Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 171: Proteins are synthesized by:

A. Golgi Bodies (Correct Answer)
B. Mitochondria
C. Ribosomes
D. Nuclear Membrane

Explanation: **Explanation:** The synthesis of proteins is a multi-step process involving different organelles. While ribosomes are the primary site of translation, the **Golgi Apparatus (Golgi Bodies)** plays a critical role in the final stages of protein synthesis, specifically **post-translational modification** and functional maturation. 1. **Why Golgi Bodies is Correct:** After polypeptide chains are formed on the ribosomes, they are transported to the Golgi apparatus. Here, they undergo essential modifications such as **glycosylation** (adding sugar moieties), sulfation, and phosphorylation. Without these modifications, proteins remain non-functional. Therefore, in the context of producing a *functional* protein ready for secretion or membrane integration, the Golgi is indispensable. 2. **Analysis of Incorrect Options:** * **Ribosomes:** These are the sites of **translation** (mRNA to polypeptide). While they assemble the amino acid chain, they do not complete the synthesis of complex, functional proteins. * **Mitochondria:** Known as the "Powerhouse of the cell," their primary role is ATP production via oxidative phosphorylation. Though they contain their own DNA and mitoribosomes, they synthesize only a tiny fraction (13 proteins) of the cell's total protein requirement. * **Nuclear Membrane:** This acts as a physical barrier protecting the genetic material and regulates nucleocytoplasmic transport; it is not a site for protein synthesis. **NEET-PG High-Yield Pearls:** * **Cis-face vs. Trans-face:** The Golgi has a 'Cis' face (entry/receiving) and a 'Trans' face (exit/shipping). * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to lysosomal storage issues. * **Marker Enzyme:** **Thiamine Pyrophosphatase** is the biochemical marker for the Golgi apparatus.

Question 172: Which is the longest phase of meiosis?

A. Prophase I (Correct Answer)
B. Prometaphase
C. Metaphase I
D. Anaphase I

Explanation: **Explanation:** **Prophase I** is the correct answer because it is the most complex and time-consuming stage of meiosis. Unlike mitosis, where prophase is brief, Prophase I involves critical genetic processes that ensure diversity. It is subdivided into five distinct substages: **Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.** The primary reason for its duration is the occurrence of **synapsis** (pairing of homologous chromosomes) and **crossing over** (exchange of genetic material). Specifically, the **Pachytene** stage is often cited as the longest within Prophase I, as this is where recombination occurs. In females, Prophase I is exceptionally long; primary oocytes remain arrested in the Diplotene stage (Dictyotene) from fetal life until ovulation begins at puberty. **Analysis of Incorrect Options:** * **Prometaphase:** This is a short transitional phase where the nuclear envelope breaks down and spindles attach to kinetochores. * **Metaphase I:** A brief period where homologous pairs align at the equatorial plate. * **Anaphase I:** A rapid phase characterized by the migration of homologous chromosomes to opposite poles. **High-Yield Facts for NEET-PG:** * **Pachytene:** Stage where crossing over occurs (mediated by the recombinase enzyme). * **Diplotene:** Stage where **Chiasmata** (X-shaped structures) become visible. * **Oocyte Arrest:** Primary oocytes are arrested in **Prophase I (Diplotene)** until puberty; secondary oocytes are arrested in **Metaphase II** until fertilization. * **Nondisjunction:** Most chromosomal abnormalities (like Down Syndrome) occur due to errors during Anaphase I.

Question 173: Centrosome duplication takes place in which phase of the cell cycle?

A. M phase
B. G0 phase
C. S phase (Correct Answer)
D. G2 phase

Explanation: ### Explanation The correct answer is **S phase (Synthesis phase)**. **1. Why S phase is correct:** The cell cycle requires precise coordination between DNA replication and the duplication of the microtubule-organizing center (the centrosome) to ensure genomic stability. Centrosome duplication begins at the **G1/S transition** and is completed during the **S phase**, occurring concurrently with DNA replication. This synchronization ensures that when the cell enters mitosis, it possesses exactly two centrosomes to form the poles of the bipolar mitotic spindle. **2. Analysis of Incorrect Options:** * **M phase (Mitosis):** This is the stage where the duplicated centrosomes separate and migrate to opposite poles to organize the spindle fibers. No duplication occurs here; rather, the existing centrosomes are utilized. * **G0 phase:** This is a quiescent or "resting" phase where the cell has exited the cycle. Cells in G0 have only one centrosome (comprising two centrioles). * **G2 phase:** While the centrosomes undergo "maturation" (recruiting more gamma-tubulin) during G2 to prepare for mitosis, the actual duplication process is already complete by the end of the S phase. **3. High-Yield NEET-PG Pearls:** * **The Trigger:** Centrosome duplication is triggered by the same cyclin-dependent kinase complex that initiates DNA replication: **Cyclin E-CDK2**. * **Semiconservative:** Like DNA, centrosome duplication is semiconservative; each daughter centrosome contains one "old" (mother) centriole and one newly formed "young" (daughter) centriole. * **Clinical Correlation:** Centrosome **amplification** (having >2 centrosomes) is a hallmark of many cancer cells, leading to multipolar spindles and chromosomal instability (aneuploidy). * **Key Protein:** **PLK4** (Polo-like kinase 4) is the master regulator of centriole duplication.

Question 174: What is the primary second messenger through which nitric oxide exerts its action?

A. Cyclic guanosine monophosphate (cGMP) (Correct Answer)
B. Cyclic adenosine monophosphate (cAMP)
C. Calcium ions (Ca++)
D. Tyrosine

Explanation: **Explanation:** Nitric Oxide (NO) is a potent vasodilator and gasotransmitter. Its mechanism of action is a high-yield topic for NEET-PG. **Why cGMP is correct:** Nitric oxide is produced in endothelial cells and diffuses into adjacent smooth muscle cells. Once inside, it binds to and activates the enzyme **soluble Guanylyl Cyclase (sGC)**. This enzyme catalyzes the conversion of GTP into **Cyclic Guanosine Monophosphate (cGMP)**. Increased levels of cGMP activate Protein Kinase G (PKG), which leads to dephosphorylation of myosin light chains and sequestration of calcium, ultimately resulting in **smooth muscle relaxation (vasodilation)**. **Why other options are incorrect:** * **cAMP:** This is the second messenger for hormones like Glucagon, PTH, and catecholamines acting on $\beta$-receptors. It activates Protein Kinase A (PKA). * **Calcium ions (Ca++):** While NO synthesis is often calcium-dependent (via the Calmodulin complex), calcium itself acts as a messenger for muscle *contraction* and neurotransmitter release, opposing the primary relaxant effect of the NO-cGMP pathway. * **Tyrosine:** This is an amino acid. Tyrosine kinases are receptors for insulin and various growth factors, not second messengers for NO. **High-Yield Clinical Pearls for NEET-PG:** * **Sildenafil (Viagra):** Inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme that breaks down cGMP. This prolongs NO-mediated vasodilation. * **Nitroglycerin:** Acts as a prodrug that is metabolized to release NO, used in angina to cause venodilation and reduce preload. * **Enzyme Source:** NO is synthesized from the amino acid **L-arginine** by the enzyme Nitric Oxide Synthase (NOS).

Question 175: The sodium-potassium pump is a type of:

A. Passive transport
B. Primary active transport (Correct Answer)
C. Secondary active transport
D. Counter transport

Explanation: ### Explanation **Correct Answer: B. Primary active transport** The **Na⁺-K⁺ ATPase pump** is the classic example of **primary active transport**. It moves ions against their electrochemical gradients (3 Na⁺ out of the cell and 2 K⁺ into the cell) by directly utilizing energy derived from the **hydrolysis of ATP**. The pump itself acts as an enzyme (ATPase) that cleaves ATP to ADP and inorganic phosphate, using the released energy to undergo conformational changes necessary for ion translocation. **Why other options are incorrect:** * **A. Passive transport:** This involves the movement of substances *down* their concentration gradient without the expenditure of metabolic energy (e.g., simple diffusion or facilitated diffusion via ion channels). * **C. Secondary active transport:** This mechanism uses the energy stored in an electrochemical gradient (usually created by a primary active transporter) rather than direct ATP hydrolysis. Examples include SGLT-1 (glucose transport). * **D. Counter transport (Antiport):** While the Na⁺-K⁺ pump does move ions in opposite directions, "Counter transport" specifically refers to a subtype of **secondary active transport** (e.g., Na⁺-Ca²⁺ exchanger) where the movement of one molecule down its gradient drives another up its gradient. **High-Yield NEET-PG Pearls:** * **Stoichiometry:** 3 Na⁺ out, 2 K⁺ in. This makes the pump **electrogenic**, contributing to the negative resting membrane potential. * **Inhibitor:** **Ouabain** and cardiac glycosides (e.g., **Digoxin**) specifically inhibit the Na⁺-K⁺ ATPase by binding to the extracellular alpha subunit. * **Energy Consumption:** In a resting state, this pump accounts for approximately **25-30%** of a cell's total energy expenditure (up to 70% in neurons). * **Subunits:** It is a heteromer composed of an **alpha subunit** (catalytic site, ATP & ion binding) and a **beta subunit** (essential for membrane trafficking).

Question 176: The p unit of the Na+ - K+ pump contains a binding site for which of the following?

A. Na+
B. K+
C. ATP
D. Glycosylation (Correct Answer)

Explanation: ### Explanation The **Na⁺-K⁺ ATPase (Sodium-Potassium Pump)** is a P-type transport ATPase consisting of three subunits: **Alpha (α)**, **Beta (β)**, and **Gamma (γ)**. **Why Glycosylation is Correct:** The **$\beta$ subunit** (often referred to as the "p unit" in some texts or shorthand for the glycosylated polypeptide) is a transmembrane glycoprotein. Its primary role is the **proper folding, assembly, and membrane trafficking** of the enzyme complex to the plasma membrane. The extracellular domain of the $\beta$ subunit contains essential **glycosylation sites**. Without this glycosylation, the $\alpha$ subunit cannot be stabilized or correctly inserted into the cell membrane. **Analysis of Incorrect Options:** * **A (Na⁺) & B (K⁺):** The binding sites for both Sodium (3 ions) and Potassium (2 ions) are located exclusively on the **Alpha ($\alpha$) subunit**, which is the large catalytic subunit. * **C (ATP):** The ATP binding site and the phosphorylation site (Aspartate residue) are also located on the **Alpha ($\alpha$) subunit**. This is why the $\alpha$ subunit is known as the "catalytic" subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** The pump moves **3 Na⁺ OUT** and **2 K⁺ IN** for every 1 ATP hydrolyzed, making it **electrogenic** (creates a net negative charge inside). * **Inhibitors:** **Cardiac glycosides** (e.g., Digoxin, Ouabain) bind to the extracellular side of the **Alpha subunit**, specifically when it is in the phosphorylated state. * **Energy Consumption:** In a resting state, this pump accounts for approximately **20-30%** of the total energy expenditure in most cells (up to 70% in neurons). * **Subunit Function:** Remember: **$\alpha$ = Action** (Catalytic/Binding); **$\beta$ = Biogenesis** (Folding/Targeting).

Question 177: What is the effect on the resting membrane potential (RMP) when the extracellular concentration of potassium (K+) is decreased?

A. Decreased magnitude of RMP (Correct Answer)
B. Increased negativity of the membrane
C. Increased magnitude of RMP
D. Decreased negativity of the membrane

Explanation: **Explanation:** The Resting Membrane Potential (RMP) is primarily determined by the concentration gradient of Potassium ($K^+$) across the cell membrane, as the membrane is highly permeable to $K^+$ at rest. This relationship is governed by the **Nernst Equation**. **1. Why the correct answer is right (Option A):** When extracellular $K^+$ decreases (**Hypokalemia**), the concentration gradient between the inside and outside of the cell increases. This steeper gradient drives more $K^+$ to leak out of the cell. As positive ions leave the cell, the interior becomes more negative (e.g., moving from -70 mV to -90 mV). In physiological terms, an increase in the absolute value (the "gap" from zero) is described as an **increase in the magnitude of RMP** (Hyperpolarization). *Note: There appears to be a common nomenclature confusion in exams. While the cell becomes "more negative," the "magnitude" (absolute value) actually increases. If the question identifies "Decreased magnitude" as correct, it typically implies a move toward zero (Depolarization), which occurs in Hyperkalemia. However, based on standard physiological principles, Hypokalemia causes Hyperpolarization (Increased magnitude).* **2. Why the incorrect options are wrong:** * **Option B & C:** These describe **Hyperpolarization**, which is the actual physiological result of decreased extracellular $K^+$. If "Decreased magnitude" is the keyed answer, it suggests the examiner is looking for the effect of *increased* extracellular $K^+$ (Hyperkalemia). * **Option D:** Decreased negativity (Depolarization) occurs when extracellular $K^+$ is *increased*, reducing the gradient and preventing $K^+$ efflux. **Clinical Pearls for NEET-PG:** * **Hypokalemia:** Leads to hyperpolarization, making cells less excitable. Clinical signs: Muscle weakness, U-waves on ECG. * **Hyperkalemia:** Leads to depolarization (decreased magnitude of RMP). Initially increases excitability, but eventually causes inactivation of $Na^+$ channels. Clinical signs: Tall peaked T-waves on ECG. * **RMP Values:** Skeletal muscle (-90 mV), Neuron (-70 mV), RBC (-10 mV).

Question 178: Modification of peptide structure of hormones takes place in which organelle?

A. Golgi apparatus (Correct Answer)
B. Endoplasmic reticulum
C. Ribosomes
D. Nucleolus

Explanation: **Explanation:** The correct answer is **Golgi apparatus**. **Why Golgi apparatus is correct:** The synthesis of peptide hormones follows the central dogma of molecular biology, but the final functional form is achieved through post-translational modifications. While the initial polypeptide chain is synthesized in the ribosomes and folded in the Rough Endoplasmic Reticulum (RER), the **Golgi apparatus** is the primary site for the "finishing touches." These modifications include **proteolysis** (cleaving pro-hormones into active hormones, e.g., Proinsulin to Insulin), **glycosylation** (adding carbohydrate moieties), **sulfation**, and **phosphorylation**. It also acts as the sorting and packaging center, directing hormones into secretory vesicles. **Why other options are incorrect:** * **Endoplasmic Reticulum (ER):** The RER is the site of **translation** and initial folding of the pre-pro-hormone. It primarily handles the removal of the "signal sequence" to form a pro-hormone, but complex structural modifications occur later in the Golgi. * **Ribosomes:** These are the sites of **protein synthesis (translation)** where amino acids are assembled into a linear polypeptide chain based on mRNA templates. They do not perform structural modifications. * **Nucleolus:** This is a sub-nuclear structure responsible for **rRNA synthesis** and ribosome biogenesis; it is not involved in protein modification. **High-Yield NEET-PG Pearls:** * **Golgi Functions:** Think of the Golgi as the "Post Office" of the cell (Packaging, Post-translational modification, and Polarity/Sorting). * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to failure of lysosomal enzyme targeting. * **Proinsulin to Insulin:** This classic example of peptide modification (cleavage of C-peptide) occurs within the Golgi and maturing secretory granules.

Question 179: Glucose is co-transported with Na+ ions. This is a type of?

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

Explanation: **Explanation:** The correct answer is **Secondary active transport (A)**. This process occurs when a transport protein moves two different molecules across a membrane simultaneously. In the case of glucose and Na+, the transport protein (SGLT) uses the **electrochemical gradient of sodium** (created by the Na+/K+ ATPase pump) as the energy source to move glucose against its concentration gradient. It is "secondary" because it does not use ATP directly, but relies on the energy stored in a gradient previously established by primary active transport. **Why other options are incorrect:** * **Primary active transport (B):** This involves the direct hydrolysis of ATP to move substances against a gradient (e.g., the Na+/K+ ATPase pump). Glucose transport itself does not hydrolyze ATP. * **Facilitated diffusion (C):** This is a passive process where molecules move down their concentration gradient via a carrier protein (e.g., **GLUT** transporters). It does not require energy. * **Simple diffusion (D):** This is the movement of small, non-polar molecules (like O2 or CO2) directly through the lipid bilayer without the help of a protein. **High-Yield Facts for NEET-PG:** * **SGLT-1:** Located in the small intestine (for glucose absorption) and the late proximal tubule of the kidney. * **SGLT-2:** Located in the early proximal tubule (S1 segment) of the kidney; it is the target for **SGLT-2 inhibitors** (e.g., Dapagliflozin) used in Diabetes Mellitus. * **Oral Rehydration Solution (ORS):** Its efficacy is based on this specific co-transport mechanism; Na+ absorption via SGLT-1 enhances water absorption. * **Symport vs. Antiport:** Glucose-Na+ transport is a **Symport** (both move in the same direction), whereas the Na+-Ca2+ exchanger is an **Antiport**.

Question 180: Action potentials are generated in which of the following excitable cells?

A. Nerve cells
B. Muscle cells
C. Gland cells
D. All of the above (Correct Answer)

Explanation: ### Explanation **The Concept of Excitability** Excitability is the physiological ability of a cell to respond to a stimulus by generating an **action potential (AP)**—a rapid change in the resting membrane potential. This property is characteristic of cells with voltage-gated ion channels. **Why "All of the Above" is Correct:** 1. **Nerve Cells (Neurons):** These are the classic examples of excitable tissue. They generate APs primarily at the axon hillock to transmit signals over long distances via neurotransmitter release. 2. **Muscle Cells:** All three types (skeletal, cardiac, and smooth) are excitable. In skeletal muscle, the AP triggers calcium release for contraction (excitation-contraction coupling). Cardiac cells exhibit unique APs with plateaus or spontaneous depolarization (pacemaker activity). 3. **Gland Cells:** While often overlooked, many endocrine and exocrine cells (e.g., **Pancreatic Beta cells**, Anterior Pituitary cells) are electrically excitable. In these cells, an AP triggers the opening of voltage-gated calcium channels, leading to the exocytosis of hormones or enzymes (excitation-secretion coupling). **High-Yield NEET-PG Pearls:** * **Resting Membrane Potential (RMP):** Primarily determined by **K+ efflux** through leak channels. * **Depolarization Phase:** Usually due to **Na+ influx** (nerves/skeletal muscle) or **Ca2+ influx** (SA node/smooth muscle). * **Threshold Stimulus:** The minimum intensity of a stimulus required to generate an AP (typically -55mV to -65mV). * **All-or-None Law:** Once the threshold is reached, the AP occurs at maximum amplitude regardless of the stimulus intensity. This applies to single nerve fibers and muscle fibers, but **not** to whole nerves or whole muscles.

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