Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 111: All cells do not divide at the same rate. Events in which phase of the cell cycle determine when a cell is going to replicate?

A. M phase
B. G1 phase (Correct Answer)
C. S phase
D. G2 phase

Explanation: **Explanation:** The cell cycle consists of Interphase (G1, S, G2) and the Mitotic (M) phase. The **G1 phase (Gap 1)** is the most variable in duration and is the primary determinant of the overall cell cycle length. During G1, the cell monitors its environment and size. The critical decision to replicate occurs at the **Restriction Point (R point)** in late G1. Once a cell passes this checkpoint, it is committed to DNA synthesis (S phase) and subsequent division, regardless of external signals. Cells that do not divide (like neurons) or enter a quiescent state (like hepatocytes) exit G1 to enter the **G0 phase**. **Analysis of Incorrect Options:** * **M phase (Mitosis):** This is the shortest phase where actual nuclear and cytoplasmic division occurs. It is a period of execution, not decision-making regarding the initiation of replication. * **S phase (Synthesis):** This is the period of DNA replication. Once the cell enters the S phase, the replication process has already been "determined" by the G1 checkpoints. * **G2 phase (Gap 2):** This phase serves as a final safety check for DNA damage and ensures all chromosomes have been replicated before mitosis. It does not determine the timing of the initial commitment to divide. **High-Yield Facts for NEET-PG:** * **G1-S Transition:** Regulated by **Cyclin D** and **CDK4/6**. These phosphorylate the Retinoblastoma (Rb) protein, releasing E2F transcription factors. * **Labile cells:** (e.g., intestinal epithelium, skin) have a very short G1 phase. * **Permanent cells:** (e.g., neurons, cardiac myocytes) remain in G0 and do not enter G1. * **Stable cells:** (e.g., liver, proximal tubules) are in G0 but can re-enter G1 upon stimulation.

Question 112: Transition from G2 to M phase of the cell cycle is controlled by which of the following cyclins?

A. Cyclin D
B. Cyclin A
C. Cyclin E
D. Cyclin B (Correct Answer)

Explanation: The cell cycle is regulated by a series of proteins called **Cyclins** and **Cyclin-Dependent Kinases (CDKs)**. These complexes act as checkpoints to ensure the cell is ready to proceed to the next phase. **Explanation of the Correct Answer:** The transition from the **G2 phase to the M phase (Mitosis)** is primarily regulated by the **Cyclin B-CDK1** complex. This complex is also known as the **Mitosis-Promoting Factor (MPF)** or Maturation-Promoting Factor. Its activation triggers essential mitotic processes such as nuclear envelope breakdown, chromosome condensation, and spindle formation. **Analysis of Incorrect Options:** * **Cyclin D (Option A):** This is the first cyclin produced in the cell cycle. It pairs with **CDK4/6** during the **G1 phase** to help the cell pass the "Restriction Point" and enter the S phase. * **Cyclin E (Option B):** This pairs with **CDK2** during the **late G1 phase** to facilitate the final transition into the S phase (DNA synthesis). * **Cyclin A (Option C):** This pairs with **CDK2** during the **S phase** (to regulate DNA replication) and with **CDK1** during the **late G2 phase** to prepare for mitosis. While it plays a role in G2, Cyclin B is the definitive regulator for the actual entry into the M phase. **NEET-PG High-Yield Pearls:** 1. **Mnemonic (DEAB):** To remember the sequence of cyclins: **D** (G1), **E** (G1/S), **A** (S/G2), **B** (G2/M). "Dog Eats A Bone." 2. **The Restriction Point:** Regulated by Cyclin D and the Retinoblastoma (Rb) protein; once passed, the cell is committed to division regardless of external growth factors. 3. **p53 Protein:** Known as the "Guardian of the Genome," it can arrest the cell cycle (usually at G1) to allow for DNA repair by inducing p21, which inhibits CDKs.

Question 113: Which of the following cell junctions allows for the exchange of cytoplasmic molecules between two cells?

A. Gap junctions (Correct Answer)
B. Tight junctions
C. Focal junctions
D. Anchoring junctions

Explanation: **Explanation:** **1. Why Gap Junctions are correct:** Gap junctions (also known as **nexus junctions**) are specialized intercellular connections that directly link the cytoplasm of two cells. They are composed of transmembrane proteins called **connexins**. Six connexins assemble to form a hemichannel called a **connexon**; when connexons from adjacent cells align, they create a continuous aqueous pore. This allows for the rapid exchange of ions (like $Ca^{2+}$), small signaling molecules (cAMP, $IP_3$), and metabolites (glucose, amino acids) between cells, facilitating electrical and metabolic coupling. **2. Why the other options are incorrect:** * **Tight Junctions (Zonula Occludens):** These act as a "seal" or barrier to prevent the paracellular movement of solutes and water. They maintain cell polarity but do not allow cytoplasmic exchange. * **Focal Junctions (Focal Adhesions):** These connect the intracellular actin cytoskeleton to the extracellular matrix (ECM) via integrins. They are involved in cell signaling and motility, not direct cell-to-cell cytoplasmic transfer. * **Anchoring Junctions:** This is a broad category including desmosomes and adherens junctions. Their primary function is to provide mechanical strength by tethering cytoskeletons together, not to facilitate molecular exchange. **3. NEET-PG High-Yield Clinical Pearls:** * **Cardiac Physiology:** Gap junctions are the structural basis of the **functional syncytium** in the heart, allowing for the rapid spread of action potentials. * **Clinical Correlation:** Mutations in connexin genes are linked to specific pathologies, such as **Connexin 26** mutations causing congenital deafness and **Connexin 32** mutations associated with Charcot-Marie-Tooth disease. * **Size Limit:** Gap junctions typically allow molecules with a molecular weight of less than **1,000 Daltons** to pass.

Question 114: Intracellular sorting and packing is performed by which organelle?

A. Endoplasmic Reticulum (ER)
B. Golgi apparatus (Correct Answer)
C. Ribosome
D. Cytoplasm

Explanation: **Explanation:** The **Golgi apparatus** is the correct answer as it functions as the "Post Office" or "Shipping Department" of the cell. Its primary role is the post-translational modification, **sorting, and packaging** of proteins and lipids received from the Endoplasmic Reticulum (ER) into membrane-bound vesicles for transport to their final destinations (lysosomes, plasma membrane, or secretion). **Analysis of Options:** * **Endoplasmic Reticulum (ER):** While the Rough ER is the site of protein synthesis and the Smooth ER handles lipid synthesis, they primarily act as a "factory" and transport network. They do not perform the final sorting and packaging. * **Ribosome:** These are the "protein factories" responsible for translation (converting mRNA into polypeptide chains). They have no role in sorting or packaging. * **Cytoplasm:** This is the aqueous medium (cytosol) and the organelles within it; it serves as the site for metabolic reactions but is not a functional organelle for protein trafficking. **High-Yield NEET-PG Pearls:** * **Polarity:** The Golgi has a **Cis-face** (entry/forming face) facing the ER and a **Trans-face** (exit/maturing face) where vesicles bud off. * **I-Cell Disease:** A high-yield clinical correlation where a deficiency in phosphorylating mannose residues (in the Golgi) leads to failure of protein trafficking to lysosomes. * **Glycosylation:** The Golgi is the major site for O-linked glycosylation and the modification of N-linked oligosaccharides. * **Silver Stain:** The Golgi apparatus is best visualized using silver salts (e.g., Cajal’s silver stain).

Question 115: Secretory proteins are synthesized in which cellular organelle?

A. Cytoplasm
B. Endoplasmic reticulum (Correct Answer)
C. Cytoplasm and then Endoplasmic Reticulum
D. Endoplasmic Reticulum and then cytoplasm

Explanation: **Explanation:** The synthesis of proteins destined for secretion, membrane integration, or lysosomal enzymes occurs specifically on the **Rough Endoplasmic Reticulum (RER)**. **1. Why the Endoplasmic Reticulum is correct:** The process begins with the "Signal Hypothesis." While translation initiates on free ribosomes in the cytoplasm, proteins destined for secretion possess a specific **N-terminal signal sequence**. This sequence is recognized by the **Signal Recognition Particle (SRP)**, which halts translation and docks the ribosome-protein complex onto the RER membrane at the **Sec61 translocon**. The protein is then synthesized directly into the RER lumen (co-translational translocation), where it undergoes folding and post-translational modifications (like N-linked glycosylation). **2. Analysis of Incorrect Options:** * **A. Cytoplasm:** Free ribosomes in the cytoplasm synthesize proteins that remain within the cell (e.g., hemoglobin, mitochondrial proteins, or nuclear proteins). * **C & D. Sequential Options:** These are distractors. While the *instruction* starts in the cytoplasm, the actual synthesis of the secretory protein bulk occurs exclusively across the RER membrane into the lumen. **High-Yield Clinical Pearls for NEET-PG:** * **Nissl Bodies:** These are large aggregations of RER found in neurons; they are responsible for synthesizing neurotransmitters (secretory proteins). * **Golgi Apparatus:** After the RER, secretory proteins move to the Golgi for "sorting and packaging" into secretory vesicles. * **I-Cell Disease:** A clinical correlation where a defect in protein tagging (mannose-6-phosphate) in the Golgi leads to the failure of secretory enzymes reaching lysosomes, causing them to be secreted extracellularly instead.

Question 116: Which ion primarily determines the resting membrane potential?

A. Sodium (Na+)
B. Potassium (K+) (Correct Answer)
C. Chloride (Cl-)
D. Magnesium (Mg++)

Explanation: **Explanation:** The resting membrane potential (RMP) of a cell is primarily determined by **Potassium (K+)** due to two main factors: **high permeability** and the **concentration gradient**. 1. **High Permeability:** At rest, the cell membrane is significantly more permeable to K+ than to any other ion. This is due to the presence of "leak channels" (specifically **Inward Rectifier K+ channels**) that remain open at rest. 2. **Concentration Gradient:** The Na+/K+ ATPase pump maintains a high intracellular K+ concentration. As K+ leaks out of the cell down its chemical gradient, it leaves behind negative charges (anions), creating an electrical potential. According to the **Nernst Equation**, the equilibrium potential for K+ is approximately **-94 mV**. Since the membrane is most permeable to K+, the RMP (typically -70 to -90 mV) sits closest to the equilibrium potential of Potassium. **Why other options are incorrect:** * **Sodium (Na+):** The membrane has very low permeability to Na+ at rest. Na+ influx is primarily responsible for the *depolarization* phase of an action potential, not the RMP. * **Chloride (Cl-):** While Cl- contributes to the RMP in some cells (like skeletal muscle), its overall influence is secondary to K+ in most excitable tissues. * **Magnesium (Mg++):** Magnesium acts as an intracellular cofactor and a blocker of certain channels (like NMDA), but it does not directly determine the RMP. **High-Yield Clinical Pearls for NEET-PG:** * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP by considering the permeability and concentration of all permeant ions (K+, Na+, and Cl-). * **Hyperkalemia:** Increases RMP (makes it less negative), bringing the cell closer to the threshold and increasing excitability initially, but eventually causing inactivation of Na+ channels. * **Gibbs-Donnan Effect:** Describes the behavior of charged particles near a semi-permeable membrane that fails to distribute evenly due to non-diffusible proteins.

Question 117: Which of the following statements is true regarding transport across a cell membrane?

A. Chloride ions are transported with glucose via symport.
B. Sodium ions are transported with glucose via antiporter.
C. Sodium ions are transported with glucose via symport. (Correct Answer)
D. Potassium ions are transported with glucose via symport.

Explanation: ### Explanation The correct answer is **C: Sodium ions are transported with glucose via symport.** **Underlying Concept:** Glucose transport across cell membranes occurs via two primary mechanisms: facilitated diffusion (GLUT transporters) and **Secondary Active Transport**. In the small intestine and proximal convoluted tubule (PCT) of the kidney, glucose is transported against its concentration gradient. This is achieved by the **SGLT (Sodium-Glucose Linked Transporter)**. This protein utilizes the electrochemical gradient of Sodium (created by the Na+-K+ ATPase) to pull glucose into the cell. Because both Sodium and Glucose move in the same direction across the membrane, the mechanism is classified as **Symport** (or Co-transport). **Analysis of Incorrect Options:** * **A & D:** Chloride and Potassium are not the primary ions coupled with glucose transport. While chloride is often involved in other symporters (like Na-K-2Cl in the Loop of Henle), it does not drive glucose uptake. * **B:** An **Antiporter** (Counter-transport) moves substances in opposite directions (e.g., the Na+-H+ exchanger). Since glucose and sodium enter the cell together, "antiporter" is physiologically incorrect for this process. **High-Yield NEET-PG Pearls:** * **SGLT-1:** Located in the **Small Intestine**; responsible for glucose absorption. Defect leads to Glucose-Galactose Malabsorption. * **SGLT-2:** Located in the **S1 segment of the PCT**; responsible for 90% of renal glucose reabsorption. * **Clinical Correlation:** **SGLT-2 Inhibitors** (e.g., Dapagliflozin) are a major class of drugs used in Type 2 Diabetes to induce glucosuria and lower blood sugar. * **Oral Rehydration Therapy (ORT):** The physiological basis of ORT is the SGLT-1 receptor; sodium is added to the solution specifically to facilitate glucose (and water) absorption.

Question 118: Which of the following cells exhibits the highest transmembrane voltage difference?

A. Smooth muscle cell
B. Hair cell (Correct Answer)
C. Purkinje fiber
D. Skeletal muscle fiber

Explanation: **Explanation:** The correct answer is **Hair cell** (Option B). This question refers to the **transmembrane voltage difference**, which is the electrical potential gradient across a cell membrane. **1. Why Hair Cells?** Inner and outer hair cells in the cochlea are unique because they are bathed in two different fluids. Their apical surface is exposed to **endolymph** (high $K^+$, $+80$ mV potential), while their basal surface is bathed in **perilymph** (low $K^+$, $0$ mV potential). The intracellular potential of a hair cell is approximately $-40$ to $-70$ mV. Therefore, the potential difference across the apical membrane (the **endocochlear potential** gradient) is roughly **120–150 mV** ($+80$ mV minus $-70$ mV). This is the highest electrical gradient found in any cell in the human body and serves as the primary driving force for sensory transduction. **2. Why other options are incorrect:** * **Smooth muscle cell (A):** These have a relatively low and unstable resting membrane potential (RMP), typically between **$-40$ to $-60$ mV**. * **Purkinje fiber (C):** These exhibit a high RMP of about **$-90$ mV**, but this is still significantly lower than the combined gradient seen in hair cells. * **Skeletal muscle fiber (D):** These have a stable RMP of approximately **$-80$ to $-90$ mV**, which is standard for excitable tissues but does not exceed the hair cell's gradient. **Clinical Pearls for NEET-PG:** * **Endolymph** is unique because it is an extracellular fluid that resembles intracellular fluid (High $K^+$, Low $Na^+$). It is secreted by the **Stria Vascularis**. * The **Endocochlear Potential (+80 mV)** is essential for hearing; its loss (e.g., due to loop diuretics like Furosemide affecting the stria vascularis) leads to sensorineural deafness. * **Highest RMP magnitude** in a single-fluid environment: Purkinje fibers/Skeletal muscle ($\approx -90$ mV).

Question 119: The cell membrane consists of various components, including integral proteins. What is a characteristic of integral proteins?

A. They are only attached to the outer leaflet.
B. They are only attached to the inner leaflet.
C. They include transmembrane proteins. (Correct Answer)
D. They are preferentially attached to the surface.

Explanation: **Explanation:** The cell membrane is a fluid mosaic of lipids and proteins. Membrane proteins are classified into two main types based on their association with the lipid bilayer: **Integral** and **Peripheral** proteins. **Why the correct answer is right:** Integral proteins are permanently anchored within the plasma membrane. They contain hydrophobic regions that interact with the fatty acid tails of the phospholipids. A major subtype of integral proteins is **transmembrane proteins**, which span the entire thickness of the lipid bilayer, protruding into both the cytosol and the extracellular fluid. These proteins serve critical functions as ion channels, carriers, and G-protein coupled receptors (GPCRs). **Analysis of incorrect options:** * **Options A & B:** Proteins attached only to the inner or outer leaflet without penetrating the hydrophobic core are typically **peripheral proteins** or lipid-anchored proteins. Integral proteins, by definition, must penetrate or span the membrane. * **Option D:** Proteins "preferentially attached to the surface" are **peripheral proteins**. They are bound to the membrane surface via weak electrostatic interactions or by attaching to integral proteins, making them easy to dissociate (e.g., Spectrin in RBCs). **High-Yield Clinical Pearls for NEET-PG:** * **Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972). * **Detergent Extraction:** Integral proteins can only be removed by disrupting the bilayer with detergents, whereas peripheral proteins can be removed using pH changes or high salt concentrations. * **Examples:** The **Na+/K+ ATPase** pump and **GLUT transporters** are classic examples of integral transmembrane proteins. * **Clinical Correlation:** Mutations in the integral protein **CFTR** (a chloride channel) lead to Cystic Fibrosis.

Question 120: Which statement is true for both pinocytosis and phagocytosis?

A. Involves the recruitment of actin filaments (Correct Answer)
B. Occurs spontaneously and nonselectively
C. Endocytotic vesicles fuse with lysosomes that release hydrolases into the vesicles
D. Is only observed in macrophages and neutrophils

Explanation: **Explanation:** Both pinocytosis ("cell drinking") and phagocytosis ("cell eating") are forms of **active transport** known as endocytosis. The fundamental mechanical requirement for both processes is the **remodeling of the cytoskeleton**. **1. Why Option A is Correct:** To internalize substances, the cell membrane must undergo structural deformation. This process is driven by the **recruitment and polymerization of actin filaments** (microfilaments) located just beneath the plasma membrane. In phagocytosis, actin drives the extension of pseudopodia; in pinocytosis, actin facilitates the invagination of the membrane to form a vesicle. **2. Why Other Options are Incorrect:** * **Option B:** While some forms of pinocytosis (fluid-phase) are nonselective, **phagocytosis is highly selective**, usually triggered by specific ligand-receptor interactions (e.g., opsonization). Neither process is "spontaneous" as both require ATP. * **Option C:** This describes the fate of the vesicle *after* endocytosis. While common in phagocytosis (forming a phagolysosome), many pinocytotic vesicles (especially in transcytosis) bypass lysosomes to release contents elsewhere or recycle them to the surface. * **Option D:** Phagocytosis is restricted to "professional phagocytes" (macrophages, neutrophils, dendritic cells). However, **pinocytosis occurs in almost all cells** of the body to take up extracellular fluid. **High-Yield NEET-PG Pearls:** * **Clathrin-dependent endocytosis:** A specific type of receptor-mediated endocytosis (e.g., LDL uptake). * **Dynamin:** The GTPase "molecular scissor" required to pinch off endocytic vesicles from the cell membrane. * **ATP Dependency:** Both processes are active and will cease if mitochondrial poisons (like cyanide) are introduced.

Practice by Chapter

Cell Membrane Structure and Function

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Membrane Transport Proteins

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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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