Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 91: What is the primary function of the Golgi apparatus?

A. Protein synthesis
B. Transcription
C. Glycosylation (Correct Answer)
D. Cholesterol synthesis

Explanation: **Explanation:** The **Golgi apparatus** acts as the "post office" or "packaging center" of the cell. Its primary role is the post-translational modification, sorting, and packaging of proteins received from the Rough Endoplasmic Reticulum (RER). **Why Glycosylation is Correct:** One of the most critical functions of the Golgi is **glycosylation**—the process of adding carbohydrate chains to proteins and lipids to form glycoproteins and glycolipids. Specifically, while N-linked glycosylation begins in the RER, **O-linked glycosylation** occurs exclusively in the Golgi. It also handles the sulfation of proteins and the formation of lysosomes. **Analysis of Incorrect Options:** * **A. Protein synthesis:** This occurs in the **Ribosomes** (either free-floating or attached to the RER). * **B. Transcription:** This is the process of DNA being copied into RNA, which takes place in the **Nucleus**. * **C. Cholesterol synthesis:** This is a primary function of the **Smooth Endoplasmic Reticulum (SER)**, which is also responsible for steroid synthesis and detoxification. **High-Yield NEET-PG Pearls:** * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating mannose residues in the Golgi leads to enzymes being secreted extracellularly rather than being sent to lysosomes. * **Cis vs. Trans:** The *Cis-face* receives vesicles from the RER, while the *Trans-face* (Trans-Golgi Network) sorts them for their final destination. * **Staining:** The Golgi apparatus can be visualized using **Silver stain** (Cajal’s method).

Question 92: Clathrin is associated with which of the following functions?

A. Pinocytosis (Correct Answer)
B. Adhesion
C. Coagulation
D. None of the above

Explanation: **Explanation:** **Clathrin** is a specialized protein that plays a critical role in the formation of coated vesicles. It is primarily associated with **Receptor-Mediated Endocytosis**, which is a highly selective form of **Pinocytosis** (cell drinking). 1. **Why Pinocytosis is Correct:** When specific molecules (ligands) bind to receptors on the cell membrane, clathrin molecules aggregate on the cytoplasmic side to form a "clathrin-coated pit." This scaffold deforms the membrane, pulling it inward to pinch off a vesicle. This process allows the cell to internalize hormones (like insulin), growth factors, and metabolites (like LDL) from the extracellular fluid. 2. **Why Other Options are Incorrect:** * **Adhesion:** Cell-to-cell or cell-to-matrix adhesion is mediated by proteins like **Integrins, Cadherins, and Selectins**, not clathrin. * **Coagulation:** This involves a cascade of clotting factors (e.g., Fibrinogen, Thrombin) and platelets to prevent blood loss. Clathrin has no role in the extracellular clotting mechanism. **High-Yield NEET-PG Pearls:** * **LDL Uptake:** The most classic example of clathrin-mediated endocytosis is the uptake of Low-Density Lipoprotein (LDL). A defect in the LDL receptor's ability to associate with clathrin-coated pits leads to **Familial Hypercholesterolemia**. * **Triskelion Structure:** Clathrin is composed of three heavy chains and three light chains, forming a geometric shape called a "triskelion." * **Dynamin:** This is the GTPase "molecular scissor" required to pinch off the clathrin-coated vesicle from the cell membrane. * **Caveolae:** These are clathrin-independent endocytic vesicles associated with the protein **caveolin**, often found in endothelial cells and adipocytes.

Question 93: What cellular structures provide cell shape and motility?

A. Microfilaments
B. Microtubules (Correct Answer)
C. Golgi apparatus
D. Mitochondria

Explanation: ### Explanation **Correct Answer: B. Microtubules** The cytoskeleton is a dynamic network of protein filaments that maintains cell structure and facilitates movement. **Microtubules**, composed of α and β-tubulin dimers, are the largest components of the cytoskeleton. They are essential for: 1. **Cell Shape:** They act as structural "girders" that resist compression. 2. **Motility:** They form the core structure of **cilia and flagella** (the axoneme), which are primary organelles for cellular locomotion. They also serve as "tracks" for intracellular transport mediated by motor proteins like dynein and kinesin. **Why other options are incorrect:** * **A. Microfilaments:** While actin microfilaments contribute to cell shape (forming the terminal web) and amoeboid movement, microtubules are the definitive structures for specialized motility organelles (cilia/flagella) and global structural integrity. * **C. Golgi apparatus:** This organelle is responsible for the modification, sorting, and packaging of proteins; it does not provide structural support or motility. * **D. Mitochondria:** Known as the "powerhouse of the cell," they generate ATP via oxidative phosphorylation but do not form the structural framework of the cell. --- ### High-Yield Clinical Pearls for NEET-PG * **Kartagener Syndrome:** A subset of Primary Ciliary Dyskinesia caused by a defect in **dynein arms** within microtubules, leading to situs inversus, chronic sinusitis, and bronchiectasis. * **Drug Targets:** Several high-yield drugs act on microtubules: * **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. * **Arrangement:** Cilia/Flagella have a **9+2** microtubule arrangement, while Centrioles/Basal bodies have a **9+0** arrangement.

Question 94: The transition temperature of lipid bilayers of a cell membrane is increased by which of the following?

A. Cholesterol
B. Saturated fatty acids (Correct Answer)
C. Hydrocarbons
D. Unsaturated fatty acids

Explanation: **Explanation:** The **transition temperature ($T_m$)** is the specific temperature at which a cell membrane changes from a rigid, ordered "gel state" to a fluid, disordered "liquid-crystalline state." A higher $T_m$ means the membrane remains solid at higher temperatures, indicating decreased fluidity. **Why Saturated Fatty Acids are Correct:** Saturated fatty acids have straight, linear hydrocarbon chains with no double bonds. This structure allows the phospholipids to pack tightly together, maximizing **van der Waals interactions**. Because they are more tightly packed, more thermal energy (higher temperature) is required to separate them and induce fluidity. Therefore, increasing the proportion of saturated fatty acids **increases the transition temperature.** **Analysis of Incorrect Options:** * **Unsaturated Fatty Acids:** These contain "kinks" or bends due to cis-double bonds. These kinks prevent tight packing, increasing membrane fluidity and **decreasing** the $T_m$. * **Cholesterol:** Often called a "fluidity buffer," cholesterol does not have a simple linear effect on $T_m$. It interferes with the movement of fatty acid chains, preventing the membrane from becoming too fluid at high temperatures and preventing it from freezing at low temperatures. It effectively **blurs or abolishes** the sharp transition temperature rather than simply increasing it. * **Hydrocarbons:** While fatty acids are hydrocarbons, the term is too broad. Short-chain hydrocarbons generally increase fluidity and decrease $T_m$. **High-Yield Clinical Pearls for NEET-PG:** * **Fluidity vs. $T_m$:** They are inversely related. Factors that increase fluidity (e.g., unsaturation, shorter chain length) decrease the transition temperature. * **Chain Length:** Longer fatty acid chains increase $T_m$ due to increased surface area for intermolecular attractions. * **Clinical Correlation:** The fluidity of the red cell membrane is vital for its **deformability** as it passes through narrow splenic sinusoids. Alterations in lipid composition (e.g., in spur cell anemia) lead to rigid membranes and hemolysis.

Question 95: Which of the following caspases is an initiator caspase?

A. Caspase 3
B. Caspase 6
C. Caspase 9 (Correct Answer)
D. None of the above

Explanation: **Explanation:** Apoptosis (programmed cell death) is mediated by a family of cysteine proteases known as **Caspases**. These are broadly categorized into two functional groups: **Initiator Caspases** and **Executioner (Effector) Caspases**. **1. Why Caspase 9 is Correct:** Caspase 9 is a classic **Initiator Caspase** associated with the **Intrinsic (Mitochondrial) Pathway**. When the mitochondria release Cytochrome c into the cytosol, it binds with Apaf-1 to form the "Apoptosome." This complex recruits and activates pro-caspase 9. Once activated, Caspase 9 triggers a proteolytic cascade that activates the executioner caspases. **2. Why the Other Options are Incorrect:** * **Caspase 3 & 6 (Options A & B):** These are **Executioner (Effector) Caspases**. Once activated by initiators, they cleave structural proteins (like nuclear lamins) and activate nucleases to cause DNA fragmentation, leading to the physical changes of cell death. Caspase 3 is considered the most important executioner caspase. **High-Yield Facts for NEET-PG:** * **Initiator Caspases:** * **Intrinsic Pathway:** Caspase 9. * **Extrinsic (Death Receptor) Pathway:** Caspase 8 and 10. * **Executioner Caspases:** Caspase 3, 6, and 7. * **Inflammatory Caspase:** Caspase 1 (involved in the formation of inflammasomes and processing of IL-1β). * **Mnemonic:** Remember **"8, 9, 10"** as the "starters" (Initiators) and **"3, 6, 7"** as the "finishers" (Executioners). * **Caspase-independent cell death:** This occurs via the release of AIF (Apoptosis Inducing Factor) from mitochondria.

Question 96: Osmotic pressure can be calculated using which law?

A. Bernoulli's law
B. Van't Hoff law (Correct Answer)
C. Ohm's law
D. Poiseuille's law

Explanation: **Explanation:** **1. Why Van’t Hoff Law is Correct:** Osmotic pressure ($\pi$) is the pressure required to prevent the inward flow of water across a semipermeable membrane. According to **Van’t Hoff’s Law**, osmotic pressure is directly proportional to the molar concentration of solutes and the absolute temperature. The formula is expressed as: $$\pi = iCRT$$ *(Where $i$ = van't Hoff factor, $C$ = molar concentration, $R$ = gas constant, and $T$ = temperature)*. In physiology, this law is fundamental to understanding how plasma proteins (like albumin) and electrolytes maintain fluid balance between the intracellular and extracellular compartments. **2. Why Other Options are Incorrect:** * **Bernoulli’s Law:** Relates to fluid dynamics, stating that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. It explains the "Venturi effect" in narrowed blood vessels. * **Ohm’s Law:** In medicine, this is applied to hemodynamics ($Q = \Delta P / R$), where blood flow ($Q$) is determined by the pressure gradient ($\Delta P$) divided by resistance ($R$). * **Poiseuille’s Law:** Describes the factors affecting the resistance to laminar flow in a cylindrical tube. It highlights that resistance is inversely proportional to the **fourth power of the radius** ($r^4$), making vessel diameter the most critical determinant of blood flow. **Clinical Pearls for NEET-PG:** * **Osmolarity vs. Osmolality:** In clinical practice, **osmolality** (mOsm/kg of water) is preferred over osmolarity (mOsm/L) because it is independent of temperature and pressure. * **Plasma Osmolality:** Normal range is **280–295 mOsm/L**. It is primarily determined by Sodium ($Na^+$), Glucose, and BUN. * **Formula:** Estimated Osmolality = $2[Na^+] + \text{Glucose}/18 + \text{BUN}/2.8$.

Question 97: What directs vesicles carrying proteins modified in the Golgi cisternae to their appropriate location within the cell?

A. A sequence of amino acids within the proteins
B. A specific carbohydrate bound to the proteins
C. Coatomer or clathrin proteins coating the vesicle
D. Complementary SNARE proteins on the vesicle and its target organelle (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Complementary SNARE proteins on the vesicle and its target organelle.** **1. Why the Correct Answer is Right:** The specificity of vesicle targeting is primarily mediated by **SNARE proteins** (Soluble NSF Attachment Protein Receptors). Once a vesicle is formed and transported toward its destination, it must recognize and fuse with the correct target membrane. This is achieved through the interaction between **v-SNAREs** (located on the vesicle membrane) and **t-SNAREs** (located on the target membrane). These proteins lock together like a "zipper," bringing the membranes into close proximity to facilitate fusion. While Rab proteins (GTPases) help in initial "tethering," the SNARE complex provides the final direction and fusion mechanism. **2. Why the Incorrect Options are Wrong:** * **Option A & B:** Amino acid sequences (like the KDEL signal) or carbohydrate tags (like Mannose-6-Phosphate for lysosomes) act as **sorting signals** that tell the Golgi *which* proteins to package into specific vesicles. However, they do not physically direct the vesicle to its destination or mediate the fusion process. * **Option C:** **Coatomer (COP I/II)** and **Clathrin** are involved in the **budding and formation** of vesicles from the donor membrane. Once the vesicle is formed, these coats usually dissociate (uncoating) before the vesicle reaches its target. They provide structure, not directional targeting. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tetanus and Botulinum Toxins:** These toxins are proteases that specifically cleave **SNARE proteins** (e.g., synaptobrevin, SNAP-25, or syntaxin) at neuromuscular junctions or inhibitory neurons, preventing neurotransmitter release. * **COP II:** Mediates **anterograde** transport (Rough ER → Golgi). * **COP I:** Mediates **retrograde** transport (Golgi → Rough ER). * **Clathrin:** Associated with receptor-mediated endocytosis and transport from the Trans-Golgi Network to lysosomes. * **I-Cell Disease:** Caused by a failure to add the Mannose-6-Phosphate tag, leading to the secretion of lysosomal enzymes into the extracellular space instead of targeting them to lysosomes.

Question 98: What is the specific role of the endoplasmic reticulum?

A. Lipid biosynthesis (Correct Answer)
B. Lipid catabolism
C. Maintenance of calcium store
D. Pentose phosphate pathway

Explanation: The **Endoplasmic Reticulum (ER)** is a multifunctional organelle divided into the Rough ER (RER) and Smooth ER (SER). ### **Why Option A is Correct** The **Smooth Endoplasmic Reticulum (SER)** is the primary site for **lipid biosynthesis**, including phospholipids, cholesterol, and steroid hormones (e.g., testosterone, estrogen, and cortisol). In cells specialized for lipid metabolism, such as hepatocytes and steroid-producing cells of the adrenal cortex, the SER is highly developed. ### **Analysis of Incorrect Options** * **B. Lipid catabolism:** This primarily occurs via **Beta-oxidation**, which takes place in the **Mitochondria** (long-chain fatty acids) and **Peroxisomes** (very-long-chain fatty acids). * **C. Maintenance of calcium store:** While the ER (specifically the Sarcoplasmic Reticulum in muscle) does store calcium, the question asks for the "specific role" in a general cellular context. In many competitive exams, lipid synthesis is considered the hallmark metabolic function of the SER. (Note: If this were a "Multiple Correct" format, C would also be valid, but A is the classic biochemical priority). * **D. Pentose phosphate pathway (PPP):** This metabolic pathway occurs entirely in the **Cytosol**. ### **High-Yield Clinical Pearls for NEET-PG** * **Rough ER (RER):** Studded with ribosomes; primary site for **protein synthesis** (secretory, lysosomal, and membrane proteins) and **N-linked glycosylation**. * **Nissl Bodies:** These are large granules of RER found in neurons; they are responsible for protein synthesis. * **Detoxification:** The SER contains the **Cytochrome P450** enzyme system, essential for the detoxification of drugs and toxins in the liver. * **Sarcoplasmic Reticulum:** A specialized SER in myocytes that regulates muscle contraction by sequestering and releasing $Ca^{2+}$ ions.

Question 99: Which of the following factors does not affect the rate of diffusion?

A. Area of diffusion
B. Concentration gradient
C. Time (Correct Answer)
D. None of the above

Explanation: ### Explanation The rate of diffusion is governed by **Fick’s First Law of Diffusion**, which mathematically describes the movement of molecules across a membrane. The law is expressed as: $$J = -D \cdot A \cdot \frac{\Delta C}{\Delta X}$$ Where: * **J** = Rate of diffusion * **D** = Diffusion coefficient (determined by temperature and molecular size) * **A** = Surface area * **ΔC** = Concentration gradient * **ΔX** = Thickness of the membrane **Why Time is the Correct Answer:** The **rate** of diffusion is defined as the amount of substance moving per unit of **time**. Therefore, time is a component of the rate itself, not a factor that determines the speed or efficiency of the process. While the *total amount* of substance diffused increases over time, the *rate* (velocity) is determined by the physical properties of the membrane and the substance. **Analysis of Incorrect Options:** * **Area of Diffusion (A):** The rate is directly proportional to the surface area. For example, the large surface area of the pulmonary alveoli facilitates rapid gas exchange. * **Concentration Gradient (B):** Diffusion is a passive process driven by a chemical or electrical gradient. A steeper gradient results in a higher rate of diffusion. * **Thickness of Membrane (ΔX):** Though not listed as an option, it is inversely proportional to the rate. **High-Yield Clinical Pearls for NEET-PG:** * **Fick’s Law in Pathology:** In **Emphysema**, the rate of gas exchange decreases because the **surface area (A)** is reduced due to alveolar wall destruction. In **Pulmonary Edema/Fibrosis**, the rate decreases because the **diffusion distance (ΔX)** increases. * **Lipid Solubility:** The diffusion coefficient (D) is highly dependent on lipid solubility. This is why $CO_2$ diffuses 20 times faster than $O_2$ across the respiratory membrane. * **Graham’s Law:** The rate of diffusion is inversely proportional to the square root of the molecular weight of the substance.

Question 100: In which organelle are the lipid and protein contents approximately equal?

A. Sarcoplasmic reticulum
B. Mitochondria (Correct Answer)
C. Myelin sheath
D. Golgi apparatus

Explanation: **Explanation:** The composition of biological membranes varies significantly depending on the organelle's specific function. In a typical cell membrane, the protein-to-lipid ratio is roughly 50:50 by weight. **Why Mitochondria is Correct:** The **Mitochondria** (specifically the inner mitochondrial membrane) is unique because it is the site of the Electron Transport Chain (ETC) and ATP synthesis. These processes require a massive density of integral and peripheral proteins (enzymes, cytochromes, and transporters). While the inner membrane is actually protein-rich (approx. 75% protein), the **overall average** for the organelle, considering both membranes, brings the ratio closest to a balanced **1:1 (50% protein, 50% lipid)** distribution compared to the extreme outliers in the other options. **Analysis of Incorrect Options:** * **Sarcoplasmic Reticulum:** This is a specialized form of smooth ER. While it contains many calcium-ATPase pumps, its protein-to-lipid ratio does not reach the near-equal balance seen in mitochondrial structures. * **Myelin Sheath:** This is the most "lipid-heavy" membrane in the body. It acts as an electrical insulator for axons and consists of approximately **80% lipids and only 20% proteins**. * **Golgi Apparatus:** Like the plasma membrane, the Golgi is primarily involved in trafficking and modification. While it contains many enzymes, its lipid content (phospholipids and cholesterol) typically outweighs its protein content compared to the mitochondria. **High-Yield Clinical Pearls for NEET-PG:** * **Highest Protein Content:** Inner Mitochondrial Membrane (approx. 3:1 protein-to-lipid ratio). * **Highest Lipid Content:** Myelin Sheath (approx. 4:1 lipid-to-protein ratio). * **Cardiolipin:** A unique phospholipid found almost exclusively in the inner mitochondrial membrane, essential for the optimal function of ETC enzymes. * **Fluid Mosaic Model:** Proposed by Singer and Nicolson, it highlights that proteins are "icebergs in a sea of lipids," but the "sea" is much shallower in mitochondria due to high protein density.

Practice by Chapter

Cell Membrane Structure and Function

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Membrane Transport Proteins

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Cellular Energetics and Metabolism

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Mitochondrial Function

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Cell Volume Regulation

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Cellular Responses to Stress

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Calcium Signaling

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Cell Cycle and Regulation

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Cellular Aging

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Apoptosis and Cell Death

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