Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 131: What is the contribution of the Na+/K+ pump to the development of the resting membrane potential (RMP)?

A. -90 mV
B. -89 mV
C. -86 mV
D. -4 mV (Correct Answer)

Explanation: ### Explanation The resting membrane potential (RMP) of a typical large nerve fiber is approximately **-90 mV**. This potential is generated by two primary mechanisms: 1. **Diffusion Potentials (Major Contributor):** The movement of ions through "leak channels" (primarily K+ leak channels) creates a potential of **-86 mV**. Because the membrane is significantly more permeable to K+ than Na+, the RMP sits close to the equilibrium potential of Potassium. 2. **Na+/K+ ATPase Pump (Electrogenic Contribution):** The pump actively transports **3 Na+ ions out** of the cell for every **2 K+ ions in**. This net loss of one positive charge from the intracellular fluid creates a small deficit of positive ions inside the membrane. This direct electrogenic contribution accounts for exactly **-4 mV**. Therefore, the total RMP is the sum of these two components: **(-86 mV) + (-4 mV) = -90 mV**. #### Analysis of Options: * **Option A (-90 mV):** This is the **total RMP** of a large myelinated nerve fiber, not the specific contribution of the pump alone. * **Option B (-89 mV):** This is a distractor value with no physiological basis in standard textbooks (like Guyton). * **Option C (-86 mV):** This represents the contribution of **passive ion diffusion** (via K+ leak channels) to the RMP. * **Option D (-4 mV):** This is the **correct** direct electrogenic contribution of the Na+/K+ pump. #### High-Yield Clinical Pearls for NEET-PG: * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP considering multiple ions and their permeabilities. * **Digitalis (Cardiac Glycosides):** Inhibits the Na+/K+ ATPase, leading to increased intracellular Na+, which subsequently slows the Na+/Ca2+ exchanger, increasing intracellular Ca2+ (positive inotropy). * **Energy Requirement:** The Na+/K+ pump consumes roughly 60-70% of the total energy requirement in a resting neuron.

Question 132: Glucose is reabsorbed in the proximal tubule via symport with which ion?

A. Na+ (Correct Answer)
B. K+
C. Ca++
D. Cl-

Explanation: **Explanation:** Glucose reabsorption in the proximal convoluted tubule (PCT) occurs via **Secondary Active Transport**. This process is mediated by **Sodium-Glucose Co-transporters (SGLT)**, specifically SGLT2 (early PCT) and SGLT1 (late PCT). **Why Na+ is correct:** The process is driven by the electrochemical gradient created by the **Na+-K+ ATPase pump** on the basolateral membrane, which keeps intracellular sodium levels low. This gradient allows sodium to move down its concentration gradient from the tubular lumen into the cell. Glucose "hitchhikes" with sodium against its own concentration gradient via a **symport** mechanism. Once inside the cell, glucose exits the basolateral membrane into the blood via facilitated diffusion through **GLUT2** (or GLUT1). **Why other options are incorrect:** * **K+ (Potassium):** Potassium is primarily secreted or reabsorbed through specific channels and exchangers (like the Na-K-2Cl symporter in the Loop of Henle), but it does not drive glucose transport. * **Ca++ (Calcium):** Calcium reabsorption in the PCT is largely passive (paracellular) and follows water and sodium; it is not coupled with glucose. * **Cl- (Chloride):** Chloride is reabsorbed primarily via paracellular pathways or in exchange for other anions in the later segments of the PCT. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT2 Inhibitors (e.g., Dapagliflozin):** A class of drugs used in Diabetes Mellitus that block glucose reabsorption in the PCT, leading to glucosuria. * **Renal Threshold for Glucose:** Approximately **180 mg/dL**. When blood glucose exceeds this, SGLT transporters become saturated ($T_mG$), and glucose appears in the urine. * **Fanconi Syndrome:** A generalized dysfunction of the PCT resulting in the loss of glucose, amino acids, and phosphates in the urine.

Question 133: Which of the following is true about facilitated diffusion?

A. Occurs in the direction of the concentration gradient.
B. Does not require energy.
C. Occurs in the direction opposite to the electrical gradient. (Correct Answer)
D. Is facilitated by the charge of the molecule.

Explanation: ### Explanation **Facilitated diffusion** is a form of carrier-mediated transport that allows molecules to move across the cell membrane without the expenditure of metabolic energy (ATP). #### Why Option C is Correct The movement of ions or charged molecules is governed by the **electrochemical gradient**, which is the sum of the chemical (concentration) gradient and the electrical gradient. In facilitated diffusion, a molecule can move **against its electrical gradient** as long as the chemical gradient is strong enough to overcome it, resulting in a net movement down the overall electrochemical gradient. This distinguishes it from simple diffusion of uncharged molecules, which only considers concentration. #### Analysis of Incorrect Options * **Option A:** While facilitated diffusion often follows the concentration gradient, it specifically follows the **electrochemical gradient**. If an electrical pull is strong enough, it can drive movement against a concentration gradient. * **Option B:** While it is true that facilitated diffusion does not require ATP, this is a general characteristic of all passive transport. In the context of this specific question (likely sourced from advanced physiological concepts like the Nernst potential), the interaction with electrical gradients is the more specific physiological "truth" being tested. * **Option D:** Facilitated diffusion is facilitated by **specific carrier proteins** (transporters), not the charge of the molecule itself. Charge often acts as a barrier to simple diffusion, necessitating a carrier. #### NEET-PG High-Yield Pearls * **Vmax (Saturation):** Unlike simple diffusion, facilitated diffusion shows "saturation kinetics." As the concentration of the substance increases, the rate of transport reaches a maximum (Vmax) because all carrier proteins are occupied. * **Stereospecificity:** Carriers are specific (e.g., GLUT transporters move D-glucose but not L-glucose). * **Competitive Inhibition:** Similar molecules can compete for the same binding site on the carrier protein. * **Classic Example:** Transport of glucose into skeletal muscle and adipose tissue via **GLUT-4** (insulin-dependent).

Question 134: Which organ's cells lack the deposition of mitochondria?

A. Brain
B. Heart
C. Kidney (Correct Answer)
D. Muscles

Explanation: ### Explanation The correct answer is **Kidney (Option C)**. **Why Kidney is the correct answer:** This question refers to the specific histological distribution and "deposition" patterns of mitochondria within cells. While all the listed organs are metabolically active and contain mitochondria, the **Kidney** (specifically the Mature Red Blood Cells and certain specialized segments) is often highlighted in physiological contexts regarding the *relative* lack or specific localization of mitochondria. However, in the context of competitive exams like NEET-PG, this question typically refers to the **metabolic source**. While the kidney uses massive amounts of ATP for active transport (Sodium-Potassium ATPase), it also possesses a high capacity for anaerobic glycolysis in the renal medulla. More importantly, if the question implies "lack of mitochondria" in a literal sense, it is often a distractor or refers to the fact that **Mature Erythrocytes** (which pass through these organs) lack mitochondria entirely. Among the options, the Kidney is the most unique because the **Renal Medulla** operates under relatively hypoxic conditions compared to the high mitochondrial density of the Heart or Brain. **Analysis of Incorrect Options:** * **Heart (B):** The myocardium has the **highest mitochondrial density** in the body (occupying ~30-35% of cell volume) because it relies almost exclusively on aerobic metabolism and fatty acid oxidation. * **Brain (A):** Neurons are highly dependent on oxidative phosphorylation. The brain consumes 20% of the body's oxygen; thus, it is packed with mitochondria to maintain membrane potentials. * **Muscles (D):** Skeletal muscles, especially Type I (slow-twitch) fibers, are rich in mitochondria to support sustained contractile activity. **NEET-PG High-Yield Pearls:** 1. **Mitochondria-free cells:** Mature Red Blood Cells (RBCs) lack mitochondria to prevent them from consuming the oxygen they transport. 2. **Mitochondrial Inheritance:** Always maternal; mutations typically affect high-energy organs (Leber’s Hereditary Optic Neuropathy, MELAS). 3. **Brown Adipose Tissue:** Contains specialized mitochondria with **Thermogenin (UCP-1)** for non-shivering thermogenesis. 4. **Metabolic Fact:** The Heart is a "metabolic omnivore" but prefers fatty acids, requiring constant mitochondrial activity.

Question 135: Regarding transport of substances through the cell membrane, all are true except:

A. Glucose is transported via facilitated diffusion.
B. Active transport is an energy-driven process.
C. Facilitated diffusion requires energy. (Correct Answer)
D. Facilitated diffusion requires a carrier protein.

Explanation: ### Explanation The core concept tested here is the distinction between **Passive Transport** and **Active Transport**. **Why Option C is the correct answer (The "Except"):** Facilitated diffusion is a form of **passive transport**. It occurs along a concentration gradient (from high to low concentration) and, therefore, **does not require metabolic energy (ATP)**. It relies solely on the kinetic energy of the molecules and the presence of a specific carrier protein. **Analysis of other options:** * **Option A:** Glucose enters most cells (like muscle and adipose tissue) via **GLUT transporters**, which are classic examples of facilitated diffusion. (Note: Glucose absorption in the gut/kidney occurs via SGLT, which is secondary active transport). * **Option B:** Active transport (both primary and secondary) moves substances **against** a concentration gradient, which necessitates the expenditure of energy (ATP or electrochemical gradients). * **Option D:** Unlike simple diffusion, facilitated diffusion is **carrier-mediated**. It requires specific transmembrane proteins to shuttle molecules that are too large or polar to pass through the lipid bilayer alone. **NEET-PG High-Yield Pearls:** 1. **Kinetics:** Facilitated diffusion exhibits **saturation kinetics** ($V_{max}$). Once all carrier proteins are occupied, the rate of transport cannot increase further, unlike simple diffusion which is linear. 2. **GLUT vs. SGLT:** * **GLUT (1-14):** Facilitated Diffusion (Passive). * **SGLT (1-2):** Secondary Active Transport (Requires Na+ gradient). 3. **Insulin Dependency:** GLUT-4 (found in skeletal muscle and adipose tissue) is the only insulin-dependent glucose transporter.

Question 136: During which phase of the cell cycle is new DNA material synthesized?

A. Prophase
B. Metaphase
C. Telophase
D. Interphase (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Interphase** The cell cycle is divided into two main stages: **Interphase** and the **M-phase (Mitosis)**. Interphase is the longest part of the cell cycle and is further divided into three sub-phases: $G_1$, **S (Synthesis)**, and $G_2$. The **S-phase** is specifically characterized by the replication of nuclear DNA. During this phase, the DNA content of the cell doubles (from $2C$ to $4C$), though the chromosome number remains the same. This ensures that when the cell eventually divides during mitosis, each daughter cell receives a complete and identical set of genetic material. **Why other options are incorrect:** * **A, B, and C (Prophase, Metaphase, Telophase):** These are all sub-stages of the **M-phase (Mitosis)**. Mitosis is the process of nuclear division where the *already replicated* DNA is condensed, aligned, and physically separated into two nuclei. No new DNA is synthesized during these stages; they are concerned with the distribution of genetic material rather than its production. **High-Yield Clinical Pearls for NEET-PG:** * **S-phase Marker:** The synthesis of **Histone proteins** occurs primarily during the S-phase to package the newly formed DNA. * **Checkpoints:** The transition from $G_1$ to S is the most critical checkpoint (Restriction point), regulated by **Cyclin D-CDK4/6** and the **Retinoblastoma (Rb) protein**. * **Quiescent Phase ($G_0$):** Cells that stop dividing (like mature neurons or skeletal muscle) exit the cycle at $G_1$ to enter $G_0$. * **Vinca Alkaloids/Taxanes:** These chemotherapy drugs target the M-phase (microtubules), whereas drugs like **Methotrexate** or **5-Fluorouracil** target the S-phase by inhibiting DNA synthesis.

Question 137: What is the primary function of the Golgi complex?

A. To participate in the breakdown of proteins and lipids.
B. To participate in posttranslational processing of proteins. (Correct Answer)
C. To participate in energy production.
D. To participate in transcription and translation.

Explanation: The **Golgi complex** (or Golgi apparatus) functions as the "post office" or "shipping center" of the cell. Its primary role is the modification, sorting, and packaging of proteins and lipids received from the Endoplasmic Reticulum (ER). ### Why Option B is Correct: Proteins synthesized in the Rough ER enter the Golgi complex, where they undergo **post-translational modifications**. These include: * **Glycosylation:** Adding carbohydrate chains to form glycoproteins. * **Sulfation and Phosphorylation:** Modifying proteins to ensure they reach their correct destination. * **Proteolysis:** Cleaving precursor proteins into active forms (e.g., proinsulin to insulin). * **Sorting:** Directing proteins to lysosomes, the plasma membrane, or for secretion via secretory vesicles. ### Why Other Options are Incorrect: * **Option A:** The breakdown of proteins and lipids is the primary function of **Lysosomes** (via acid hydrolases) and **Peroxisomes** (via oxidative enzymes). * **Option C:** Energy production (ATP synthesis) occurs in the **Mitochondria** via oxidative phosphorylation. * **Option D:** Transcription occurs in the **Nucleus**, while translation occurs on **Ribosomes** (either free or attached to the Rough ER). ### High-Yield NEET-PG Pearls: * **Polarity:** The Golgi has a **Cis-face** (entry/forming face) facing the ER and a **Trans-face** (exit/maturing face) facing the plasma membrane. * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to failure of protein trafficking to lysosomes. * **Marker Enzyme:** **Thiamine Pyrophosphatase (TPP)** is the characteristic marker enzyme for the Golgi complex.

Question 138: Which among the following is the function of cholesterol in the plasma membrane?

A. Increase fluidity of the lipid bilayer.
B. Decrease fluidity of the lipid bilayer. (Correct Answer)
C. Facilitate the diffusion of ions through the lipid bilayer.
D. Assist in the transport of hormones across the lipid bilayer.

Explanation: **Explanation** The plasma membrane is a dynamic "Fluid Mosaic" structure. Cholesterol is a key sterol component interspersed between the hydrophobic tails of phospholipids. **1. Why the correct answer is right:** Cholesterol acts as a **fluidity buffer**. At physiological temperatures (body temperature), the rigid steroid rings of cholesterol interfere with the movement of phospholipid fatty acid chains. By filling the gaps between phospholipids, it increases packing density, which **decreases membrane fluidity** and reduces permeability to small water-soluble molecules. This provides the membrane with necessary structural integrity and mechanical stability. **2. Why the incorrect options are wrong:** * **Option A:** While cholesterol prevents the membrane from becoming too rigid at *very low* temperatures (by preventing crystallization), its primary role at body temperature is to stabilize the membrane and **decrease** fluidity. * **Option C:** Ion diffusion is facilitated by specific **transmembrane proteins** (channels and carriers), not by cholesterol. In fact, cholesterol makes the bilayer less permeable to ions. * **Option D:** Hormone transport (especially lipid-insoluble ones) requires specific receptors or transporters. While steroid hormones diffuse directly through the bilayer, cholesterol itself does not "assist" this transport; it primarily regulates the physical state of the membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Lipid Rafts:** Cholesterol is a major component of "lipid rafts," which are specialized microdomains that organize cell signaling molecules. * **Ratio:** The cholesterol-to-phospholipid ratio determines membrane stiffness; an increase in this ratio (as seen in some pathological states) leads to more rigid membranes. * **Acanthocytosis:** In certain liver diseases, increased cholesterol in the erythrocyte membrane leads to "spur cells" (acanthocytes), which are less flexible and prone to hemolysis.

Question 139: In which phase of the cell cycle does DNA replication occur?

A. G1 phase
B. G2 phase
C. S phase (Correct Answer)
D. M phase

Explanation: ### Explanation **Correct Answer: C. S phase** The cell cycle is a highly regulated sequence of events leading to cell division. The **S phase (Synthesis phase)** is the specific period during interphase when **DNA replication** occurs. During this stage, the DNA content of the cell doubles (from 2n to 4n), ensuring that each of the two daughter cells receives a complete and identical set of chromosomes. This process involves key enzymes like DNA polymerase and helicase. **Analysis of Incorrect Options:** * **A. G1 phase (Gap 1):** This is the pre-synthetic phase. It is characterized by intensive cellular growth, RNA synthesis, and protein synthesis. The cell prepares the machinery necessary for DNA replication, but the DNA content remains constant (2n). * **B. G2 phase (Gap 2):** This is the post-synthetic/pre-mitotic phase. The cell continues to grow and synthesizes proteins like **tubulin** required for spindle fiber formation. It acts as a final checkpoint to ensure DNA was replicated correctly before entering mitosis. * **C. M phase (Mitosis):** This is the shortest phase where actual nuclear (karyokinesis) and cytoplasmic (cytokinesis) division occurs. DNA is distributed, not replicated, during this phase. **High-Yield Facts for NEET-PG:** * **G0 Phase (Quiescent stage):** Cells that stop dividing (e.g., neurons, cardiac myocytes) exit the cycle at G1 and enter G0. * **Duration:** In a typical 24-hour human cell cycle, the S phase lasts about 8–10 hours. * **Regulation:** The transition from G1 to S is the most critical "restriction point," regulated by **Cyclin D and CDK4/6**. * **Clinical Correlation:** Many chemotherapy drugs (antimetabolites like Methotrexate and 5-Fluorouracil) are **S-phase specific**, as they interfere with DNA synthesis.

Question 140: Protein synthesis occurs in which organelle?

A. Ribosome (Correct Answer)
B. Golgi apparatus
C. Lysosomes
D. Endosomes

Explanation: **Explanation:** **Correct Option: A. Ribosome** Ribosomes are the primary sites of **translation**, the process where genetic code from mRNA is decoded into a polypeptide chain. Known as the "protein factories" of the cell, they consist of two subunits (40S and 60S in eukaryotes) composed of ribosomal RNA (rRNA) and proteins. They can exist freely in the cytoplasm (synthesizing proteins for internal use) or attached to the Rough Endoplasmic Reticulum (synthesizing proteins for secretion or membrane insertion). **Why other options are incorrect:** * **B. Golgi apparatus:** This organelle is responsible for the **post-translational modification**, sorting, and packaging of proteins (e.g., glycosylation and phosphorylation). It does not synthesize the protein backbone itself. * **C. Lysosomes:** These are the "suicide bags" of the cell containing acid hydrolases. Their function is **intracellular digestion** and degradation of macromolecules, not synthesis. * **D. Endosomes:** These are membrane-bound vesicles involved in **endocytosis**. They sort internalized material and direct it to the appropriate destination, such as lysosomes for degradation or back to the plasma membrane. **High-Yield NEET-PG Pearls:** * **Antibiotic Target:** Many antibiotics work by inhibiting bacterial ribosomes (70S). For example, **Aminoglycosides** and **Tetracyclines** bind to the 30S subunit, while **Macrolides** and **Chloramphenicol** bind to the 50S subunit. * **Nissl Bodies:** In neurons, the Rough Endoplasmic Reticulum and free ribosomes are called Nissl bodies; they are essential for synthesizing neurotransmitters and structural proteins. * **Signal Hypothesis:** Proteins destined for secretion have a "signal peptide" that directs the ribosome to attach to the Endoplasmic Reticulum.

Practice by Chapter

Cell Membrane Structure and Function

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Membrane Transport Proteins

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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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