Cellular Physiology Practice Questions

Q: An increase in extracellular K+ concentration leads to what change in the resting membrane potential (RMP)?

Decrease in RMP. **Explanation:** The Resting Membrane Potential (RMP) of a cell is primarily determined by the concentration gradient of Potassium ($K^+$) across the cell membrane, as the membrane is highly permeable to $K^+$ at rest. This relationship is governed by the **Nernst Equation**. **1. Why Option A is Correct:** Under normal conditions, $K^+$ concentration is much higher inside the cell (~140 mEq/L) than outside (~4 mEq/L). This steep gradient drives $K^+$ to leak out, leaving the interior electronegative (approx. –70 to –90 mV). When **extracellular $K^+$ increases** (Hyperkalemia), the concentration gradient between the inside and outside of the cell decreases. Consequently, less $K^+$ leaves the cell, and the interior becomes less negative (e.g., moving from –90 mV to –70 mV). In physiological terms, moving toward zero is a **decrease in the magnitude of the RMP** (Depolarization). **2. Why Other Options are Incorrect:** * **Options B & D:** While the RMP does change, the relationship is logarithmic (as per the Nernst Equation), not a fixed linear increase of –10 or –20 mV per 10 mEq/L. Furthermore, an "increase" in RMP usually implies becoming more negative (hyperpolarization), which is the opposite of what occurs in hyperkalemia. * **Option C:** RMP is extremely sensitive to extracellular $K^+$ levels; therefore, "no change" is physiologically impossible. **Clinical Pearls for NEET-PG:** * **Hyperkalemia:** Leads to partial depolarization, making cells initially more excitable, but eventually causes inactivation of $Na^+$ channels, leading to cardiac arrhythmias (Tall T-waves on ECG). * **Hypokalemia:** Increases the concentration gradient, causing $K^+$ to exit the cell more rapidly, leading to **Hyperpolarization** (RMP becomes more negative/increases in magnitude). * **Goldman-Hodgkin-Katz Equation:** Used instead of Nernst when considering multiple ions ($Na^+, K^+, Cl^-$) simultaneously.

Q: What is the role of the membrane protein clathrin?

Receptor-mediated endocytosis. ### Explanation **Correct Answer: D. Receptor-mediated endocytosis** **Mechanism:** Clathrin is a specialized protein that plays a critical role in the formation of **coated vesicles**. During receptor-mediated endocytosis, specific ligands bind to their respective surface receptors. This triggers the recruitment of clathrin molecules from the cytoplasm to the inner surface of the plasma membrane. Clathrin assembles into a geometric, lattice-like structure called a **triskelion**, which physically deforms the membrane to form a "clathrin-coated pit." This pit eventually invaginates and pinches off (aided by the GTPase **dynamin**) to become an intracellular vesicle. **Analysis of Incorrect Options:** * **A & B (Cell Motility and Shape):** These functions are primarily governed by the **cytoskeleton**, specifically actin microfilaments (motility/lamellipodia) and intermediate filaments or microtubules (structural integrity/shape). * **C (Exocytosis):** While exocytosis involves vesicle fusion, it is generally mediated by **SNARE proteins** (v-SNARE and t-SNARE) rather than clathrin. Clathrin is specifically associated with the *internalization* of membranes. **High-Yield NEET-PG Pearls:** * **Dynamin:** The "molecular scissor" that pinches off the clathrin-coated vesicle from the cell membrane. * **LDL Uptake:** The classic example of clathrin-mediated endocytosis is the cellular uptake of **Low-Density Lipoprotein (LDL)** and **Transferrin**. * **Triskelion:** The basic structural unit of clathrin, consisting of three heavy chains and three light chains. * **Caveolae:** An alternative pathway for endocytosis involving the protein **caveolin** (often associated with lipid rafts), distinct from the clathrin pathway.

Q: Movement of substances across the intercellular space is prevented by which of the following structures?

Tight junction. **Explanation:** The correct answer is **Tight Junction (Zonula Occludens)**. **1. Why Tight Junction is correct:** Tight junctions are the most apical components of the junctional complex. They are formed by the fusion of the outer leaflets of adjacent cell membranes via transmembrane proteins like **claudins and occludins**. Their primary function is to act as a **selective barrier**, sealing the intercellular space (paracellular pathway). This prevents the free movement of water, ions, and macromolecules between cells, thereby maintaining distinct chemical environments on either side of the epithelial sheet. **2. Why other options are incorrect:** * **Zona Adherens (Adherens Junction):** These are anchoring junctions located below tight junctions. They connect the actin cytoskeletons of adjacent cells using **cadherins** but do not seal the intercellular space. * **Gap Junction (Nexus):** These are communicating junctions composed of **connexons**. They allow the direct passage of small ions and signaling molecules *between* cells (intracellularly), rather than preventing movement in the space *around* them. * **Desmosomes (Macula Adherens):** These provide strong mechanical adhesion by linking intermediate filaments (keratin) of adjacent cells. They are like "spot welds" and do not form a continuous seal to block paracellular transport. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood-Brain Barrier (BBB):** The physiological basis of the BBB is the presence of extensive tight junctions between capillary endothelial cells. * **Magnesium Reabsorption:** In the Thick Ascending Limb (TAL) of the Loop of Henle, magnesium is reabsorbed paracellularly through tight junctions containing **Claudin-16**. * **Pemphigus Vulgaris:** A clinical condition involving antibodies against **Desmoglein** (a component of desmosomes), leading to skin blistering.

Q: Which of the following does not belong to the molecular motor family?

Actin. **Explanation:** Molecular motors are specialized proteins that utilize the chemical energy of **ATP hydrolysis** to generate mechanical work and movement along cytoskeletal tracks. **Why Actin is the correct answer:** Actin is a **structural protein** that forms microfilaments. It serves as the **track** or "railway" upon which molecular motors (specifically Myosin) move. While actin is essential for motility and muscle contraction, it lacks intrinsic ATPase activity to function as a motor itself. Therefore, it is a component of the cytoskeleton, not a molecular motor. **Analysis of incorrect options:** * **Kinesin:** An ATPase motor protein that typically moves toward the **plus-end** (periphery) of microtubules. It is responsible for **anterograde axonal transport**. * **Dynein:** An ATPase motor protein that moves toward the **minus-end** (centrosome) of microtubules. It mediates **retrograde axonal transport** and the beating of cilia/flagella. * **Myosin:** A large family of motor proteins (e.g., Myosin II in muscle) that move along **actin filaments**. They convert ATP into mechanical force for muscle contraction and vesicle transport. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Dynein arms**, leading to immobile cilia, bronchiectasis, and situs inversus. * **Axonal Transport:** Remember **"K"**inesin for **"K"**arrying away from the cell body (Anterograde) and **D**ynein for **D**riving back to the cell body (Retrograde). * **Microtubules** are the tracks for Kinesin and Dynein; **Microfilaments (Actin)** are the tracks for Myosin.

Q: Cell volume and shape are maintained by which of the following?

Gibbs-Donnan effect. **Explanation:** The **Gibbs-Donnan effect** (or Donnan Equilibrium) describes the behavior of charged particles near a semi-permeable membrane when one of the ions is "non-diffusible" (e.g., intracellular proteins). Because these negatively charged proteins cannot leave the cell, they influence the distribution of diffusible ions (like $Na^+$, $K^+$, and $Cl^-$). This creates an osmotic gradient that draws water into the cell. To counteract this and maintain constant **cell volume and shape**, the cell utilizes the $Na^+\text{-}K^+$ ATPase pump to actively extrude sodium. Without the balance provided by the Gibbs-Donnan effect and active transport, cells would swell and rupture. **Analysis of Options:** * **A. Goldman effect:** This refers to the **Goldman-Hodgkin-Katz (GHK) equation**, which is used to calculate the resting membrane potential based on the permeability and concentration gradients of multiple ions. It relates to electrical excitability, not volume maintenance. * **C. Singer’s effect:** This is a distractor. S.J. Singer is famous for the **Singer-Nicolson Fluid Mosaic Model**, which describes the structure of the cell membrane (phospholipid bilayer with embedded proteins), but there is no specific "Singer’s effect" related to cell volume. **High-Yield Clinical Pearls for NEET-PG:** * **Gibbs-Donnan Equation:** At equilibrium, the product of diffusible ions on one side equals the product of diffusible ions on the other ($[K^+]_i \times [Cl^-]_i = [K^+]_o \times [Cl^-]_o$). * **Oncotic Pressure:** The Gibbs-Donnan effect is responsible for about 6-7 mmHg of the total plasma oncotic pressure due to the presence of albumin. * **Regulatory Volume Increase/Decrease (RVI/RVD):** While Gibbs-Donnan sets the baseline, acute volume changes are managed by $Na^+\text{-}H^+$ exchangers and $K^+\text{-}Cl^-$ cotransporters.

Q: All of the following act as cell adhesion molecules, EXCEPT:

Lecithin. **Explanation:** Cell Adhesion Molecules (CAMs) are specialized transmembrane proteins located on the cell surface that facilitate cell-to-cell and cell-to-matrix interactions. They are essential for tissue integrity, inflammation, and wound healing. **Why Lecithin is the correct answer:** **Lecithin (Phosphatidylcholine)** is a **phospholipid**, not a protein. It is a major structural component of cell membranes and acts as a surfactant in the lungs. It does not function as an adhesion molecule. In the context of biochemistry, it is also a source of choline for acetylcholine synthesis. **Analysis of other options:** * **Integrins (Option A):** These are heterodimeric receptors that primarily mediate **cell-matrix adhesion** (e.g., binding to fibronectin or collagen). They also play a crucial role in "inside-out" signaling and the firm adhesion phase of leukocyte extravasation. * **Selectins (Option B):** These are carbohydrate-binding lectins responsible for the initial **"rolling"** phase of leukocytes along the vascular endothelium during inflammation. (e.g., P-selectin, E-selectin). * **Cadherins (Option C):** These are **calcium-dependent** homophilic adhesion molecules. They are vital for maintaining tissue architecture (e.g., E-cadherin in epithelial desmosomes). Loss of E-cadherin is a hallmark of Epithelial-Mesenchymal Transition (EMT) in cancer metastasis. **High-Yield Clinical Pearls for NEET-PG:** * **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a defect in **Integrins** (CD18). Presents with delayed umbilical cord separation and recurrent infections without pus. * **LAD Type 2:** Caused by a defect in **Sialyl-Lewis X** (ligand for Selectins). * **Pemphigus Vulgaris:** An autoimmune disease where antibodies target **Desmoglein** (a type of Cadherin), leading to skin blistering.

Q: Transport of neutral substances across the cell membrane occurs by which mechanism?

Diffusion. **Explanation:** **Mechanism of Transport for Neutral Substances:** The cell membrane is a semi-permeable lipid bilayer. **Diffusion** (specifically simple diffusion) is the primary mechanism for the transport of **neutral, non-polar, and lipid-soluble substances** (e.g., $O_2$, $CO_2$, $N_2$, and steroid hormones). These substances dissolve in the hydrophobic lipid bilayer and move down their concentration gradient without the need for carrier proteins or energy. While water is polar, it is small enough to also move via simple diffusion (osmosis) through the lipid matrix. **Analysis of Incorrect Options:** * **A. Porins:** These are large transmembrane proteins found in the outer membranes of Gram-negative bacteria and mitochondria. They form aqueous channels that allow the passage of hydrophilic molecules, not neutral lipid-soluble substances. * **B. Ionophores:** These are lipid-soluble molecules (often antibiotics like Valinomycin) that facilitate the transport of specific **ions** (charged particles) across the membrane, not neutral substances. * **C. Lipopolysaccharides (LPS):** These are structural components of the outer membrane of Gram-negative bacteria (endotoxins). They are not transport mechanisms. **High-Yield Clinical Pearls for NEET-PG:** * **Fick’s Law of Diffusion:** The rate of net diffusion is proportional to the concentration gradient, surface area, and membrane permeability, but inversely proportional to the membrane thickness. * **Facilitated Diffusion:** Unlike simple diffusion, this requires a carrier protein (e.g., **GLUT-4** for glucose) and exhibits **stereospecificity** and **saturation kinetics** ($V_{max}$). * **Gases:** $O_2$ and $CO_2$ transport across the alveolar-capillary membrane is the classic physiological example of simple diffusion.

Q: Cell matrix adhesion are mediated by?

Integrins. **Explanation:** Cell adhesion molecules (CAMs) are essential proteins that facilitate interactions between cells and their environment. They are broadly categorized into two types: those mediating **cell-cell** adhesion and those mediating **cell-matrix** adhesion. **Why Integrins are correct:** Integrins are transmembrane heterodimers (composed of $\alpha$ and $\beta$ subunits) that primarily function as receptors for the **extracellular matrix (ECM)**. They bind to components like fibronectin, laminin, and collagen. By linking the ECM to the intracellular actin cytoskeleton, integrins play a crucial role in signal transduction and structural stability (e.g., in hemidesmosomes) [1]. **Analysis of Incorrect Options:** * **Cadherins:** These are calcium-dependent proteins primarily involved in **cell-to-cell** adhesion. They are the key components of *zonula adherens* and *desmosomes* [1]. * **Selectins:** These facilitate transient **cell-to-cell** interactions, specifically the "rolling" of leukocytes on vascular endothelium during inflammation. * **Calmodulin:** This is an intracellular calcium-binding messenger protein, not an adhesion molecule. It modulates the activities of various enzymes and proteins in response to calcium levels. **High-Yield Clinical Pearls for NEET-PG:** * **Glanzmann Thrombasthenia:** Caused by a deficiency of Integrin $\alpha$IIb$\beta$3 (GPIIb/IIIa), leading to defective platelet aggregation. * **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a defect in the $\beta$2-integrin (CD18), resulting in impaired leukocyte migration and recurrent infections. * **Pemphigus Vulgaris:** An autoimmune condition where antibodies attack **Desmoglein** (a type of Cadherin), leading to loss of cell-cell adhesion (acantholysis).

Question 1

The cell membrane is permeable to three ions, X, Y, and Z. The equilibrium potentials for ions X and Y are -50 mV and -30 mV, respectively. If the resting membrane potential (RMP) is such that there is no net electrogenic transfer, what is the equilibrium potential for ion Z?

Accepted Answer

80

Answer

-130

Answer

10

Answer

-50

Question 2

What is the resting membrane potential of a cell?

Accepted Answer

Dependent on the permeability of the cell membrane to K+ being greater than to Na+.

Answer

Falls to zero if Na+/K+ ATPase in the membrane is inhibited.

Answer

Equal to the equilibrium potential for K+.

Answer

Equal to the equilibrium potential of Na+.

Question 3

An increase in extracellular K+ concentration leads to what change in the resting membrane potential (RMP)?

Accepted Answer

Decrease in RMP

Answer

Increase in RMP by –10 mV for each 10 mEq/L increase in K+

Answer

No change in RMP

Answer

Increase in RMP by –20 mV for each 10 mEq/L increase in K+

Question 4

What is the role of the membrane protein clathrin?

Accepted Answer

Receptor-mediated endocytosis

Answer

Cell motility

Answer

Cell shape

Answer

Exocytosis

Question 5

Movement of substances across the intercellular space is prevented by which of the following structures?

Accepted Answer

Tight junction

Answer

Zona adherens

Answer

Gap junction

Answer

Desmosomes

Question 6

Which of the following does not belong to the molecular motor family?

Accepted Answer

Actin

Answer

Kinesin

Answer

Dynein

Answer

Myosin

Question 7

Cell volume and shape are maintained by which of the following?

Accepted Answer

Gibbs-Donnan effect

Answer

Goldman effect

Answer

Singer's effect

Answer

None of the above

Question 8

All of the following act as cell adhesion molecules, EXCEPT:

Accepted Answer

Lecithin

Answer

Integrin

Answer

Selectin

Answer

Cadherin

Question 9

Transport of neutral substances across the cell membrane occurs by which mechanism?

Accepted Answer

Diffusion

Answer

Porins

Answer

Ionophores

Answer

Lipopolysaccharides

Question 10

Cell matrix adhesion are mediated by?

Accepted Answer

Integrins

Answer

Cadherins

Answer

Selectins

Answer

Calmodulin

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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