Cellular Physiology Practice Questions

Q: Which of the following are force-generating proteins?

Dynein and kinesin. **Explanation:** The question focuses on **molecular motors**, which are specialized proteins that convert chemical energy (ATP) into mechanical work to generate force and movement within cells. **Why Option B is Correct:** **Dynein and Kinesin** are the primary microtubule-based motor proteins. * **Kinesins** generally move cargo (organelles, vesicles) toward the **plus-end** of microtubules (anterograde transport, e.g., from the cell body to the axon terminal). * **Dyneins** move cargo toward the **minus-end** (retrograde transport) and are also responsible for the beating of cilia and flagella. * *Note:* **Myosin** is also a force-generating protein (actin-based), but it is paired incorrectly in Option A. **Why Other Options are Incorrect:** * **Option A:** While Myosin is a motor protein, **Myoglobin** is an iron-containing protein found in muscle cells that functions solely for oxygen storage, not force generation. * **Option C:** **Calmodulin** is a calcium-binding messenger protein that regulates various enzymatic activities (like MLCK). **G proteins** are molecular switches involved in signal transduction. Neither generates mechanical force. * **Option D:** **Troponin** is a regulatory protein complex (consisting of Troponin C, I, and T) that controls the interaction between actin and myosin. It acts as a "switch" but does not generate force itself. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **dynein arms**, leading to immotile cilia, bronchiectasis, and situs inversus. * **Axonal Transport:** Fast axonal transport (400 mm/day) utilizes kinesin and dynein, whereas slow transport (0.5–10 mm/day) carries structural proteins like tubulin. * **Myosin II** is the specific type of myosin responsible for muscle contraction.

Q: Which of the following statements regarding exocytosis is correct?

All of the above.. **Explanation:** Exocytosis is the process by which a cell transports secretory vesicles to the cell membrane to release their contents into the extracellular space. It is a fundamental mechanism for neurotransmitter release, hormone secretion, and membrane protein integration. **Why "All of the Above" is correct:** * **Calcium-dependence (Option A):** Regulated exocytosis (e.g., neurotransmitter release at the synapse) is triggered by a rise in intracellular $Ca^{2+}$. When an action potential reaches the terminal, voltage-gated calcium channels open; the influx of $Ca^{2+}$ acts as the signal for vesicle fusion. * **Constitutive vs. Non-constitutive (Option B):** * **Constitutive exocytosis** is continuous and occurs in all cells (e.g., secretion of extracellular matrix components). It does not require an external signal. * **Non-constitutive (Regulated) exocytosis** occurs in specialized cells (e.g., endocrine cells, neurons) and requires a specific stimulus or second messenger. * **SNARE Proteins (Option C):** Fusion requires the interaction between **v-SNAREs** (on the vesicle membrane, e.g., Synaptobrevin) and **t-SNAREs** (on the target/plasma membrane, e.g., Syntaxin and SNAP-25). These proteins form a complex that "zips" the membranes together. **High-Yield Clinical Pearls for NEET-PG:** 1. **Tetanus and Botulinum Toxins:** These act by proteolytically cleaving SNARE proteins, thereby inhibiting neurotransmitter release. * *Tetanus toxin* affects inhibitory interneurons (Renshaw cells), leading to spastic paralysis. * *Botulinum toxin* affects excitatory cholinergic neurons, leading to flaccid paralysis. 2. **Synaptotagmin:** This is the specific $Ca^{2+}$ sensor on the vesicle membrane that triggers the final fusion step. 3. **ATP Requirement:** Exocytosis is an active process requiring energy (ATP).

Q: Exocytosis requires which ion?

Ca+. **Explanation:** **1. Why Calcium (Ca²⁺) is the Correct Answer:** Exocytosis is the process by which a cell transports secretory vesicles to the cell membrane to release their contents into the extracellular space. This process is fundamentally **Calcium-dependent**. When an action potential reaches a nerve terminal or a secretory cell, it triggers the opening of **voltage-gated calcium channels**. The resulting influx of Ca²⁺ ions acts as a second messenger. These ions bind to specific calcium-sensing proteins (such as **synaptotagmin**) located on the vesicle membrane. This binding triggers the **SNARE complex** to facilitate the fusion of the vesicle with the plasma membrane, leading to the release of neurotransmitters, hormones, or enzymes. **2. Why Other Options are Incorrect:** * **Na⁺ (Sodium):** Primarily involved in the depolarization phase of the action potential. While it initiates the electrical signal, it does not directly trigger the fusion of vesicles. * **K⁺ (Potassium):** Responsible for the repolarization phase and maintaining the resting membrane potential. High extracellular K⁺ can cause depolarization, but it is the subsequent Ca²⁺ entry that causes exocytosis. * **Mg²⁺ (Magnesium):** Magnesium actually acts as a **natural calcium channel blocker**. High levels of Mg²⁺ can inhibit exocytosis by competing with Ca²⁺ at the presynaptic terminals (e.g., inhibiting Acetylcholine release at the neuromuscular junction). **3. NEET-PG High-Yield Pearls:** * **SNARE Proteins:** Remember **V-SNARE** (Synaptobrevin) on the vesicle and **T-SNARE** (Syntaxin, SNAP-25) on the target membrane. * **Toxins:** *Clostridium botulinum* and *Clostridium tetani* toxins work by cleaving these SNARE proteins, thereby preventing exocytosis of neurotransmitters. * **Lambert-Eaton Syndrome:** Antibodies against voltage-gated Ca²⁺ channels lead to reduced exocytosis of Acetylcholine, causing muscle weakness.

Q: Trace B best describes the kinetics of which of the following events?

Transport of O2 across an artificial lipid bilayer. ***Transport of O2 across an artificial lipid bilayer*** - Shows **linear, non-saturable kinetics** because O2 transport occurs via **simple diffusion** without requiring carriers or pumps. - The rate of transport is directly proportional to the concentration gradient, producing a **straight-line relationship** characteristic of passive diffusion. *Transport of Ca++ into the sarcoplasmic reticulum of a smooth muscle cell* - Involves **active transport** via **Ca-ATPase pumps** which exhibit **saturable kinetics** with a hyperbolic curve. - Transport rate plateaus at high Ca++ concentrations due to **carrier saturation**, unlike the linear trace shown. *Transport of K+ into a muscle cell* - Occurs through **carrier-mediated transport** via **Na+/K+-ATPase** or potassium channels, showing **saturable kinetics**. - Exhibits **Michaelis-Menten kinetics** with a hyperbolic curve that reaches maximum velocity (Vmax) at high concentrations. *Transport of Na+ out of a nerve cell* - Mediated by **Na+/K+-ATPase pump** which demonstrates **saturable, carrier-mediated kinetics** with a hyperbolic curve. - Shows **concentration-dependent saturation** where transport rate levels off at high Na+ concentrations, not linear kinetics.

Q: Which of the following events does not occur in the M phase of mitosis?

Replication of the genome. **Explanation:** The cell cycle is divided into two primary phases: **Interphase** (the preparatory phase) and the **M phase** (Mitosis/Meiosis) [1], [2]. **Why "Replication of the genome" is the correct answer:** DNA replication (genome duplication) occurs exclusively during the **S phase (Synthesis phase)** of Interphase, not the M phase [1], [2]. By the time a cell enters mitosis, it already contains a double set of DNA (4n). The purpose of the M phase is to segregate this pre-replicated genetic material into two daughter cells, not to synthesize it. **Analysis of Incorrect Options:** * **A. Condensation of chromosomes:** This occurs during **Prophase** (the first stage of M phase) [1]. Chromatin fibers coil and tighten to become distinct chromosomes, making them visible under a light microscope. * **C. Fragmentation of the nuclear envelope:** This occurs during **Prometaphase**. The nuclear lamina breaks down to allow spindle fibers access to the chromosomes. * **D. Alignment of chromatids along the equatorial plate:** This is the hallmark of **Metaphase** [1]. Sister chromatids line up at the cell's center (metaphase plate) to ensure equal distribution. **NEET-PG High-Yield Pearls:** * **G0 Phase:** Cells that cease dividing (e.g., neurons, cardiac myocytes) enter a quiescent state called G0 [2]. * **Checkpoints:** The **G1/S checkpoint** (Restriction point) is the most critical rate-limiting step of the cell cycle [2]. * **Colchicine:** A high-yield pharmacological link; it inhibits mitosis by interfering with microtubule polymerization, arresting cells in **Metaphase**. * **Karyotyping:** Always performed during **Metaphase** because chromosomes are at their maximum state of condensation.

Q: The binding site for which of the following is present on the $\beta$ subunit of the Na+ - K+ pump?

Glycosylation. The **Na+-K+ ATPase pump** is an electrogenic transmembrane protein essential for maintaining resting membrane potential. It is a heteromultimer consisting primarily of an **$\alpha$ subunit** and a **$\beta$ subunit**. ### **Why Glycosylation is Correct** The **$\beta$ subunit** is a glycoprotein. Its primary functions are the assembly of the pump complex and its subsequent trafficking/targeting to the plasma membrane. The extracellular domain of the $\beta$ subunit contains multiple **glycosylation sites**. Without this glycosylation, the pump cannot be properly integrated into the cell membrane. ### **Analysis of Incorrect Options** The **$\alpha$ subunit** is the "catalytic" or functional subunit. It contains the binding sites for: * **Option A (Na+):** Three Na+ binding sites are located on the intracellular side of the $\alpha$ subunit. * **Option B (K+):** Two K+ binding sites are located on the extracellular side of the $\alpha$ subunit. * **Option C (ATP):** The ATP binding and phosphorylation site is located on the intracellular loop of the $\alpha$ subunit. * *Note:* The binding site for **Digitalis (Ouabain)** is also located on the extracellular surface of the $\alpha$ subunit. ### **NEET-PG High-Yield Pearls** * **Stoichiometry:** The pump moves **3 Na+ OUT** and **2 K+ IN** for every 1 ATP hydrolyzed. * **Electrogenicity:** It creates a net deficit of positive charges inside the cell, contributing directly to the negativity of the Resting Membrane Potential (RMP). * **Inhibitors:** It is inhibited by **Cardiac Glycosides** (Digoxin/Ouabain), which bind to the $\alpha$ subunit in the E2-P state. * **Subunit Roles:** * $\alpha$: Catalytic (Na, K, ATP, Digoxin binding). * $\beta$: Structural/Trafficking (Glycosylation). * $\gamma$: Regulatory (found in specific tissues like the kidney).

Q: Which are the most common cells to exhibit ameboid locomotion in the human body?

WBC. **Explanation:** **Ameboid locomotion** is a type of crawling-like movement characterized by the protrusion of **pseudopodia** (false feet). This process involves the continuous rearrangement of the actin cytoskeleton, where actin polymerization at the leading edge pushes the cell membrane forward. **Why WBCs are the correct answer:** White Blood Cells (WBCs), particularly **neutrophils and macrophages**, are the primary cells in the human body that utilize ameboid movement. This mechanism is essential for **chemotaxis**—the movement of cells toward a chemical stimulus (such as bacterial toxins or inflammatory cytokines). It allows WBCs to undergo **diapedesis** (squeezing through capillary walls) to reach the site of infection or tissue injury in the extravascular space. **Analysis of Incorrect Options:** * **A. RBCs:** Red Blood Cells are passive travelers within the circulatory system. They lack a nucleus and the complex cytoskeletal machinery required for independent locomotion. Their primary structural feature is flexibility (deformability) to pass through narrow capillaries, not active movement. * **C. Platelets:** While platelets can change shape and extend "filopodia" during activation and clot retraction, they do not exhibit true ameboid locomotion to migrate through tissues like WBCs. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Ameboid movement depends on **ATP**, **calcium ions**, and the interaction between **actin and myosin** filaments. * **Other cells:** Apart from WBCs, **fibroblasts** (during wound healing) and **embryonic cells** (during morphogenesis) also exhibit ameboid-like movement. * **Clinical Correlation:** Deficiencies in leukocyte adhesion or cytoskeletal remodeling (e.g., **LAD - Leukocyte Adhesion Deficiency**) result in impaired ameboid movement, leading to recurrent infections.

Question 1

Which of the following are force-generating proteins?

Accepted Answer

Dynein and kinesin

Answer

Myosin and myoglobin

Answer

Calmodulin and G protein

Answer

Troponin

Question 2

Which organ's cells lack the deposition of mitochondria?

Accepted Answer

Kidney

Answer

Brain

Answer

Heart

Answer

Muscles

Question 3

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

Accepted Answer

Decrease fluidity of the lipid bilayer.

Answer

Increase fluidity of the lipid bilayer.

Answer

Facilitate the diffusion of ions through the lipid bilayer.

Answer

Assist in the transport of hormones across the lipid bilayer.

Question 4

Which of the following statements regarding exocytosis is correct?

Accepted Answer

All of the above.

Answer

It is a calcium-dependent process.

Answer

It can be constitutive or non-constitutive.

Answer

It requires SNARE proteins.

Question 5

Exocytosis requires which ion?

Accepted Answer

Ca+

Answer

Na+

Answer

K+

Answer

Mg+2

Question 6

Which factor favors transport across a cell membrane?

Accepted Answer

High concentration gradient

Answer

Thick membrane

Answer

Large particle size

Answer

Polar substance

Question 7

Trace B best describes the kinetics of which of the following events?

Accepted Answer

Transport of O2 across an artificial lipid bilayer

Answer

Transport of Ca++ into the sarcoplasmic reticulum of a smooth muscle cell

Answer

Transport of K+ into a muscle cell

Answer

Transport of Na+ out of a nerve cell

Question 8

Which of the following events does not occur in the M phase of mitosis?

Accepted Answer

Replication of the genome

Answer

Condensation of the chromosomes

Answer

Fragmentation of the nuclear envelope

Answer

Alignment of the chromatids along the equatorial plate

Question 9

The binding site for which of the following is present on the $\beta$ subunit of the Na+ - K+ pump?

Accepted Answer

Glycosylation

Answer

Na+

Answer

K+

Answer

ATP

Question 10

Which are the most common cells to exhibit ameboid locomotion in the human body?

Accepted Answer

WBC

Answer

RBC

Answer

Platelets

Answer

None of the above

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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