Cellular Physiology — MCQs

If a cell membrane were permeable only to K+, what would be the effect of reducing the extracellular K+ concentration from 5 mM to 2.5 mM on the membrane potential, given the following intracellular and extracellular ion concentrations: Intracellular (mM): K+ 140, Na+ 12, Cl- 5, Ca++ 0.0001; Extracellular (mM): K+ 5, Na+ 145, Cl- 125, Ca++ 5?

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 81: What is/are the function(s) of a microtubule?

A. Providing strength to the cell skeleton
B. Serving as a base for cilia arising from the cell
C. Facilitating the movement of cytoplasm within the cell
D. All of the above (Correct Answer)

Explanation: **Explanation:** Microtubules are the largest components of the cytoskeleton, composed of polymers of **alpha and beta-tubulin**. They are dynamic structures essential for maintaining cell architecture and facilitating intracellular transport. * **Option A (Strength):** Microtubules provide structural rigidity and shape to the cell. Along with intermediate filaments and microfilaments, they form the "scaffold" of the cell, allowing it to resist compression and maintain its 3D conformation. * **Option B (Base for Cilia):** Microtubules are the core components of cilia and flagella. The **basal body** (kinetosome), which anchors these structures to the cell membrane, is composed of a specialized arrangement of microtubule triplets (9+0 pattern). * **Option C (Cytoplasmic Movement):** Microtubules act as "railway tracks" for intracellular transport. Molecular motor proteins—**Kinesin** (anterograde) and **Dynein** (retrograde)—move organelles, vesicles, and chromosomes along these tubules, effectively facilitating the movement of contents within the cytoplasm. Since all three functions are primary roles of microtubules, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Dynein arms**, leading to immobile cilia, bronchiectasis, and situs inversus. * **Pharmacology Link:** Several drugs target microtubules to inhibit cell division (Mitotic inhibitors): * **Vinca alkaloids (Vincristine/Vinblastine):** Inhibit microtubule polymerization. * **Taxanes (Paclitaxel):** Stabilize microtubules (prevent depolymerization). * **Colchicine:** Inhibits microtubule assembly (used in Gout). * **Griseofulvin:** Antifungal that interferes with microtubule function. * **Structure:** Cilia/Flagella have a **9+2** microtubule arrangement, while Centrioles/Basal bodies have a **9+0** arrangement.

Question 82: If at resting potential, the concentrations of permeable ions X and Y are represented as -50 and -30 respectively, what is the value of ion Z?

A. 20
B. -20
C. 80 (Correct Answer)
D. -80

Explanation: ### Explanation **Concept: The Principle of Electroneutrality** The core concept behind this question is the **Principle of Electroneutrality**, which states that in any biological compartment (intracellular or extracellular fluid), the sum of positive charges (cations) must equal the sum of negative charges (anions) to maintain electrical balance. Mathematically, this is expressed as: **$\sum \text{Cations} + \sum \text{Anions} = 0$** In this scenario, we are given the values of three ions (X, Y, and Z) contributing to the resting potential. To maintain equilibrium: $X + Y + Z = 0$ $(-50) + (-30) + Z = 0$ $-80 + Z = 0$ **$Z = +80$** Therefore, for the cell to maintain its electrochemical balance at resting potential, Ion Z must carry a positive charge of 80 to offset the combined negative charge of 80 from ions X and Y. --- ### Analysis of Options * **C (80): Correct.** As calculated above, +80 is the only value that satisfies the law of electroneutrality, bringing the net charge to zero. * **A (20): Incorrect.** This value would result in a net negative charge of -60, leaving the system unstable. * **B (-20): Incorrect.** This would increase the net negativity to -100. * **D (-80): Incorrect.** This would result in a total charge of -160. This is a common "distractor" for students who confuse the sum of magnitudes with the balance of charges. --- ### High-Yield Clinical Pearls for NEET-PG * **Gibbs-Donnan Equilibrium:** Describes the distribution of permeable ions across a membrane when an impermeable charged ion (like intracellular protein) is present. * **Resting Membrane Potential (RMP):** Primarily determined by the **K+ efflux** through leak channels. In most neurons, RMP is approximately **-70 mV**. * **Goldman-Hodgkin-Katz Equation:** Unlike the Nernst equation (which calculates the equilibrium potential for a *single* ion), this equation determines the RMP by considering the **permeability** and concentration gradients of all ions (Na+, K+, and Cl-).

Question 83: Protein synthesis occurs in which part of the cell?

A. Smooth endoplasmic reticulum
B. Rough endoplasmic reticulum (Correct Answer)
C. Golgi body
D. All of the above

Explanation: **Explanation:** Protein synthesis is primarily the function of **ribosomes**. The **Rough Endoplasmic Reticulum (RER)** is characterized by the presence of ribosomes attached to its outer surface (giving it a "rough" appearance). These ribosomes are specifically responsible for synthesizing proteins destined for secretion, incorporation into the plasma membrane, or storage within lysosomes. Once synthesized, these proteins enter the lumen of the RER for folding and initial post-translational modifications. **Analysis of Options:** * **Smooth Endoplasmic Reticulum (SER):** Lacks ribosomes. Its primary functions include lipid and steroid synthesis, detoxification of drugs (especially in hepatocytes), and calcium storage (as sarcoplasmic reticulum in muscles). * **Golgi Body:** Does not synthesize proteins. It acts as the "post office" of the cell, responsible for modifying, sorting, and packaging proteins received from the RER into vesicles. * **All of the above:** Incorrect, as protein synthesis is localized to ribosomes (either free in the cytosol or bound to the RER). **High-Yield Facts for NEET-PG:** * **Free Ribosomes:** Synthesize proteins that remain *inside* the cell (e.g., hemoglobin, mitochondrial enzymes, cytoplasmic proteins). * **Nissl Bodies:** These are large granules of RER found in neurons; they are the site of protein synthesis for neurotransmitters. * **Clinical Correlation:** Plasma cells (which secrete antibodies) and pancreatic acinar cells (which secrete digestive enzymes) are rich in RER due to their high protein-synthetic activity.

Question 84: All of the following events occur during mitosis EXCEPT:

A. Cytokinesis
B. Chromatids separate
C. DNA replication (Correct Answer)
D. The kinetochore becomes evident

Explanation: **Explanation:** The cell cycle is divided into two main phases: **Interphase** (preparation) and **M-phase** (mitosis/division). **Why DNA Replication is the correct answer:** DNA replication occurs during the **S-phase (Synthetic phase)** of Interphase, not during Mitosis. During this stage, the DNA content of the cell doubles (from 2n to 4n) to ensure that each daughter cell receives an identical set of chromosomes. By the time a cell enters Mitosis (Prophase), the DNA has already been replicated. **Analysis of other options:** * **Cytokinesis (Option A):** This is the final step of the M-phase, involving the physical division of the cytoplasm into two distinct daughter cells. * **Chromatids separate (Option B):** This is the hallmark of **Anaphase**. The centromeres split, and sister chromatids are pulled toward opposite poles of the cell. * **Kinetochore becomes evident (Option D):** Kinetochores are protein structures on chromatids where spindle fibers attach. They become clearly visible and functional during **Prometaphase/Metaphase**. **High-Yield Clinical Pearls for NEET-PG:** * **G0 Phase:** A quiescent stage where cells (like neurons or mature muscle cells) exit the cycle and stop dividing. * **Checkpoints:** The **G1-S checkpoint** (Rate-limiting step) is regulated by p53; mutations here are linked to most human cancers. * **Colchicine:** A drug used in Gout that inhibits mitosis by interfering with microtubule formation (arrests cells in Metaphase). * **Mnemonic for Mitosis:** **PMAT** (Prophase, Metaphase, Anaphase, Telophase).

Question 85: All of the following statements about cadherins are true, EXCEPT:

A. Adhesion is Ca++-dependent and homophilic.
B. When mixed together, cells expressing different cadherins sort out into homotypic cell aggregates.
C. Cadherins associate with the actin cytoskeleton.
D. Cadherins interact with components of the basement membrane. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because cadherins are primarily involved in **cell-to-cell** adhesion, not cell-to-matrix adhesion. Interaction with the basement membrane (extracellular matrix) is the function of **Integrins**, which anchor cells to laminin and fibronectin. **Analysis of Options:** * **Option A:** Cadherins are named for "**Ca**lcium-dependent **adher**ence." They facilitate **homophilic** binding, meaning a cadherin on one cell binds to an identical cadherin molecule on an adjacent cell. * **Option B:** Due to their homophilic nature, cadherins are essential for **tissue sorting** during embryogenesis. Cells expressing E-cadherin will preferentially aggregate with other E-cadherin-expressing cells, leading to homotypic clusters. * **Option C:** The cytoplasmic tail of cadherins links to the **actin cytoskeleton** via adapter proteins called **catenins** (alpha, beta, and gamma). This linkage is vital for the structural integrity of Adherens junctions. **High-Yield NEET-PG Pearls:** * **E-cadherin:** Found in epithelial tissues. A loss of E-cadherin is a hallmark of **Epithelial-Mesenchymal Transition (EMT)**, facilitating cancer metastasis. * **Pemphigus Vulgaris:** An autoimmune condition where antibodies target **Desmogleins** (a type of cadherin in desmosomes), leading to acantholysis (loss of cell-cell adhesion). * **Integrins vs. Cadherins:** Remember: **C**adherins = **C**ell-to-**C**ell; **I**ntegrins = **I**nterface (Cell-to-Matrix).

Question 86: Cell drinking is also known as:

A. Phagocytosis
B. Pinocytosis (Correct Answer)
C. Endocytosis
D. None of the above

Explanation: **Explanation:** The correct answer is **Pinocytosis (Option B)**. **Why Pinocytosis is correct:** Pinocytosis (derived from the Greek word *pino*, meaning "to drink") is a form of endocytosis where the cell membrane invaginates to ingest extracellular fluid and dissolved solutes. Unlike receptor-mediated processes, it is generally non-specific. The membrane forms small vesicles (pinosomes) that pinch off into the cytoplasm, allowing the cell to sample the surrounding fluid environment. This is a constitutive process occurring in almost all eukaryotic cells. **Analysis of Incorrect Options:** * **A. Phagocytosis:** Known as **"Cell Eating."** This is the ingestion of large particulate matter (e.g., bacteria, dead cells) by specialized cells like macrophages and neutrophils. It involves the formation of large vesicles called phagosomes. * **C. Endocytosis:** This is a **broad category** that encompasses both phagocytosis and pinocytosis. While pinocytosis is a type of endocytosis, the specific term for "cell drinking" is pinocytosis. * **D. None of the above:** Incorrect, as pinocytosis is the standard physiological term. **High-Yield Facts for NEET-PG:** * **ATP Dependency:** Both pinocytosis and phagocytosis are **active transport** processes requiring energy (ATP). * **Clathrin-Mediated Endocytosis:** A specific type of pinocytosis used for the uptake of specific molecules like LDL and iron (transferrin). * **Dynamin:** The GTPase enzyme responsible for "pinching off" the vesicle from the cell membrane. * **Macropinocytosis:** A larger-scale, non-specific form of fluid uptake often triggered by growth factors.

Question 87: Which law states that the amount of a substance crossing a given area is directly proportional to the area available for diffusion, concentration gradient, and diffusion coefficient?

A. Universal gas law
B. Dalton's law
C. Fick's law (Correct Answer)
D. All of the above

Explanation: ### Explanation **Correct Answer: C. Fick’s Law** **Why it is correct:** Fick’s First Law of Diffusion describes the rate at which a substance moves across a membrane. Mathematically, it is expressed as: **J = -DA (ΔC/ΔX)** Where: * **J** = Rate of diffusion * **D** = Diffusion coefficient (depends on the nature of the substance and temperature) * **A** = Surface area available for diffusion * **ΔC/ΔX** = Concentration gradient In medical physiology, this law explains how gases (like $O_2$ and $CO_2$) exchange across the alveolar-capillary membrane. The rate of exchange increases if the surface area or concentration gradient increases, and decreases if the membrane thickness increases. **Why the other options are incorrect:** * **A. Universal Gas Law ($PV = nRT$):** Relates pressure, volume, and temperature of an ideal gas; it does not describe the movement of substances across membranes. * **B. Dalton’s Law:** States that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the individual gases. It explains how partial pressures are calculated but not the rate of diffusion. **High-Yield Clinical Pearls for NEET-PG:** * **Emphysema:** Reduces the surface area (**A**) for diffusion due to alveolar wall destruction, leading to hypoxemia (Fick’s Law). * **Pulmonary Edema/Fibrosis:** Increases the diffusion distance (membrane thickness), thereby reducing the rate of gas exchange. * **Graham’s Law:** Often confused with Fick’s; it states that the diffusion rate of a gas is inversely proportional to the square root of its molecular weight. This explains why $CO_2$ (heavier) diffuses slower than $O_2$ in air, even though $CO_2$ is more soluble in liquids.

Question 88: Regarding brown adipose tissue, which of the following statements is false?

A. High ATP synthase activity (Correct Answer)
B. High vascularity
C. Reduced or absent in the obese
D. High content of mitochondria

Explanation: ### Explanation **Correct Option: A. High ATP synthase activity** The primary function of **Brown Adipose Tissue (BAT)** is **non-shivering thermogenesis**, not ATP production. This process is mediated by **Uncoupling Protein-1 (UCP-1)**, also known as **Thermogenin**, located in the inner mitochondrial membrane. Thermogenin acts as a proton channel that allows protons to leak back into the mitochondrial matrix, bypassing **ATP synthase (Complex V)**. Consequently, the electrochemical gradient is dissipated as **heat** rather than being used to synthesize ATP. Therefore, BAT is characterized by **low ATP synthase activity** despite having high oxidative capacity. **Analysis of Other Options:** * **B. High vascularity:** BAT is highly vascularized to ensure a steady supply of oxygen for oxidation and to efficiently distribute the generated heat to the rest of the body via the bloodstream. * **C. Reduced or absent in the obese:** Clinical studies (including PET-CT scans) show that BAT activity is inversely correlated with Body Mass Index (BMI). Obese individuals have significantly lower levels of active brown fat compared to lean individuals. * **D. High content of mitochondria:** The "brown" color of BAT is due to the high density of mitochondria (which contain iron-rich cytochromes) and extensive capillary networks. --- ### High-Yield Clinical Pearls for NEET-PG * **Location:** In adults, BAT is found in the cervical, supraclavicular, axillary, and paravertebral regions. In neonates, it is prominent (interscapular) to prevent hypothermia. * **Regulation:** BAT is activated by the **Sympathetic Nervous System** via **$\beta_3$-adrenergic receptors**. * **Imaging:** Active BAT is a common cause of "false positives" on **FDG-PET scans** because it highly consumes glucose. * **Molecular Marker:** **UCP-1 (Thermogenin)** is the definitive molecular marker for brown and "beige" adipocytes.

Question 89: Which organelle plays a pivotal role in apoptosis?

A. Cytoplasm
B. Golgi complex
C. Mitochondria (Correct Answer)
D. Nucleus

Explanation: **Explanation:** The **Mitochondrion** is the central executioner of the intrinsic (mitochondrial) pathway of apoptosis. This process is initiated in response to internal cellular stress, such as DNA damage or oxidative stress. The key event is the increase in **Mitochondrial Outer Membrane Permeability (MOMP)**, regulated by the Bcl-2 family of proteins. Pro-apoptotic proteins (Bax/Bak) create pores in the membrane, leading to the leakage of **Cytochrome c** into the cytoplasm. Once released, Cytochrome c binds with Apaf-1 to form the **apoptosome**, which activates Caspase-9, triggering the executioner caspase cascade. **Analysis of Incorrect Options:** * **A. Cytoplasm:** While the later stages of apoptosis (caspase activation) occur here, the cytoplasm serves as the medium rather than the regulatory "pivotal" organelle. * **B. Golgi Complex:** Primarily involved in protein modification, sorting, and packaging; it does not play a primary role in initiating or regulating programmed cell death. * **C. Nucleus:** Although nuclear changes (chromatin condensation and DNA fragmentation by CAD—Caspase Activated DNase) are hallmarks of apoptosis, they are downstream effects rather than the initiating trigger. **NEET-PG High-Yield Pearls:** * **Anti-apoptotic proteins:** Bcl-2, Bcl-xL (they stabilize the mitochondrial membrane). * **Pro-apoptotic proteins:** Bax, Bak, Bim, Bad, PUMA. * **Mitochondrial markers:** Cytochrome c release is the definitive sign of the intrinsic pathway. * **Extrinsic Pathway:** Initiated by Death Receptors (e.g., Fas, TNF-R) and involves **Caspase-8**.

Question 90: If a cell membrane were permeable only to K+, what would be the effect of reducing the extracellular K+ concentration from 5 mM to 2.5 mM on the membrane potential, given the following intracellular and extracellular ion concentrations: Intracellular (mM): K+ 140, Na+ 12, Cl- 5, Ca++ 0.0001; Extracellular (mM): K+ 5, Na+ 145, Cl- 125, Ca++ 5?

A. 19 millivolts depolarization
B. 19 millivolts hyperpolarization (Correct Answer)
C. 38 millivolts depolarization
D. 38 millivolts hyperpolarization

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The membrane potential ($E_m$) of a cell permeable only to $K^+$ is determined by the **Nernst Equation**: $$E_K = -61 \times \log_{10} \frac{[K^+]_{in}}{[K^+]_{out}}$$ * **Initial State:** With $[K^+]_{out} = 5\text{ mM}$ and $[K^+]_{in} = 140\text{ mM}$, the potential is approximately $-88\text{ mV}$. * **Final State:** When $[K^+]_{out}$ is reduced to $2.5\text{ mM}$, the concentration gradient for $K^+$ increases. According to the Nernst equation, a **two-fold change** in concentration (at body temperature) results in a change of approximately **18–19 mV** ($61 \times \log_{10} 2 \approx 18.36\text{ mV}$). * **Direction:** Since the extracellular concentration decreased, the gradient driving $K^+$ out of the cell becomes steeper. More positive charges leave the cell, making the interior more negative. This change from $-88\text{ mV}$ to approximately $-107\text{ mV}$ is a **19 mV hyperpolarization**. **2. Why the Other Options are Wrong:** * **A & C (Depolarization):** These are incorrect because reducing extracellular $K^+$ increases the efflux of $K^+$, making the membrane potential more negative (hyperpolarization), not less negative (depolarization). * **D (38 mV):** This would occur if the concentration changed by a factor of four (e.g., from 10 mM to 2.5 mM), as $\log_{10} 4 \approx 0.6$, leading to a $61 \times 0.6 \approx 37\text{ mV}$ shift. **3. Clinical Pearls & High-Yield Facts:** * **Hypokalemia and Excitability:** In clinical practice, acute hypokalemia hyperpolarizes the resting membrane potential, moving it further from the threshold. This makes it harder to trigger an action potential, explaining the **muscle weakness and paralysis** seen in hypokalemic periodic paralysis. * **Nernst vs. Goldman Equation:** Use the Nernst equation for a single ion; use the Goldman-Hodgkin-Katz (GHK) equation when the membrane is permeable to multiple ions ($Na^+, K^+, Cl^-$). * **Rule of Thumb:** For a monovalent ion at $37^\circ\text{C}$, a 10-fold change in the concentration gradient shifts the potential by **61 mV**.

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