Cellular Physiology Practice Questions

Q: Emeiocytosis or reverse pinocytosis requires which ion?

Ca++. ### Explanation **Emeiocytosis** (also known as **exocytosis** or reverse pinocytosis) is the process by which a cell releases substances (like hormones, neurotransmitters, or enzymes) from intracellular vesicles into the extracellular space. **Why Ca++ is the Correct Answer:** Calcium ions ($Ca^{2+}$) act as the universal "coupling agent" in stimulus-secretion coupling. When a cell is stimulated, intracellular calcium levels rise—either through influx via voltage-gated channels or release from the endoplasmic reticulum. This increase in $Ca^{2+}$ triggers the fusion of secretory vesicle membranes with the plasma membrane. Specifically, calcium binds to proteins like **synaptotagmin**, which facilitates the **SNARE complex** to pull the membranes together, allowing the contents to be expelled. Without $Ca^{2+}$, the final fusion and release step cannot occur. **Why Other Options are Incorrect:** * **Na+ (Sodium):** Primarily involved in depolarization and maintaining osmotic balance, but does not directly trigger vesicle fusion. * **K+ (Potassium):** Essential for maintaining the resting membrane potential and repolarization; high extracellular $K+$ can cause depolarization, but it is the subsequent $Ca^{2+}$ influx that drives emeiocytosis. * **Mg++ (Magnesium):** Often acts as a calcium antagonist. High levels of $Mg^{2+}$ can actually inhibit exocytosis by blocking calcium channels (e.g., in the neuromuscular junction). **High-Yield Clinical Pearls for NEET-PG:** * **Classic Example:** The release of **Insulin** from the Beta cells of the pancreas occurs via emeiocytosis in response to $Ca^{2+}$ influx. * **Neurotransmission:** At the synaptic cleft, the release of neurotransmitters (like Acetylcholine) is strictly $Ca^{2+}$-dependent. * **Botulinum Toxin:** Acts by cleaving SNARE proteins, thereby preventing $Ca^{2+}$-mediated emeiocytosis of Acetylcholine, leading to flaccid paralysis.

Q: Regarding the cell membrane, which of the following statements is true?

Proteins are displaced laterally.. The cell membrane is best described by the **Fluid Mosaic Model** (proposed by Singer and Nicolson), which characterizes the membrane as a dynamic, fluid structure rather than a static one. ### **Explanation of the Correct Option** **C. Proteins are displaced laterally:** The lipid bilayer acts as a two-dimensional solvent. Membrane proteins (both integral and peripheral) are not fixed in one position; they can move or "float" laterally within the plane of the membrane. This lateral mobility is crucial for cellular processes like signal transduction, receptor clustering, and cell-to-cell junctions. ### **Explanation of Incorrect Options** * **A. Lipids are regularly arranged:** This is incorrect because the membrane is **fluid**, not crystalline. The fatty acid chains are in constant motion (flexion, rotation, and lateral diffusion), leading to a "disordered" or liquid-crystalline state. * **B. Lipids are symmetrical:** This is a common misconception. The cell membrane is **asymmetrical**. The outer leaflet is rich in phosphatidylcholine and sphingomyelin, while the inner (cytoplasmic) leaflet contains phosphatidylserine (negatively charged) and phosphatidylethanolamine. Carbohydrates (glycolipids) are found exclusively on the outer surface. ### **High-Yield NEET-PG Pearls** * **Flippases & Floppases:** While lateral movement is rapid, "flip-flop" movement (transverse diffusion from one leaflet to another) is rare and requires specific enzymes (Flippases/Scramblases). * **Membrane Fluidity:** Increased by **unsaturated fatty acids** (kinks in tails) and higher temperatures. **Cholesterol** acts as a fluidity buffer, stabilizing the membrane at high temperatures and preventing freezing at low temperatures. * **Lipid Rafts:** These are specialized microdomains rich in cholesterol and sphingolipids that serve as platforms for cell signaling.

Q: Na+ uptake at the basolateral surface of apical cells occurs via which transport mechanism?

Active transport. ### Explanation The transport of sodium (Na+) across epithelial cells (such as those in the renal tubules or intestinal mucosa) is a polarized process. While Na+ enters the cell at the **apical surface** down its electrochemical gradient (passive), its exit at the **basolateral surface** occurs against a steep gradient. **1. Why Active Transport is Correct:** The movement of Na+ from the intracellular compartment to the interstitial fluid at the basolateral membrane is mediated by the **Na+/K+-ATPase pump**. This pump utilizes energy from ATP hydrolysis to move 3 Na+ ions out of the cell and 2 K+ ions into the cell. Because this process moves sodium against both a concentration and electrical gradient, it is defined as **Primary Active Transport**. This pump is the "engine" that maintains low intracellular Na+ concentrations, driving secondary transport processes elsewhere. **2. Why Other Options are Incorrect:** * **Passive Transport & Diffusion:** These involve the movement of solutes down a gradient without the expenditure of metabolic energy. While Na+ enters the **apical** side via diffusion (through channels like ENaC), it cannot leave the basolateral side this way because the extracellular Na+ concentration is significantly higher than the intracellular concentration. * **Osmosis:** This refers specifically to the movement of **water** (solvent) across a semipermeable membrane, not the movement of solutes like sodium. **High-Yield Clinical Pearls for NEET-PG:** * **The "Engine" of the Nephron:** The basolateral Na+/K+-ATPase is the primary consumer of oxygen in the kidney. * **Ouabain/Digitalis:** These drugs specifically inhibit the Na+/K+-ATPase pump by binding to the extracellular site. * **Secondary Active Transport:** The gradient created by this basolateral pump is what allows for the apical reabsorption of glucose (SGLT) and amino acids.

Question 1

Emeiocytosis or reverse pinocytosis requires which ion?

Accepted Answer

Ca++

Answer

Na+

Answer

K+

Answer

Mg++

Question 2

Regarding the cell membrane, which of the following statements is true?

Accepted Answer

Proteins are displaced laterally.

Answer

Lipids are regularly arranged.

Answer

Lipids are symmetrical.

Answer

None of the above.

Question 3

What is the rate-limiting step in facilitated diffusion?

Accepted Answer

Conformational change in transporter

Answer

Availability of contransporter

Answer

Synthesis of carrier protein

Answer

Polarization of solute

Question 4

Which of the following is an example of secondary active transport?

Accepted Answer

Sodium-amino acid transport

Answer

Potassium transport

Answer

Water transport

Answer

Transport of oxygen

Question 5

Na+ uptake at the basolateral surface of apical cells occurs via which transport mechanism?

Accepted Answer

Active transport

Answer

Passive transport

Answer

Diffusion

Answer

Osmosis

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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