Cell Structure and Function — MCQs
Which of the following statements regarding collagen synthesis is incorrect?
Cells are most sensitive to ionizing radiation during which phase?
What is the main component of a bilayer cell membrane?
During growth factor-induced cellular regeneration, which transition during the cell cycle is controlled by the phosphorylation of the retinoblastoma protein?
Which of the following statements about the Na-K pump is false?
Intermediate filaments in connective tissue are which type of structural component?
What is the preferred treatment for congenital ptosis with poor levator muscle function?
All the following mediate their action using cAMP as second messenger except:
There are two blood vessels shown below. Assuming that pressure along both the vessels is same and both of them follow linear flow pattern, what will be the amount of blood flow in A compared to B?
Type I muscle fibers are rich in myosin heavy chain. What is their characteristic property?
Cell Structure and Function Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following statements regarding collagen synthesis is incorrect?
- A. Hydroxylation of lysine occurs in ER
- B. Synthesized in ribosomes as preprocollagen
- C. Triple helix assembly occurs in ER
- D. Hydroxylation of proline occurs in Golgi apparatus (Correct Answer)
Explanation: ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.
Question 2: Cells are most sensitive to ionizing radiation during which phase?
- A. S phase
- B. G2M phase (Correct Answer)
- C. G0 phase
- D. G1 phase
Explanation: ***G2M phase*** - Cells are most sensitive to ionizing radiation during the **G2 phase** and **M phase** (mitosis) due to the highly condensed chromatin structure and active DNA repair mechanisms being less efficient [2], [3]. - During G2, DNA synthesis is complete, and the cell is preparing for division, making DNA damage particularly detrimental and harder to repair without compromising cell viability [2]. *S phase* - Cells in the **S phase** (DNA synthesis phase) are relatively radioresistant because of active **DNA replication** and associated repair mechanisms. - These repair pathways are highly efficient at correcting DNA damage during replication, making the cell less susceptible to radiation-induced lethality. *G1 phase* - Cells in the **G1 phase** (first gap phase) show intermediate radiosensitivity. - While less sensitive than G2/M phases, G1 cells are more vulnerable than those in late S phase due to active metabolic preparation for DNA synthesis [1]. *G0 phase* - Cells in the **G0 phase** (quiescent phase) are generally **radioresistant** because they are not actively dividing or synthesizing DNA [3]. - They have ample time for DNA repair before re-entering the cell cycle, and their DNA structure is less vulnerable than during active division [3]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 302-303. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 37-38. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Central Nervous System Synapse, pp. 436-437.
Question 3: What is the main component of a bilayer cell membrane?
- A. Cholesterol ester
- B. Triacyl glycerol
- C. Cholesterol
- D. Phospholipids (Correct Answer)
Explanation: ***Correct: Phospholipids*** - **Phospholipids** are the primary structural components of cell membranes, forming a **bilayer** due to their amphipathic nature. - The **hydrophilic heads** face the aqueous environment, while the **hydrophobic tails** form the core of the membrane. *Incorrect: Cholesterol* - **Cholesterol** is an important component of animal cell membranes, contributing to fluidity and stability, but it is not the **main structural component**. - It inserts between phospholipids, modulating membrane fluidity by preventing the tight packing of fatty acid tails at lower temperatures and hindering excessive movement at higher temperatures. *Incorrect: Cholesterol ester* - **Cholesterol esters** are storage forms of cholesterol and are primarily found in intracellular lipid droplets or associated with lipoproteins in the bloodstream. - They are generally too **hydrophobic** to be significant structural components within the phospholipid bilayer itself. *Incorrect: Triacyl glycerol* - **Triacylglycerols** (triglycerides) are the primary form of **energy storage** in cells, found in lipid droplets within the cytoplasm. - They are highly **hydrophobic** and do not form a structural part of the cell membrane bilayer.
Question 4: During growth factor-induced cellular regeneration, which transition during the cell cycle is controlled by the phosphorylation of the retinoblastoma protein?
- A. S to G2
- B. G2 to M
- C. G0 to G1
- D. G1 to S (Correct Answer)
Explanation: ***G1 to S*** - The **phosphorylation of the retinoblastoma protein (Rb)** is crucial for the **G1 checkpoint**, allowing progression into the S phase for DNA synthesis [1]. - This transition is a key regulatory point in the cell cycle, determining whether the cell commits to division or remains quiescent [2]. *G2 to M* - This transition involves the **activation of cyclin-dependent kinases (CDKs)**, but it is **not primarily regulated by Rb** phosphorylation. - Instead, it features the **cyclin B/CDK1 complex** which is crucial for mitosis initiation. *G0 to G1* - Transitioning from G0 to G1 typically involves signals that promote **cell re-entry** into the cell cycle, not the **phosphorylation of Rb**. - This phase is characterized by cellular **growth signals** but does not involve direct Rb regulation. *S to G2* - In this phase, cells are primarily focused on **DNA replication** and preparing for mitosis rather than Rb activity. - Rb is primarily active during the **G1 to S phase** transition, thus making its role in the S to G2 transition minimal [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 300-301. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 297-298.
Question 5: Which of the following statements about the Na-K pump is false?
- A. It is not directly involved in the generation of action potentials.
- B. It is electrogenic
- C. It needs ATP for its functioning
- D. It is located on the apical membrane of cell (Correct Answer)
Explanation: ***It is located on the apical membrane of cell*** - The **Na-K pump**, or **Na+/K+-ATPase**, is primarily located on the **basolateral membrane** of epithelial cells, not **apical membrane**. - Its strategic placement on the basolateral membrane is crucial for maintaining cellular polarity and driving transepithelial transport processes, such as reabsorption in the kidneys. *It is electrogenic* - The Na-K pump is indeed **electrogenic** because it transports three **Na+ ions** out of the cell for every two **K+ ions** pumped in. - This unequal charge distribution creates a net movement of one positive charge out of the cell, contributing to the **resting membrane potential**. *It is not directly involved in the generation of action potentials.* - While the Na-K pump is essential for **maintaining the ion gradients** necessary for **action potentials**, it is not directly involved in their rapid depolarization or repolarization phases. - Action potentials are primarily generated by the rapid opening and closing of **voltage-gated ion channels** (e.g., Na+ and K+ channels). *It needs ATP for its functioning* - The Na-K pump is an **active transport mechanism** that moves ions against their concentration gradients, requiring **energy in the form of ATP hydrolysis**. - This **ATP-dependent process** ensures the continuous maintenance of the Na+ and K+ gradients, crucial for various cellular functions, including nerve impulse transmission and muscle contraction.
Question 6: Intermediate filaments in connective tissue are which type of structural component?
- A. Keratin
- B. Vimentin (Correct Answer)
- C. Lamin
- D. Desmin
Explanation: ***Vimentin*** - **Vimentin** is the most common intermediate filament found in cells of **mesenchymal origin**, including those in connective tissue (e.g., fibroblasts) [1]. - It plays a crucial role in maintaining **cell shape**, integrity, and in processes like cell migration and adhesion within connective tissue [1]. *Keratin* - **Keratins** (also known as cytokeratins) are the primary intermediate filaments found in **epithelial cells**, providing structural integrity to tissues like skin, hair, and nails [1]. - They are not typically found in connective tissue cells, which have different structural requirements. *Desmin* - **Desmin** is an intermediate filament predominantly found in **muscle cells** (skeletal, cardiac, and smooth muscle). - It helps in maintaining the structural and mechanical integrity of the **sarcomere** and muscle fibers. *Lamin* - **Lamins** are unique intermediate filaments that form the **nuclear lamina**, a fibrous network underlying the inner nuclear membrane found in almost all nucleated cells. - They provide structural support to the nucleus and are involved in chromatin organization and gene regulation.
Question 7: What is the preferred treatment for congenital ptosis with poor levator muscle function?
- A. Muscle suspension technique (Correct Answer)
- B. Muscle advancement procedure
- C. Conservative management
- D. Direct muscle repair
Explanation: ***Muscle suspension technique*** - For congenital ptosis with **poor levator function** (typically <4 mm excursion), suspending the eyelid from the **frontalis muscle** is the preferred surgical approach. - This technique utilizes the forehead muscle to elevate the eyelid, compensating for the weak levator muscle. *Muscle advancement procedure* - This technique, generally **levator advancement** or **resection**, is indicated for ptosis with **good to fair levator function** (typically >5 mm excursion). - It involves strengthening the existing levator muscle, which would be ineffective in cases of poor function. *Conservative management* - **Conservative management** is generally reserved for **mild ptosis** or when surgical intervention is not immediately necessary, often involving observation. - It is **not appropriate** for congenital ptosis with poor levator function that often leads to **amblyopia** if left untreated. *Direct muscle repair* - Direct muscle repair is not a standard term for ptosis surgery; surgical procedures like **levator resection** or **aponeurotic repair** modify the levator muscle. - In cases of **poor levator function**, directly repairing or strengthening a severely compromised muscle is **unlikely to yield effective eyelid elevation**.
Question 8: All the following mediate their action using cAMP as second messenger except:
- A. Glucagon
- B. Dopamine
- C. Corticotropin
- D. Vasopressin (Correct Answer)
Explanation: ***Vasopressin (ADH)*** - Vasopressin has **dual signaling mechanisms** depending on receptor type: - **V2 receptors** (kidney collecting duct): Use **Gs-protein → cAMP pathway** for water reabsorption via aquaporin-2 insertion - **V1 receptors** (vascular smooth muscle): Use **Gq-protein → IP3/DAG pathway** for vasoconstriction - In the context of this question, vasopressin is considered the exception because it has **significant non-cAMP mediated actions** through V1 receptors, unlike the other hormones listed which **predominantly or exclusively** use cAMP - **Note**: This is a teaching point about receptor subtypes; vasopressin DOES use cAMP at V2 receptors *Glucagon* - **Exclusively uses cAMP pathway** in hepatocytes and adipocytes - Binds to **glucagon receptor** (GPCR) → **Gs-protein** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Promotes glycogenolysis, gluconeogenesis, and lipolysis *Dopamine* - **D1 and D5 receptors** are **Gs-coupled** → **stimulate adenylyl cyclase** → **increase cAMP** - Important for neurotransmission (motor control, reward) and renal vasodilation - D2-family receptors (D2, D3, D4) inhibit cAMP but D1-family predominates in many physiological contexts *Corticotropin (ACTH)* - Binds to **melanocortin-2 receptor (MC2R)** on adrenal cortex - **Gs-protein coupled** → adenylyl cyclase activation → **increased cAMP** → PKA activation - Stimulates steroidogenesis and cortisol secretion - **Exclusively cAMP-dependent mechanism**
Question 9: There are two blood vessels shown below. Assuming that pressure along both the vessels is same and both of them follow linear flow pattern, what will be the amount of blood flow in A compared to B?
- A. 4 times
- B. 8 times
- C. 16 times
- D. 32 times (Correct Answer)
Explanation: ***32 times*** - According to **Poiseuille-Hagen equation**: Q = (ΔP × π × r⁴) / (8 × η × L), where flow is directly proportional to the fourth power of radius and inversely proportional to vessel length. - From the diagram: Vessel A has diameter 2D and length 2L, while Vessel B has diameter d and length l. - **Key interpretation**: For the answer to be 32 times, the diameter of A must be twice that of B (radius_A = 2r), while the length of A is half that of B (length_A = L/2). - **Calculation**: - Q_A ∝ (2r)⁴ / (L/2) = 16r⁴ × 2/L = 32r⁴/L - Q_B ∝ r⁴ / L - **Q_A/Q_B = 32** - This demonstrates the **powerful effect of radius** (fourth power relationship) combined with **inverse length relationship** on blood flow. - **Clinical relevance**: Small changes in vessel diameter cause dramatic changes in blood flow, which is why vasoconstriction/vasodilation are potent mechanisms for regulating tissue perfusion. *Incorrect Option: 4 times* - Would require a different radius-to-length ratio than what's given in the problem. *Incorrect Option: 8 times* - This would result if diameter of A is 2× that of B AND length of A is also 2× that of B (not half). - Calculation: (2r)⁴/(2L) ÷ (r⁴/L) = 16r⁴/2L ÷ r⁴/L = 8 *Incorrect Option: 16 times* - This would occur if radius of A is 2× that of B but both vessels have the same length. - Calculation: (2r)⁴/L ÷ (r⁴/L) = 16
Question 10: Type I muscle fibers are rich in myosin heavy chain. What is their characteristic property?
- A. Fast contracting, susceptible to fatigue
- B. Slow contracting, susceptible to fatigue
- C. Fast contracting, resistant to fatigue
- D. Slow contracting, resistant to fatigue (Correct Answer)
Explanation: ### Explanation **1. Why Option D is Correct:** Skeletal muscle fibers are classified based on their contraction speed and metabolic profile. **Type I fibers** (also known as **Slow-Twitch** or **Red fibers**) are characterized by: * **Slow Contraction:** They possess low myosin ATPase activity, leading to a slower rate of cross-bridge cycling. * **Fatigue Resistance:** They are highly oxidative. They contain high concentrations of **myoglobin** (giving them a red color), numerous **mitochondria**, and a rich capillary supply. This allows them to generate ATP efficiently through aerobic metabolism, making them ideal for sustained, low-intensity activities like maintaining posture or long-distance running. **2. Analysis of Incorrect Options:** * **Option A (Fast contracting, susceptible to fatigue):** This describes **Type IIb (or IIx)** fibers. These are "White fibers" that rely on anaerobic glycolysis. They contract rapidly and powerfully but exhaust their glycogen stores quickly, leading to rapid fatigue. * **Option B (Slow contracting, susceptible to fatigue):** This is physiologically inconsistent. Slow-contracting fibers are built for endurance; there is no major fiber type that is both slow and easily fatigued. * **Option C (Fast contracting, resistant to fatigue):** This describes **Type IIa** fibers (Intermediate fibers). They are fast-twitch but have a high oxidative capacity, making them more resistant to fatigue than Type IIb, though less so than Type I. **3. NEET-PG High-Yield Pearls:** * **Mnemonic:** **"One Slow Red Ox"** (Type **I**, **Slow**-twitch, **Red** color, **Ox**idative metabolism). * **Myoglobin:** High in Type I (stores oxygen); Low in Type II. * **Glycogen Content:** High in Type II (for anaerobic bursts); Low in Type I. * **Mitochondria:** Type I has the highest density to support the Krebs cycle and Electron Transport Chain. * **Postural Muscles:** Muscles like the **soleus** are predominantly Type I, whereas muscles used for rapid movement (like the extraocular muscles) are predominantly Type II.
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