system, essential for detoxifying drugs and toxins. * **Nissl Bodies:** These are large granules of RER found in neurons, responsible for high levels of protein synthesis.

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Cell Membrane Structure and Function

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Calcium Signaling

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Cell Cycle and Regulation

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Cellular Aging

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Apoptosis and Cell Death

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