Cellular Physiology Practice Questions

Q: The graph given below shows which of the following transport mechanisms?

Facilitated diffusion. ***Facilitated diffusion*** - The graph shows a **hyperbolic curve** with a **Vmax plateau**, indicating **saturation kinetics** characteristic of carrier-mediated transport. - Transport occurs **down the concentration gradient** without energy requirement, but is limited by **carrier protein availability**. *Simple diffusion* - Would show a **linear relationship** between concentration and transport rate with no saturation plateau. - Transport is directly proportional to **concentration gradient** without involvement of **carrier proteins**. *Primary active transport* - Requires **ATP hydrolysis** to transport substances **against their concentration gradient**. - Shows saturation kinetics but can maintain transport even when **concentration gradient favors reverse direction**. *Secondary active transport* - Uses **ion gradients** (usually Na+) established by primary active transport to drive **co-transport** or **counter-transport**. - Can transport substances **against their gradient** using energy from **coupled ion movement** down its gradient.

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. **1. Why Dynein and Kinesin are correct:** Dynein and kinesin are the primary force-generating motor proteins associated with **microtubules**. * **Kinesin** moves cargo (vesicles/organelles) toward the (+) end of the microtubule (**anterograde transport**). * **Dynein** moves cargo toward the (-) end (**retrograde transport**) and is also responsible for the beating motion of cilia and flagella. * *Note:* While **Myosin** is also a force-generating protein (associated with actin), it is paired incorrectly in Option A. **2. Why other options are incorrect:** * **Option A:** Myosin is a motor protein, but **Myoglobin** is an oxygen-binding protein found in muscle sarcoplasm; it does not generate force. * **Option C:** **Calmodulin** is a calcium-binding messenger protein (crucial for smooth muscle contraction), and **G proteins** are molecular switches involved in signal transduction. Neither acts as a motor protein. * **Option D:** **Troponin** is a regulatory protein complex (consisting of T, I, and C subunits) that controls the interaction between actin and myosin. It 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 anterograde transport (kinesin-mediated) is used by viruses like **Herpes Simplex**, while retrograde transport (dynein-mediated) is used by the **Rabies virus** and **Tetanus toxin** to reach the CNS. * **Myosin II** is the specific isoform responsible for skeletal muscle contraction.

Q: Which factor does not affect the diffusion of a substance through the cell membrane?

ATP generating capacity on both sides of the membrane. **Explanation:** The core concept tested here is the difference between **Passive Diffusion** and **Active Transport**. **Why Option C is correct:** Diffusion is a passive process driven by the kinetic energy of molecules moving down a concentration or electrochemical gradient. According to **Fick’s Law of Diffusion**, this process does not require metabolic energy (ATP). Therefore, the ATP-generating capacity of the cell or its surroundings has no impact on the rate of simple or facilitated diffusion. ATP is only required for primary active transport (e.g., Na⁺-K⁺ ATPase pump). **Analysis of Incorrect Options:** * **A. Molecular size:** Smaller molecules diffuse more rapidly than larger ones. According to Graham’s Law, the diffusion rate is inversely proportional to the square root of the molecular weight. * **B. Lipid solubility:** The cell membrane is a phospholipid bilayer. Substances with high lipid solubility (e.g., O₂, CO₂, steroid hormones) can dissolve directly through the lipid matrix, significantly increasing their diffusion rate. * **C. Distribution of channels:** For non-lipid soluble substances (ions like Na⁺, K⁺), diffusion depends on the presence and density of specific protein channels or carriers (facilitated diffusion). **High-Yield NEET-PG Pearls:** * **Fick’s Law Equation:** $J = -DA (\Delta C / \Delta X)$. Diffusion is directly proportional to Surface Area (A), Diffusion Coefficient (D), and Concentration Gradient ($\Delta C$), but inversely proportional to Membrane Thickness ($\Delta X$). * **Rate-limiting factor:** For simple diffusion, the limit is the concentration gradient; for facilitated diffusion, the limit is the **Vmax** (saturation of carrier proteins). * **Gases:** Oxygen and Carbon dioxide diffusion is strictly perfusion-limited under normal physiological conditions.

Q: What is the action of the alpha subunit of a G protein?

Hydrolysis of GTP to GDP. **Explanation:** The G-protein complex is a heterotrimer consisting of **alpha (α), beta (β), and gamma (γ)** subunits. The **alpha subunit** is the functional engine of this complex and possesses intrinsic enzymatic activity. **1. Why Option A is Correct:** The alpha subunit acts as a **GTPase**. Once the G-protein is activated (bound to GTP), it triggers downstream effectors. To prevent continuous signaling, the alpha subunit eventually **hydrolyzes the bound GTP into GDP** and inorganic phosphate. This hydrolysis acts as a "molecular switch" that turns the protein off, allowing the alpha subunit to re-associate with the βγ complex and return to its inactive state. **2. Why Other Options are Incorrect:** * **Option B:** The exchange of GDP for GTP is facilitated by the **activated receptor** (acting as a Guanine Nucleotide Exchange Factor or GEF), not the alpha subunit itself. This step activates the G-protein. * **Option C:** Receptor internalization (downregulation) is typically mediated by **β-arrestins** and clathrin-coated pits following phosphorylation by G-protein-coupled receptor kinases (GRKs). * **Option D:** Agonist binding occurs at the **extracellular domain** of the G-Protein Coupled Receptor (GPCR), which then induces a conformational change to activate the intracellular G-protein. **High-Yield Clinical Pearls for NEET-PG:** * **Cholera Toxin:** Inhibits the GTPase activity of the **Gs alpha subunit**, leading to constitutive activation of Adenylyl Cyclase and permanent "on" state (causing secretory diarrhea). * **Pertussis Toxin:** Prevents the GDP-GTP exchange on the **Gi alpha subunit**, keeping it in the "off" state (inhibiting the inhibitor). * **Gs** stimulates Adenylyl Cyclase (increases cAMP); **Gi** inhibits it; **Gq** activates Phospholipase C (increases $IP_3/DAG$).

Q: What is the extracellular binding site on the Na+ - K+ pump?

Ouabain. **Explanation:** The **Na⁺-K⁺ ATPase pump** is an electrogenic transmembrane protein that maintains the resting membrane potential by pumping 3 Na⁺ ions out and 2 K⁺ ions into the cell against their concentration gradients. This process requires energy derived from ATP hydrolysis. **Why Ouabain is Correct:** The Na⁺-K⁺ pump has specific binding sites located on its **alpha (α) subunit**. While Na⁺ and ATP bind on the **intracellular** side, the **extracellular** side contains binding sites for K⁺ and **cardiac glycosides** (like Ouabain and Digitalis). Ouabain acts as a potent inhibitor by binding to the extracellular domain, preventing the dephosphorylation of the pump and halting its activity. **Analysis of Incorrect Options:** * **A. Na⁺:** Sodium ions bind to three high-affinity sites located on the **intracellular** surface of the pump. * **B. ATP:** The catalytic site for ATP binding and hydrolysis is located on the **intracellular** loop of the alpha subunit. * **C. PO₄ (Phosphate):** During the pump cycle, the terminal phosphate group from ATP binds to an aspartate residue on the **intracellular** side, causing a conformational change. **Clinical Pearls for NEET-PG:** * **Mechanism of Action:** By inhibiting the Na⁺-K⁺ pump, cardiac glycosides increase intracellular Na⁺. This decreases the activity of the Na⁺-Ca²⁺ exchanger (NCX), leading to increased intracellular Ca²⁺ and increased cardiac contractility (**Positive Inotropy**). * **Stoichiometry:** 3 Na⁺ Out, 2 K⁺ In, 1 ATP consumed. * **Inhibitors:** Ouabain (G-strophanthin) and Digoxin are the classic competitive inhibitors at the K⁺ binding site.

Q: Release of neurotransmitter from presynaptic vesicles is an example of?

Exocytosis. ### Explanation **Correct Answer: D. Exocytosis** **Mechanism:** The release of neurotransmitters at the synapse is a classic example of **regulated exocytosis**. When an action potential reaches the presynaptic terminal, it triggers the opening of **voltage-gated calcium channels**. The resulting influx of $Ca^{2+}$ ions promotes the fusion of neurotransmitter-filled synaptic vesicles with the presynaptic membrane. This process is mediated by **SNARE proteins** (Synaptobrevin on the vesicle; Syntaxin and SNAP-25 on the plasma membrane). Once fused, the vesicle contents are expelled into the synaptic cleft. **Why other options are incorrect:** * **Simple Diffusion (A):** This involves the passive movement of small, non-polar molecules (like $O_2$ or $CO_2$) directly through the lipid bilayer along a concentration gradient. Neurotransmitters are often polar and stored in bulk, requiring a vesicular mechanism. * **Facilitated Diffusion (B):** This is a passive process requiring specific carrier proteins or channels (e.g., GLUT transporters) to move substances down a gradient. It does not involve vesicular fusion. * **Active Transport (C):** This involves moving solutes *against* a concentration gradient using ATP (Primary) or electrochemical gradients (Secondary). While neurotransmitters are *loaded* into vesicles via active transport (VMAT), their *release* into the cleft is exocytosis. **High-Yield Clinical Pearls for NEET-PG:** * **Synaptotagmin:** Acts as the primary $Ca^{2+}$ sensor that triggers the final fusion step of exocytosis. * **Toxins:** *Clostridium botulinum* (Botulinum toxin) and *Clostridium tetani* (Tetanus toxin) act by proteolytically cleaving **SNARE proteins**, thereby inhibiting neurotransmitter exocytosis. * **Lambert-Eaton Myasthenic Syndrome (LEMS):** Antibodies against presynaptic voltage-gated calcium channels inhibit the $Ca^{2+}$ influx necessary for exocytosis, leading to muscle weakness.

Q: Active transport and facilitated diffusion share which of the following characteristics?

Follows Michaelis-Menten kinetics. **Explanation:** The core similarity between **Active Transport** and **Facilitated Diffusion** is that both are **carrier-mediated transport** mechanisms. **Why Option D is Correct:** Because both processes rely on specific carrier proteins, they exhibit **saturation kinetics**. There are a finite number of binding sites on the carrier proteins; once all sites are occupied, the transport rate reaches a maximum ($V_{max}$). This relationship between solute concentration and transport rate follows **Michaelis-Menten kinetics**, similar to enzyme-substrate interactions. Therefore, both show a "plateau" effect, unlike simple diffusion which is linear. **Analysis of Incorrect Options:** * **Option A:** Since both use specific binding sites, they are subject to **competitive inhibition** by structurally similar molecules that compete for the same carrier slot. * **Option B:** Carrier proteins are **highly specific** for the molecules they transport (e.g., GLUT transporters for glucose), not non-specific. * **Option C:** Only **Active Transport** moves solutes against a concentration gradient (requiring energy). Facilitated diffusion is a passive process that moves solutes **down** a concentration gradient. **High-Yield Clinical Pearls for NEET-PG:** * **Stereospecificity:** Carrier proteins can distinguish between optical isomers (e.g., D-glucose is transported, but L-glucose is not). * **GLUT Transporters:** Classic examples of facilitated diffusion. * **SGLT (Sodium-Glucose Linked Transporter):** Example of Secondary Active Transport (Symport). * **Key mnemonic:** All carrier-mediated transport shows **S.S.C.** (Saturation, Specificity, and Competition).

Question 1

Across the cell membrane of a skeletal muscle cell in the resting state with a non-electrogenic pump (1 Na+ : 1K+), which of the following statements is true?

Accepted Answer

The rate of sodium influx is equal to the rate of potassium efflux.

Answer

Both sodium and potassium are in electrochemical equilibrium.

Answer

The rate of sodium influx is much less than the rate of potassium efflux.

Answer

The rate of sodium influx is much greater than the rate of potassium efflux.

Question 2

All of the following are true about gap junctions except:

Accepted Answer

Opening of gap junctions does not depend on transmembrane voltage.

Answer

Permits passage of molecules up to 1000 Da molecular weight.

Answer

Halothane closes gap junctions reversibly.

Answer

A gap junction is made up of connexons, each made of six identical connexins.

Question 3

All of the following statements about phagocytosis are true, except:

Accepted Answer

Used to ingest particles < 0.5 microns in size.

Answer

Amoeba and other unicellular protozoa make their living out of this process.

Answer

Used to ingest particles > 0.5 microns in size.

Answer

Digestion occurs within phagolysosomes.

Question 4

The graph given below shows which of the following transport mechanisms?

Accepted Answer

Facilitated diffusion

Answer

Simple diffusion

Answer

Primary active transport

Answer

Secondary active transport

Question 5

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 6

Which factor does not affect the diffusion of a substance through the cell membrane?

Accepted Answer

ATP generating capacity on both sides of the membrane

Answer

Molecular size of the substance

Answer

Lipid solubility of the membrane

Answer

Distribution of channels for the substance in the membrane

Question 7

What is the action of the alpha subunit of a G protein?

Accepted Answer

Hydrolysis of GTP to GDP

Answer

Exchange of GDP for GTP

Answer

Receptor internalization

Answer

Agonist binding to the receptor

Question 8

What is the extracellular binding site on the Na+ - K+ pump?

Accepted Answer

Ouabain

Answer

Na+

Answer

ATP

Answer

PO4

Question 9

Release of neurotransmitter from presynaptic vesicles is an example of?

Accepted Answer

Exocytosis

Answer

Simple diffusion

Answer

Facilitated diffusion

Answer

Active transport

Question 10

Active transport and facilitated diffusion share which of the following characteristics?

Accepted Answer

Follows Michaelis-Menten kinetics

Answer

Competitive inhibition does not occur

Answer

Requires a non-specific carrier protein

Answer

Transport of solute against concentration gradient

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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