Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 161: All the following are characteristics of the plasma membrane except?

A. It serves as a selective transport route through the facilitation of both active and passive transport.
B. It possesses a thickness of 1 to 2 mm. (Correct Answer)
C. It is involved in the generation of ionic gradients between the cytoplasm and the external environment.
D. It serves as a sensor of signals from the extracellular environment.

Explanation: **Explanation:** The plasma membrane is a dynamic, semi-permeable lipid bilayer essential for cellular homeostasis. **Why Option B is the Correct Answer (The False Statement):** The plasma membrane is an ultra-thin structure, typically measuring **7.5 to 10 nanometers (nm)** in thickness. The option states a thickness of 1 to 2 millimeters (mm), which is anatomically impossible for a microscopic cellular component (1 mm is visible to the naked eye and is roughly 100,000 times thicker than an actual cell membrane). **Analysis of Other Options:** * **Option A:** The membrane contains specialized proteins (channels, carriers, and pumps) that facilitate **passive transport** (e.g., glucose via GLUT) and **active transport** (e.g., Na⁺-K⁺ ATPase) to maintain cellular composition. * **Option C:** By being selectively permeable and utilizing active transport pumps, the membrane creates vital **electrochemical gradients** (e.g., high intracellular K⁺ and high extracellular Na⁺), which are fundamental for secondary active transport and excitability. * **Option D:** The membrane is embedded with **receptors** (GPCRs, Enzyme-linked receptors) that act as sensors, transducing extracellular chemical or mechanical signals into intracellular responses. **High-Yield NEET-PG Pearls:** * **Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972), it describes the membrane as a "sea of lipids with a mosaic of proteins." * **Composition:** Primarily consists of phospholipids, proteins, and cholesterol. Cholesterol acts as a "fluidity buffer." * **Carbohydrates:** Found only on the **outer surface** as the glycocalyx, important for cell recognition. * **Lipid Rafts:** Specialized microdomains rich in sphingolipids and cholesterol involved in signal transduction.

Question 162: The sodium-potassium pump is an example of which type of cellular transport?

A. Active transport (Correct Answer)
B. Passive transport
C. Facilitated diffusion
D. Osmosis

Explanation: **Explanation:** The **Sodium-Potassium Pump (Na⁺/K⁺-ATPase)** is the classic example of **Primary Active Transport**. It is "active" because it moves ions against their electrochemical gradients—pumping 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. This process requires the direct hydrolysis of **ATP** to provide the necessary energy. It is "electrogenic" as it creates a net charge imbalance across the membrane, contributing to the resting membrane potential. **Analysis of Incorrect Options:** * **Passive Transport:** This involves the movement of substances down their concentration gradient without the expenditure of energy. * **Facilitated Diffusion:** A form of passive transport that uses carrier proteins or channels (e.g., GLUT transporters) to move molecules down a gradient. It does not require ATP. * **Osmosis:** The specific passive movement of water molecules across a semi-permeable membrane from an area of low solute concentration to high solute concentration. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Stoichiometry:** 3 Na⁺ out, 2 K⁺ in. Remember: **"K-IN"** (Potassium goes in). 2. **Inhibition:** The pump is specifically inhibited by **Cardiac Glycosides** (e.g., **Digoxin** and Ouabain). This inhibition increases intracellular Na⁺, which subsequently slows the Na⁺/Ca²⁺ exchanger, increasing intracellular Ca²⁺ and myocardial contractility. 3. **Energy Consumption:** In a resting state, this pump can consume up to 30-40% of a cell's total ATP production (even more in neurons). 4. **Function:** It is essential for maintaining cell volume; if the pump fails, the cell swells and bursts (hydropic swelling).

Question 163: Which of the following is NOT a receptor for integrins?

A. Fibronectin
B. Glycoprotein on platelet surface
C. Leukocyte adhesion molecule
D. Platelet-derived growth factor (Correct Answer)

Explanation: **Explanation:** **Integrins** are transmembrane heterodimeric proteins (composed of $\alpha$ and $\beta$ subunits) that function as primary receptors for the **Extracellular Matrix (ECM)** and cell-cell adhesion. **Why Option D is Correct:** **Platelet-derived growth factor (PDGF)** is a soluble cytokine/growth factor, not an adhesion molecule. It binds to specific **Receptor Tyrosine Kinases (RTKs)**, not integrins. While PDGF signaling can influence integrin activity (crosstalk), it does not serve as a ligand or receptor for integrins. **Analysis of Incorrect Options:** * **A. Fibronectin:** This is a major ECM protein. Integrins (specifically $\alpha_5\beta_1$) act as the primary receptors for fibronectin via the **RGD (Arg-Gly-Asp)** sequence. * **B. Glycoprotein on platelet surface:** The most famous integrin is **GPIIb/IIIa** ($\alpha_{IIb}\beta_3$), found on platelets. It binds fibrinogen and von Willebrand factor to facilitate platelet aggregation. * **C. Leukocyte adhesion molecule:** Integrins like **LFA-1** ($\alpha_L\beta_2$) and **VLA-4** ($\alpha_4\beta_1$) are expressed on leukocytes. They bind to ligands like ICAM-1 and VCAM-1 on endothelial cells, crucial for leukocyte extravasation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Glanzmann Thrombasthenia:** A bleeding disorder caused by a deficiency or defect in the integrin **GPIIb/IIIa**. 2. **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a defect in the **$\beta_2$ subunit (CD18)** of integrins, leading to impaired neutrophil migration and delayed umbilical cord separation. 3. **RGD Sequence:** Many integrins recognize the Arginine-Glycine-Aspartate (RGD) tripeptide sequence in their ligands (e.g., fibronectin, vitronectin).

Question 164: Cellular and flagellar movement is carried out by all of the following except?

A. Intermediate filaments (Correct Answer)
B. Actin
C. Tubulin
D. Myosin

Explanation: **Explanation:** The cytoskeleton of a cell consists of three primary components: **Microtubules (Tubulin)**, **Microfilaments (Actin/Myosin)**, and **Intermediate Filaments**. The key to this question lies in distinguishing between structural support and active motility. **1. Why Intermediate Filaments (Correct Answer) are the exception:** Intermediate filaments (e.g., Keratin, Vimentin, Desmin, Neurofilaments, and Lamins) are primarily **structural** in nature. They provide mechanical strength to cells and tissues, helping them withstand shearing forces. Unlike microtubules and microfilaments, intermediate filaments are **non-polar** and lack associated "motor proteins." Therefore, they do not participate in cellular or flagellar movement. **2. Why the other options are involved in movement:** * **Tubulin (Microtubules):** These are the structural units of **cilia and flagella** (arranged in a 9+2 pattern). Movement is generated by the motor protein **Dynein**, which slides microtubules against each other. * **Actin & Myosin:** These are responsible for **amoeboid movement**, muscle contraction, and cytokinesis (cell cleavage). Actin (microfilaments) interacts with Myosin (motor protein) to generate contractile forces required for cellular locomotion. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Dynein arms** within microtubules, leading to immotile cilia, situs inversus, and infertility. * **Colchicine & Vinca Alkaloids:** These drugs act by inhibiting **Tubulin** polymerization, arresting cell division. * **Diagnostic Marker:** Intermediate filaments are tissue-specific and used in pathology to identify tumor origins (e.g., **Cytokeratin** for Carcinomas, **Vimentin** for Sarcomas, **PSA** is not an IF, but **Desmin** is for myogenic tumors). * **Smallest to Largest:** Microfilaments (7nm) < Intermediate Filaments (10nm) < Microtubules (25nm).

Question 165: Cell motility is due to which protein?

A. Motilin
B. Centromere
C. Microtubule (Correct Answer)
D. Microfilament

Explanation: **Explanation:** **Why Microtubules are the Correct Answer:** Cell motility, specifically the movement of the entire cell (like sperm) or the movement of organelles within a cell, is primarily driven by **microtubules**. These are hollow cylinders made of alpha and beta-tubulin. They form the structural core of **cilia and flagella** (in a 9+2 arrangement), which are the primary organelles for locomotion. Microtubules also act as "tracks" for motor proteins like **dynein** (retrograde transport) and **kinesin** (anterograde transport), facilitating intracellular motility. **Analysis of Incorrect Options:** * **Motilin (A):** This is a 22-amino acid polypeptide hormone secreted by M cells in the duodenum. It regulates the **Migrating Motor Complex (MMC)** in the gut; it is a chemical signal, not a structural protein for cellular movement. * **Centromere (B):** This is a specialized DNA sequence of a chromosome that links a pair of sister chromatids. While it is the site where the kinetochore forms, it is a genetic region, not a motility protein. * **Microfilament (D):** Composed of actin, these are primarily responsible for cell shape, muscle contraction, and **amoeboid movement** (pseudopodia). While they contribute to "crawling," the standard answer for generalized cell motility in a structural context (especially regarding cilia/flagella) is microtubules. **High-Yield NEET-PG Pearls:** * **Kartagener Syndrome:** A triad of situs inversus, chronic sinusitis, and bronchiectasis caused by a defect in **dynein arms** within microtubules, leading to immotile cilia and male infertility. * **Drugs acting on Microtubules:** Remember the mnemonic **"Microtubules Get Constructed Very Terribly"** (Mebendazole, Griseofulvin, Colchicine, Vincristine/Vinblastine, Paclitaxel). * **Axonal Transport:** Fast axonal transport is microtubule-dependent. Kinesin moves "away" from the cell body (+ end), while Dynein moves "towards" it (- end).

Question 166: Peripheral cell membrane proteins are involved in which of the following functions?

A. Active transport pumps
B. Ion channels
C. Cellular adhesion (Correct Answer)
D. Enzymatic activity at the cell surface

Explanation: **Explanation:** Cell membrane proteins are classified into two main types based on their association with the lipid bilayer: **Integral (Transmembrane)** and **Peripheral** proteins. **Why Cellular Adhesion is Correct:** Peripheral proteins do not penetrate the lipid bilayer; instead, they are loosely attached to the inner or outer surfaces of the membrane via electrostatic interactions. On the extracellular surface, peripheral proteins (often as part of the glycocalyx) play a critical role in **cellular adhesion** and cell-to-cell recognition. They act as "anchors" or "linkers" that connect the cell membrane to the extracellular matrix or to the internal cytoskeleton (e.g., Spectrin and Ankyrin in RBCs). **Analysis of Incorrect Options:** * **A & B (Active transport pumps and Ion channels):** These functions require a continuous pathway across the hydrophobic lipid bilayer. Therefore, they are exclusively performed by **Integral/Transmembrane proteins**, which span the entire thickness of the membrane. * **D (Enzymatic activity):** While some peripheral proteins have enzymatic roles, this is not their primary defining function in the context of membrane structural organization compared to adhesion. Furthermore, most major membrane-bound signaling enzymes (like Adenylyl Cyclase) are integral proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Spectrin & Ankyrin:** These are classic examples of peripheral proteins on the inner membrane of RBCs. A deficiency in these proteins leads to **Hereditary Spherocytosis**. * **Integral Proteins:** These can only be removed by disrupting the bilayer with detergents, whereas peripheral proteins can be removed using high-salt solutions or pH changes. * **Fluid Mosaic Model:** Remember that proteins provide the "mosaic" pattern, while lipids provide the "fluidity."

Question 167: Which of the following is NOT a characteristic of facilitated diffusion?

A. Inhibition
B. Saturability
C. Specificity
D. Uphill transport (Correct Answer)

Explanation: ### Explanation **Facilitated diffusion** is a form of carrier-mediated transport that allows molecules to move across the cell membrane along their electrochemical gradient. **Why "Uphill transport" is the correct answer:** Facilitated diffusion is a **passive process**, meaning it does not require metabolic energy (ATP). It follows the concentration gradient (from high to low concentration), which is known as **downhill transport**. "Uphill transport" refers to movement against a gradient, which is the hallmark of **Active Transport** (Primary or Secondary). **Analysis of Incorrect Options:** Since facilitated diffusion relies on specific **carrier proteins**, it exhibits the following kinetics: * **Specificity (C):** Carrier proteins have binding sites that recognize only specific molecules or isomers (e.g., GLUT transporters are specific for D-glucose, not L-glucose). * **Saturability (B):** There are a finite number of carriers. Once all binding sites are occupied, the transport rate reaches a maximum ($V_{max}$), showing a plateau phase. * **Inhibition (A):** Because it involves protein binding, the process can be inhibited by competitive molecules that structurally resemble the substrate. **NEET-PG High-Yield Pearls:** * **Classic Example:** Transport of glucose into skeletal muscle and adipose tissue via **GLUT-4** (insulin-dependent). * **Kinetics:** Facilitated diffusion follows **Michaelis-Menten kinetics**, similar to enzyme-substrate interactions. * **Comparison:** Simple diffusion is the only transport mechanism that is **not** saturable (it is linear and does not show $V_{max}$). * **Key Distinction:** Both facilitated diffusion and active transport are carrier-mediated, but only active transport can achieve a "Target-to-Source" ratio greater than 1 (uphill).

Question 168: In dividing cells, the spindle is formed by which of the following components?

A. Ubiquitin
B. Tubulin (Correct Answer)
C. Laminin
D. Keratin

Explanation: **Explanation:** The correct answer is **Tubulin (Option B)**. The mitotic spindle is a complex structure composed of **microtubules**, which are essential for segregating chromosomes during cell division (mitosis and meiosis). Microtubules are polymers made of globular protein subunits called **$\alpha$ and $\beta$-tubulin**. During the M-phase of the cell cycle, these tubulin dimers polymerize to form spindle fibers that attach to the kinetochores of chromosomes, facilitating their movement to opposite poles. **Analysis of Incorrect Options:** * **Ubiquitin (A):** This is a small regulatory protein involved in the degradation of defective or unneeded proteins via the **Ubiquitin-Proteasome Pathway**. It does not have a structural role in the spindle. * **Laminin (C):** This is a major glycoprotein component of the **basal lamina** (extracellular matrix). It helps in cell adhesion and signaling, not intracellular division. * **Keratin (D):** This is a type of **intermediate filament** found primarily in epithelial cells, providing mechanical strength to tissues (e.g., skin, hair, nails). **Clinical Pearls for NEET-PG:** * **Drug Link:** Several anti-cancer drugs target tubulin to inhibit the spindle. **Vinca alkaloids** (Vincristine/Vinblastine) inhibit tubulin polymerization, while **Taxanes** (Paclitaxel) inhibit depolymerization (stabilize microtubules). * **Colchicine:** Used in Gout, it also works by inhibiting tubulin polymerization, thereby interfering with neutrophil migration. * **Structure:** Microtubules follow a **"9+2" arrangement** in cilia/flagella and a **"9+0" arrangement** in centrioles.

Question 169: Which of the following statements regarding Nitric oxide (NO) is true?

A. NO is synthesized from arginine. (Correct Answer)
B. NO is spontaneously produced from NO2.
C. NO causes vasoconstriction.
D. NO is released from mitochondria.

Explanation: **Explanation:** Nitric Oxide (NO), formerly known as Endothelium-Derived Relaxing Factor (EDRF), is a key gaseous signaling molecule in cellular physiology. **1. Why Option A is Correct:** NO is synthesized through the enzymatic conversion of the amino acid **L-arginine** into **L-citrulline**. This reaction is catalyzed by the enzyme **Nitric Oxide Synthase (NOS)** in the presence of oxygen and several cofactors (NADPH, FAD, FMN, and Tetrahydrobiopterin/BH4). **2. Why Incorrect Options are Wrong:** * **Option B:** NO is a highly reactive gas with a short half-life (seconds). It is synthesized enzymatically on demand, not spontaneously from NO2. * **Option C:** NO is a potent **vasodilator**. It diffuses into vascular smooth muscle cells, activates **soluble Guanylyl Cyclase (sGC)**, increases **cGMP** levels, and leads to smooth muscle relaxation. * **Option D:** NO is primarily synthesized in the **cytoplasm** (by eNOS in endothelial cells or nNOS in neurons) or is associated with the plasma membrane, rather than being a mitochondrial release product. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms of NOS:** There are three types: **nNOS** (Type 1, Neuronal), **iNOS** (Type 2, Inducible/Macrophage-related), and **eNOS** (Type 3, Endothelial). * **Mechanism of Action:** NO → ↑ cGMP → Protein Kinase G (PKG) activation → Dephosphorylation of Myosin Light Chain → Vasodilation. * **Therapeutic Link:** Nitroglycerin works by releasing NO to treat angina. Sildenafil (Viagra) inhibits Phosphodiesterase-5 (PDE-5), preventing the breakdown of cGMP, thereby prolonging NO-mediated vasodilation.

Question 170: Which of the following statements about the cell membrane is incorrect?

A. Lipids are regularly arranged.
B. Lipids are arranged symmetrically. (Correct Answer)
C. Proteins can be displaced laterally.
D. None of the above.

Explanation: ### Explanation The cell membrane is best described by the **Fluid Mosaic Model** (proposed by Singer and Nicolson). Understanding its structural organization is crucial for grasping how cells interact with their environment. **1. Why Option B is the Correct Answer (Incorrect Statement):** Lipids in the cell membrane are **arranged asymmetrically**, not symmetrically. The composition of the outer leaflet differs significantly from the inner leaflet: * **Outer Leaflet:** Rich in Phosphatidylcholine and Sphingomyelin. * **Inner Leaflet:** Rich in Phosphatidylserine (carries a negative charge) and Phosphatidylethanolamine. This asymmetry is vital for cell signaling and apoptosis (e.g., the flipping of phosphatidylserine to the outer leaflet is a "eat-me" signal for macrophages). **2. Analysis of Other Options:** * **Option A (Lipids are regularly arranged):** This is a **correct** statement. Lipids are organized into a regular **bilayer** with hydrophilic heads facing the aqueous environment and hydrophobic tails pointing inward. * **Option C (Proteins can be displaced laterally):** This is a **correct** statement. The membrane is "fluid," allowing integral proteins to move laterally within the lipid sea unless they are anchored by the cytoskeleton. **High-Yield Clinical Pearls for NEET-PG:** * **Flippases & Floppases:** These are ATP-dependent enzymes that maintain membrane asymmetry. **Scramblases** are ATP-independent and mix lipids during apoptosis. * **Carbohydrates:** Always located on the **external surface** (Glycocalyx), never on the cytoplasmic side. * **Cholesterol:** Acts as a "fluidity buffer." It increases fluidity at low temperatures and decreases it at high temperatures. * **Lipid Rafts:** Specialized microdomains rich in cholesterol and sphingolipids that serve as platforms for receptor signaling.

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