Cellular Physiology Practice Questions

Q: The plasma membrane of a cell is bounded to the cytoskeleton by which of the following molecules?

Ankyrin. **Explanation:** The plasma membrane is anchored to the underlying cytoskeleton through a specialized protein network that provides structural integrity and maintains cell shape. **Why Ankyrin is correct:** **Ankyrin** acts as a primary "adapter" or "bridge" protein. It possesses specific binding sites that allow it to attach integral membrane proteins (such as the **Band 3 anion exchanger** or Na⁺-K⁺ ATPase) to the underlying **spectrin** cytoskeleton. By tethering the lipid bilayer to the meshwork of the cytoskeleton, ankyrin ensures the membrane remains stable under mechanical stress. **Analysis of Incorrect Options:** * **Spectrin (Option A):** While spectrin is a major component of the membrane skeleton, it forms the horizontal meshwork *underneath* the membrane. It does not bind directly to the lipid bilayer; it requires ankyrin to facilitate that connection. * **Tubulin (Option B):** Tubulin is the protein subunit of **microtubules**. Microtubules are involved in intracellular transport and cell division (mitotic spindle) but are not the primary molecules anchoring the plasma membrane to the cortical cytoskeleton. * **Laminin (Option D):** Laminin is a major glycoprotein of the **extracellular matrix (basal lamina)**. It is involved in cell adhesion to external structures, not the internal anchoring of the membrane to the cytoskeleton. **Clinical Pearls for NEET-PG:** * **Hereditary Spherocytosis:** A deficiency or mutation in **Ankyrin** (most common cause, ~50-60% of cases) or Spectrin leads to a loss of membrane surface area. This results in spherical, fragile RBCs that are prematurely destroyed in the spleen. * **Band 3 Protein:** This is the specific integral membrane protein that Ankyrin binds to in the erythrocyte membrane. * **Mnemonic:** **A**nkyrin **A**nchors the membrane.

Q: Which ion is required for exocytosis?

Calcium. **Explanation:** **Why Calcium is Correct:** Exocytosis is the process by which a cell transports secretory products out of the cytoplasm via membrane-bound vesicles. **Calcium ($Ca^{2+}$)** is the essential trigger for this process. When an action potential reaches a nerve terminal or a secretory cell, voltage-gated calcium channels open, leading to an influx of $Ca^{2+}$. This increase in intracellular calcium concentration acts as a signal for **SNARE proteins** (Synaptobrevin, Syntaxin, and SNAP-25) to facilitate the docking and fusion of the vesicle with the plasma membrane, releasing its contents into the extracellular space. **Why Incorrect Options are Wrong:** * **Potassium ($K^+$):** Primarily responsible for maintaining the resting membrane potential and the repolarization phase of the action potential. * **Sodium ($Na^+$):** Primarily responsible for the depolarization phase of the action potential but does not directly trigger the fusion of secretory vesicles. * **Magnesium ($Mg^{2+}$):** Often acts as a calcium antagonist. High levels of magnesium can actually inhibit exocytosis (e.g., inhibiting acetylcholine release at the neuromuscular junction) by competing with calcium for entry into the presynaptic terminal. **High-Yield NEET-PG Pearls:** * **Synaptotagmin:** This is the specific protein on the vesicle membrane that acts as the **calcium sensor** to trigger fusion. * **Toxins:** Botulinum and Tetanus toxins work by cleaving SNARE proteins, thereby preventing $Ca^{2+}$-induced exocytosis of neurotransmitters. * **Lambert-Eaton Syndrome:** Antibodies against voltage-gated calcium channels prevent $Ca^{2+}$ influx, leading to impaired exocytosis of Acetylcholine and muscle weakness.

Q: Which of the following statements is true regarding secondary active transport?

It directly uses an ATP molecule for transport.. ### Explanation **Correct Answer: C. It directly uses an ATP molecule for transport.** **Underlying Medical Concept:** Secondary active transport (also known as coupled transport) is a process where the transport of a solute against its electrochemical gradient is powered by the energy stored in the electrochemical gradient of another solute (usually $Na^+$). While it does not hydrolyze ATP directly, it **indirectly** relies on ATP because the primary active transporter (like the $Na^+$-$K^+$ ATPase pump) must use ATP to establish the initial concentration gradient that drives the secondary process. **Analysis of Options:** * **Option A & B (Incorrect):** Secondary active transport can occur in two forms: **Symport (Cotransport)**, where both solutes move in the same direction (e.g., SGLT-1 in the intestine), or **Antiport (Counter-transport)**, where solutes move in opposite directions (e.g., $Na^+$-$Ca^{2+}$ exchanger). It is not restricted to one direction. * **Option D (Incorrect):** All forms of active transport (primary and secondary) are **carrier-mediated**. They require specific transmembrane proteins to facilitate the movement of solutes across the lipid bilayer. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 & SGLT-2:** Classic examples of secondary active transport (Symport) used in glucose reabsorption in the kidneys and absorption in the gut. * **Digitalis Mechanism:** Digoxin inhibits the $Na^+$-$K^+$ ATPase (Primary). This leads to an increase in intracellular $Na^+$, which subsequently inhibits the $Na^+$-$Ca^{2+}$ exchanger (Secondary), increasing intracellular $Ca^{2+}$ and cardiac contractility. * **Oral Rehydration Therapy (ORT):** Works on the principle of $Na^+$-Glucose cotransport (SGLT-1); $Na^+$ absorption enhances water reabsorption.

Q: Which of the following is correct about the image shown below?

X=Alpha subunit, A=Beta subunit. ***X=Alpha subunit, A=Beta subunit*** - The image displays the **Na+/K+-ATPase pump**, a crucial enzyme in maintaining cellular membrane potential and ionic gradients. - **X represents the alpha subunit**, the larger catalytic subunit responsible for **ATP hydrolysis, ion binding sites (3 Na+ and 2 K+), ouabain binding, and ion translocation** across the membrane. - **A represents the beta subunit**, the smaller glycoprotein subunit that assists in **proper folding, membrane insertion, and stabilization** of the alpha subunit. - The alpha subunit contains approximately 1000 amino acids with 10 transmembrane domains, while the beta subunit is smaller with a single transmembrane domain. *X=Beta subunit, A=Alpha subunit* - This incorrectly reverses the subunit identities. The **larger, catalytically active subunit (X) is the alpha subunit**, not the beta subunit. - The beta subunit is smaller and does not contain the ATP binding site or perform ion translocation. *X=Beta subunit, A=Delta subunit* - This is incorrect on both counts. **X is the alpha subunit**, not beta, and there is **no delta subunit** in the standard Na+/K+-ATPase structure. - While a gamma subunit exists in some tissues (FXYD proteins), it is not typically labeled as "delta" and is not shown in this basic diagram. *X=Alpha subunit, A=Delta subunit* - While correctly identifying **X as the alpha subunit**, this incorrectly labels A as a delta subunit. - **A is the beta subunit**, which is essential for proper enzyme function and membrane localization.

Q: Identify the modality of intercellular communication shown below.

Paracrine. ***Paracrine*** - The image shows a **signaling cell** releasing **signaling molecules** (red dots) into the extracellular space, which then act on a nearby **target cell**. This local signaling is characteristic of paracrine communication. - In **paracrine signaling**, the molecules travel short distances through the interstitial fluid to influence neighboring cells, without entering the bloodstream. *Autocrine* - In **autocrine signaling**, a cell releases signaling molecules that then act on **receptors on the same cell** that produced them. The image clearly depicts communication between two different cells. - The signaling molecules are shown moving from one cell (signaling cell) to another distinct cell (target cell), rather than acting back on the originating cell. *Synaptic* - **Synaptic signaling** involves specialized structures called **synapses** where neurons transmit signals using **neurotransmitters** across a synaptic cleft to a target cell (another neuron, muscle cell, or gland cell). The image does not show a neuronal structure or a synapse. - This type of communication is highly specific to the nervous system and involves electrical impulses followed by chemical transmission, which is not represented here. *Gap junction* - **Gap junction communication** involves direct passage of signaling molecules between adjacent cells through specialized protein channels called **gap junctions**. - The image depicts signaling molecules being released into the extracellular space and binding to receptors on the target cell, rather than passing directly between the cytoplasms of two cells.

Q: Which of these is true about the highlighted transporter?

One molecule goes in, other molecule goes out. ***One molecule goes in, other molecule goes out*** - The highlighted transporter is the **Na+/K+ ATPase**, which actively pumps 3 **Na+ ions out** of the cell and 2 **K+ ions into** the cell, maintaining an electrochemical gradient. - This counter-transport (one molecule type going in and another going out) is characteristic of an **antiporter** pump. *Both molecules go in* - This option would describe a **symporter** mechanism where two different molecules move in the **same direction** across the membrane. - The Na+/K+ ATPase explicitly shows Na+ moving out and K+ moving in, which contradicts simultaneous inward movement. *Both molecules go out* - This would mean two molecules are expelled from the cell. The Na+/K+ ATPase, however, has K+ entering the cell. - While Na+ is pumped out by this transporter, K+ is actively transported inward. *One molecule goes in and two exit* - The Na+/K+ ATPase transports three Na+ ions out of the cell and two K+ ions into the cell, which is a 3:2 ratio and not one in and two out. - This option incorrectly describes the stoichiometry and directionality of ions for the Na+/K+ ATPase.

Q: Which of the following is not a component of the process shown in the image?

Actin. ***Actin*** - The image depicts **microtubule-based transport** involving motor proteins (dynein and kinesin) moving cargo along microtubules. - **Actin filaments** are a completely separate cytoskeletal system and are NOT components of the microtubule-based process shown. - This is the most fundamental distinction - actin vs. microtubule cytoskeletal systems. *Dynein* - **Dynein** is clearly shown in the image, transporting **retrograde cargo** toward the minus end of the microtubule. - It is a key motor protein for minus-end directed movement along microtubules. *Kinesin* - **Kinesin** is depicted in the image, transporting **anterograde cargo** toward the plus end of the microtubule. - It is the primary motor protein for plus-end directed movement along microtubules. *Myosin* - While **myosin** typically works with actin filaments rather than microtubules, it belongs to the same functional category as dynein and kinesin (motor proteins). - Some myosin isoforms can even associate with microtubule-based processes in specific contexts. - **Actin** is the better answer as it represents a completely different cytoskeletal system, whereas myosin is still a motor protein like the others listed.

Q: The following graph shows the transport kinetics of a solute transferred across cell membrane. What is the likely mechanism of transport of this solute?

Carrier-mediated transport. ***Carrier-mediated transport*** - The graph shows **saturation kinetics**, where the velocity of transport increases with extracellular concentration up to a maximum (Vmax). This is characteristic of transport systems that rely on a **finite number of carrier proteins**. - The presence of a **Vmax** and the curve indicating 1/2 Vmax (which corresponds to Km for Michaelis-Menten kinetics) are hallmarks of **carrier-mediated processes**, such as **facilitated diffusion** or **active transport**. - As substrate concentration increases, all available carriers become saturated, leading to a plateau in transport rate. *Simple diffusion* - Would show **linear kinetics** with no saturation, as the rate of transport continues to increase proportionally with concentration gradient. - Does not involve carrier proteins or demonstrate Vmax. *Osmosis* - Refers specifically to **water movement** across a semipermeable membrane. - Also shows linear kinetics dependent on osmotic gradient, not saturation kinetics. *Filtration* - A **pressure-driven process** that moves solutes through pores based on hydrostatic pressure. - Does not demonstrate saturation kinetics or involve specific carrier proteins.

Q: Cystic fibrosis leads to defect in which of the following channels?

Cl-. ***Cl-*** - Cystic fibrosis is caused by a mutation in the **CFTR gene**, which encodes for the **Cystic Fibrosis Transmembrane Conductance Regulator protein**. - This protein functions primarily as a **chloride channel**, and its dysfunction leads to impaired chloride transport across epithelial cell membranes. *K+* - While potassium channels are crucial for many physiological processes, their primary dysfunction is **not directly linked to the pathogenesis of cystic fibrosis**. - Defects in potassium channels are associated with conditions like **long QT syndrome** or certain forms of epilepsy. *Ca2+* - **Calcium channels play a role in various cellular signaling pathways**, but their direct defect is not the underlying cause of cystic fibrosis. - Conditions like **Lambert-Eaton myasthenic syndrome** involve antibodies affecting presynaptic calcium channels. *Na+* - **Sodium channels are involved in maintaining membrane potential and fluid balance**, and while they interact with CFTR, their primary defect is not the cause of cystic fibrosis. - Dysregulation of sodium transport can occur secondary to CFTR dysfunction, leading to **dehydrated mucus**, but the initial defect is in chloride.

Q: Which of the following ion plays a role in exocytosis?

Calcium. ***Calcium*** - **Calcium ions** are crucial for initiating the fusion of **secretory vesicles** with the plasma membrane during **exocytosis**. - An increase in intracellular calcium concentration, often due to an influx from outside the cell, triggers the release of neurotransmitters, hormones, and other substances. *Potassium* - **Potassium ions** are primarily involved in maintaining the **resting membrane potential** and repolarization during action potentials. - While essential for neuronal function, they do not directly trigger the **vesicle fusion** step of exocytosis. *Sodium* - **Sodium ions** are vital for depolarizing the membrane and initiating **action potentials**, as well as for certain co-transport mechanisms. - However, they do not directly bind to proteins involved in **exocytosis** to trigger the release mechanism. *Magnesium* - **Magnesium ions** serve as **cofactors** for many enzymes, including ATPases, and play a role in stabilizing nucleic acids and proteins. - While important for overall cellular function, magnesium does not directly initiate or regulate the **fusion events** of exocytosis.

Question 1

The plasma membrane of a cell is bounded to the cytoskeleton by which of the following molecules?

Accepted Answer

Ankyrin

Answer

Spectrin

Answer

Tubulin

Answer

Laminin

Question 2

Which ion is required for exocytosis?

Accepted Answer

Calcium

Answer

Potassium

Answer

Sodium

Answer

Magnesium

Question 3

Which of the following statements is true regarding secondary active transport?

Accepted Answer

It directly uses an ATP molecule for transport.

Answer

It occurs only in the same direction as the coupled solute.

Answer

It occurs only in the opposite direction of the coupled solute.

Answer

It does not require a carrier protein.

Question 4

Which of the following is correct about the image shown below?

Accepted Answer

X=Alpha subunit, A=Beta subunit

Answer

X=Beta subunit, A=Alpha subunit

Answer

X=Beta subunit, A=Delta subunit

Answer

X=Alpha subunit, A=Delta subunit

Question 5

Identify the modality of intercellular communication shown below.

Accepted Answer

Paracrine

Answer

Autocrine

Answer

Synaptic

Answer

Gap junction

Question 6

Which of these is true about the highlighted transporter?

Accepted Answer

One molecule goes in, other molecule goes out

Answer

Both molecules go in

Answer

Both molecules go out

Answer

One molecule goes in and two exit

Question 7

Which of the following is not a component of the process shown in the image?

Accepted Answer

Actin

Answer

Dynein

Answer

Kinesin

Answer

Myosin

Question 8

The following graph shows the transport kinetics of a solute transferred across cell membrane. What is the likely mechanism of transport of this solute?

Accepted Answer

Carrier-mediated transport

Answer

Simple diffusion

Answer

Osmosis

Answer

Filtration

Question 9

Cystic fibrosis leads to defect in which of the following channels?

Accepted Answer

Cl-

Answer

K+

Answer

Ca2+

Answer

Na+

Question 10

Which of the following ion plays a role in exocytosis?

Accepted Answer

Calcium

Answer

Potassium

Answer

Sodium

Answer

Magnesium

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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