Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 141: What is the function of cholesterol in the plasma membrane?

A. Helps in the transport of ions
B. Helps in exocytosis
C. Maintains fluidity (Correct Answer)
D. Helps in the synthesis of bile acids

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Cholesterol is a crucial component of the eukaryotic plasma membrane, acting as a **"fluidity buffer."** It inserts itself between the hydrophobic tails of phospholipids. * **At high temperatures:** It stabilizes the membrane and raises the melting point, preventing it from becoming too fluid or "leaky." * **At low temperatures:** It prevents the phospholipid tails from packing too tightly and crystallizing, thereby maintaining membrane flexibility. This dual role ensures that the membrane remains functional across a range of physiological conditions. **2. Why the Other Options are Incorrect:** * **Option A:** Ion transport is primarily the function of **integral membrane proteins** (channels and pumps like the $Na^+/K^+$ ATPase), not lipids. * **Option B:** Exocytosis is a complex process involving vesicular transport and **SNARE proteins**. While membrane fluidity is necessary for fusion, cholesterol itself is not the primary driver of this mechanism. * **Option C:** While cholesterol is indeed the precursor for bile acid synthesis, this process occurs exclusively in the **liver (hepatocytes)** and is not a function of cholesterol *within* the plasma membrane structure. **3. High-Yield Facts for NEET-PG:** * **Lipid Rafts:** Cholesterol, along with sphingolipids, forms "lipid rafts"—specialized microdomains that serve as platforms for cell signaling and receptor trafficking. * **Prokaryotic Exception:** Bacterial membranes (except *Mycoplasma*) lack cholesterol; they use sterol-like molecules called **hopanoids** instead. * **Ratio:** In most mammalian plasma membranes, the molar ratio of cholesterol to phospholipids is approximately **1:1**.

Question 142: Trace B best describes the kinetics of which of the following events?

A. Transport of Ca++ into the sarcoplasmic reticulum of a smooth muscle cell
B. Transport of K+ into a muscle cell
C. Transport of Na+ out of a nerve cell
D. Transport of O2 across an artificial lipid bilayer (Correct Answer)

Explanation: ***Transport of O2 across an artificial lipid bilayer*** - Shows **linear, non-saturable kinetics** because O2 transport occurs via **simple diffusion** without requiring carriers or pumps. - The rate of transport is directly proportional to the concentration gradient, producing a **straight-line relationship** characteristic of passive diffusion. *Transport of Ca++ into the sarcoplasmic reticulum of a smooth muscle cell* - Involves **active transport** via **Ca-ATPase pumps** which exhibit **saturable kinetics** with a hyperbolic curve. - Transport rate plateaus at high Ca++ concentrations due to **carrier saturation**, unlike the linear trace shown. *Transport of K+ into a muscle cell* - Occurs through **carrier-mediated transport** via **Na+/K+-ATPase** or potassium channels, showing **saturable kinetics**. - Exhibits **Michaelis-Menten kinetics** with a hyperbolic curve that reaches maximum velocity (Vmax) at high concentrations. *Transport of Na+ out of a nerve cell* - Mediated by **Na+/K+-ATPase pump** which demonstrates **saturable, carrier-mediated kinetics** with a hyperbolic curve. - Shows **concentration-dependent saturation** where transport rate levels off at high Na+ concentrations, not linear kinetics.

Question 143: Which transport mechanism requires a carrier protein but no energy?

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

Explanation: **Explanation:** The correct answer is **Facilitated Diffusion**. This process is a form of passive transport, meaning it occurs along a concentration gradient (from high to low concentration) and therefore requires **no metabolic energy (ATP)**. However, because the molecules involved are often large or polar (e.g., glucose), they cannot pass through the lipid bilayer alone and require specific **integral membrane carrier proteins** or channels to "facilitate" their passage. **Analysis of Options:** * **Simple Diffusion:** Movement occurs directly through the phospholipid bilayer or through non-gated channels. It requires neither energy nor a specific carrier protein. * **Osmosis:** This is the specific term for the simple diffusion of water molecules across a semi-permeable membrane. It does not require carrier proteins or energy. * **Active Transport:** This mechanism moves substances *against* their electrochemical gradient. It requires both a **carrier protein** (pump) and **energy** (Primary active transport uses ATP directly; Secondary active transport uses the gradient established by primary transport). **NEET-PG High-Yield Pearls:** 1. **Stereospecificity:** Carrier proteins in facilitated diffusion are highly specific (e.g., GLUT transporters for D-glucose, not L-glucose). 2. **Saturation Kinetics ($V_{max}$):** Unlike simple diffusion, facilitated diffusion is "saturable." Once all carrier proteins are occupied, the rate of transport reaches a plateau ($V_{max}$). 3. **Competitive Inhibition:** Similar molecules can compete for the same carrier binding site, reducing the transport rate of the primary substance. 4. **Classic Example:** The transport of glucose into skeletal muscle and adipose tissue via **GLUT-4** (which is insulin-dependent) is a hallmark example of facilitated diffusion.

Question 144: Which mechanism maintains the action potential across the cell membrane?

A. Na+/K+ pump (Correct Answer)
B. cAMP
C. Ca++
D. Phosphodiesterase

Explanation: **Explanation:** The **Na+/K+ ATPase pump** is the primary mechanism for maintaining the ionic gradients necessary for the resting membrane potential (RMP) and the long-term restoration of the action potential. It is an active transport mechanism that pumps **3 Na+ ions out** of the cell and **2 K+ ions into** the cell against their concentration gradients, using energy from ATP hydrolysis. This process is "electrogenic," as it creates a net loss of positive charges from the intracellular compartment, contributing to the negative internal environment. Without this pump, the concentration gradients would dissipate due to "leak" channels, rendering the cell unable to fire subsequent action potentials. **Why other options are incorrect:** * **cAMP:** This is a second messenger involved in intracellular signal transduction (e.g., GPCR pathways). It regulates cellular metabolism and protein kinase activation but does not directly maintain the membrane potential. * **Ca++:** While calcium is crucial for the plateau phase of the cardiac action potential and neurotransmitter release, it is not the primary ion responsible for maintaining the baseline resting potential or the Na+/K+ balance. * **Phosphodiesterase:** This is an enzyme that breaks down cyclic nucleotides (cAMP/cGMP). It is a target for drugs (like Sildenafil or Caffeine) but has no direct role in maintaining ionic gradients across the membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** 3 Na+ Out / 2 K+ In. * **Inhibitor:** **Ouabain** and **Digitalis (Digoxin)** inhibit the Na+/K+ pump, leading to increased intracellular Na+, which subsequently increases intracellular Ca++ via the Na+/Ca++ exchanger (positive inotropic effect). * The Na+/K+ pump accounts for approximately **60-70% of the total energy consumption** in neurons.

Question 145: Which of the following is NOT an example of ultrafiltration?

A. Filtration at systemic capillaries
B. Formation of cerebrospinal fluid
C. Filtration at the glomerulus
D. Secretion of bile by hepatocytes (Correct Answer)

Explanation: **Explanation:** **Ultrafiltration** is a passive process where fluid and small solutes are forced through a semi-permeable membrane under the influence of a **hydrostatic pressure gradient**, while larger molecules (like proteins) are retained. **Why Option D is Correct:** The **secretion of bile by hepatocytes** is an **active secretory process**, not a passive filtration process. It involves the active transport of bile acids, bilirubin, and electrolytes into the bile canaliculi against concentration gradients, followed by the osmotic movement of water. Since it requires cellular energy (ATP) and specific transport proteins rather than simple hydrostatic pressure, it is not classified as ultrafiltration. **Why Other Options are Incorrect:** * **Option A (Systemic Capillaries):** Fluid moves out of systemic capillaries into the interstitium based on Starling forces (Hydrostatic vs. Oncotic pressure). This is a classic example of ultrafiltration. * **Option B (Cerebrospinal Fluid):** CSF is formed at the **choroid plexus** primarily through the ultrafiltration of plasma across the capillary wall, followed by active transport of specific ions. * **Option C (Glomerulus):** This is the most prominent example of ultrafiltration in the body. High hydrostatic pressure in the glomerular capillaries forces water and small solutes into Bowman’s space, filtered by the glomerular filtration barrier. **High-Yield Clinical Pearls for NEET-PG:** * **Starling’s Law:** Governs ultrafiltration. Net Filtration = $K_f \times [(P_c - P_i) - \sigma(\pi_c - \pi_i)]$. * **Bile Secretion:** The rate-limiting step is the active transport of bile salts into the canaliculi via the **Bile Salt Export Pump (BSEP)**. * **Glomerular Filtration Barrier:** Comprises fenestrated endothelium, basement membrane (negative charge), and podocyte slit diaphragms (Nephrin). It restricts molecules based on both **size** (>70 kDa) and **charge** (polyanionic).

Question 146: Which of the following events does not occur in the M phase of mitosis?

A. Condensation of the chromosomes
B. Replication of the genome (Correct Answer)
C. Fragmentation of the nuclear envelope
D. Alignment of the chromatids along the equatorial plate

Explanation: **Explanation:** The cell cycle is divided into two primary phases: **Interphase** (the preparatory phase) and the **M phase** (Mitosis/Meiosis) [1], [2]. **Why "Replication of the genome" is the correct answer:** DNA replication (genome duplication) occurs exclusively during the **S phase (Synthesis phase)** of Interphase, not the M phase [1], [2]. By the time a cell enters mitosis, it already contains a double set of DNA (4n). The purpose of the M phase is to segregate this pre-replicated genetic material into two daughter cells, not to synthesize it. **Analysis of Incorrect Options:** * **A. Condensation of chromosomes:** This occurs during **Prophase** (the first stage of M phase) [1]. Chromatin fibers coil and tighten to become distinct chromosomes, making them visible under a light microscope. * **C. Fragmentation of the nuclear envelope:** This occurs during **Prometaphase**. The nuclear lamina breaks down to allow spindle fibers access to the chromosomes. * **D. Alignment of chromatids along the equatorial plate:** This is the hallmark of **Metaphase** [1]. Sister chromatids line up at the cell's center (metaphase plate) to ensure equal distribution. **NEET-PG High-Yield Pearls:** * **G0 Phase:** Cells that cease dividing (e.g., neurons, cardiac myocytes) enter a quiescent state called G0 [2]. * **Checkpoints:** The **G1/S checkpoint** (Restriction point) is the most critical rate-limiting step of the cell cycle [2]. * **Colchicine:** A high-yield pharmacological link; it inhibits mitosis by interfering with microtubule polymerization, arresting cells in **Metaphase**. * **Karyotyping:** Always performed during **Metaphase** because chromosomes are at their maximum state of condensation.

Question 147: Excitable tissue at rest is least permeable to which of the following ions?

A. Na+ (Correct Answer)
B. K+
C. Ca++
D. Cl-

Explanation: **Explanation:** The resting membrane potential (RMP) of an excitable cell is determined by the selective permeability of the cell membrane to various ions. At rest, the membrane is primarily permeable to ions for which "leak channels" are open. **Why Na+ is the Correct Answer:** In a resting state, the cell membrane is **least permeable to Sodium (Na+)**. The number of open Na+ leak channels is significantly lower than those for Potassium. Specifically, the membrane is **50 to 100 times more permeable to K+ than to Na+**. Because the membrane is relatively "tight" to Na+, this ion cannot easily enter the cell down its electrochemical gradient, which is crucial for maintaining the negative RMP (typically -70 to -90 mV). **Analysis of Incorrect Options:** * **B. K+:** This is the ion the membrane is **most permeable** to at rest. K+ efflux through leak channels is the primary determinant of the RMP, bringing it close to the equilibrium potential of Potassium (-94 mV). * **C. Ca++:** While the membrane has very low permeability to Calcium, in the context of standard physiological teaching for excitable tissues (nerve/muscle), Na+ is traditionally cited as the least permeable cation compared to K+ and Cl-. Furthermore, Ca++ levels are strictly regulated by active transport rather than simple leak permeability. * **D. Cl-:** Most excitable membranes are moderately permeable to Chloride. In some cells, Cl- permeability is quite high, and its equilibrium potential often sits near the RMP. **High-Yield Clinical Pearls for NEET-PG:** * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP by considering the permeability and concentration gradients of all major ions (Na+, K+, Cl-). * **Gibbs-Donnan Effect:** Refers to the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly due to the presence of non-diffusible proteins. * **Na+-K+ ATPase:** This is an electrogenic pump (3 Na+ out/2 K+ in) that maintains the concentration gradient but contributes only about -4 to -5 mV directly to the RMP.

Question 148: Which of the following intracellular structures does NOT have a cell membrane?

A. Mitochondria
B. Nucleus
C. Nucleolus (Correct Answer)
D. Endoplasmic reticulum

Explanation: **Explanation:** The correct answer is **C. Nucleolus**. **1. Why Nucleolus is the correct answer:** In cellular physiology, organelles are classified as membrane-bound or non-membrane-bound. The **nucleolus** is a dense, spherical structure located within the nucleus. It is essentially a large aggregate of macromolecules (RNA, proteins, and chromatin) and is **not enclosed by a lipid bilayer membrane**. It functions as the site for ribosomal RNA (rRNA) synthesis and ribosome subunit assembly. Because it lacks a membrane, its contents are in direct contact with the nucleoplasm. **2. Why the other options are incorrect:** * **Mitochondria (A):** These are double-membrane-bound organelles. They possess an outer membrane and a folded inner membrane (cristae), essential for the electron transport chain. * **Nucleus (B):** The nucleus is enclosed by the **nuclear envelope**, which consists of two concentric lipid bilayer membranes perforated by nuclear pores. * **Endoplasmic Reticulum (D):** The ER is an extensive network of membrane-enclosed sacs (cisternae) and tubules. It is a single-membrane-bound organelle continuous with the outer nuclear membrane. **3. NEET-PG High-Yield Facts:** * **Non-membrane bound structures:** Nucleolus, Ribosomes, Centrioles/Centrosomes, Cytoskeleton (microtubules, microfilaments), and Inclusion bodies. * **Double-membrane bound structures:** Nucleus, Mitochondria, and Chloroplasts (in plants). * **Single-membrane bound structures:** ER, Golgi apparatus, Lysosomes, and Peroxisomes. * **Clinical Pearl:** The size and number of nucleoli increase in cells with high protein synthesis requirements (e.g., cancer cells or plasma cells), a feature often used by pathologists to grade malignancy.

Question 149: The binding site for which of the following is present on the $\beta$ subunit of the Na+ - K+ pump?

A. Na+
B. K+
C. ATP
D. Glycosylation (Correct Answer)

Explanation: The **Na+-K+ ATPase pump** is an electrogenic transmembrane protein essential for maintaining resting membrane potential. It is a heteromultimer consisting primarily of an **$\alpha$ subunit** and a **$\beta$ subunit**. ### **Why Glycosylation is Correct** The **$\beta$ subunit** is a glycoprotein. Its primary functions are the assembly of the pump complex and its subsequent trafficking/targeting to the plasma membrane. The extracellular domain of the $\beta$ subunit contains multiple **glycosylation sites**. Without this glycosylation, the pump cannot be properly integrated into the cell membrane. ### **Analysis of Incorrect Options** The **$\alpha$ subunit** is the "catalytic" or functional subunit. It contains the binding sites for: * **Option A (Na+):** Three Na+ binding sites are located on the intracellular side of the $\alpha$ subunit. * **Option B (K+):** Two K+ binding sites are located on the extracellular side of the $\alpha$ subunit. * **Option C (ATP):** The ATP binding and phosphorylation site is located on the intracellular loop of the $\alpha$ subunit. * *Note:* The binding site for **Digitalis (Ouabain)** is also located on the extracellular surface of the $\alpha$ subunit. ### **NEET-PG High-Yield Pearls** * **Stoichiometry:** The pump moves **3 Na+ OUT** and **2 K+ IN** for every 1 ATP hydrolyzed. * **Electrogenicity:** It creates a net deficit of positive charges inside the cell, contributing directly to the negativity of the Resting Membrane Potential (RMP). * **Inhibitors:** It is inhibited by **Cardiac Glycosides** (Digoxin/Ouabain), which bind to the $\alpha$ subunit in the E2-P state. * **Subunit Roles:** * $\alpha$: Catalytic (Na, K, ATP, Digoxin binding). * $\beta$: Structural/Trafficking (Glycosylation). * $\gamma$: Regulatory (found in specific tissues like the kidney).

Question 150: What is the primary function of the Golgi apparatus?

A. Storing and packing of proteins (Correct Answer)
B. Breaking of fatty acids
C. Steroid synthesis
D. DNA replication

Explanation: **Explanation:** The **Golgi apparatus** acts as the "post office" or "shipping center" of the cell. Its primary function is the post-translational modification, sorting, and packaging of proteins received from the Rough Endoplasmic Reticulum (RER). Proteins are processed (e.g., glycosylation, sulfation) and then packaged into secretory vesicles for transport to their final destinations, such as the plasma membrane, lysosomes, or extracellular secretion. **Analysis of Options:** * **Option A (Correct):** The Golgi complex organizes proteins into membrane-bound vesicles. This "storing and packing" is essential for maintaining cellular structure and facilitating exocytosis. * **Option B (Incorrect):** The breakdown of long-chain fatty acids (beta-oxidation) occurs primarily in the **Mitochondria** and **Peroxisomes**. * **Option C (Incorrect):** Steroid and lipid synthesis is the hallmark function of the **Smooth Endoplasmic Reticulum (SER)**, which is abundant in cells of the adrenal cortex and gonads. * **Option D (Incorrect):** DNA replication occurs exclusively within the **Nucleus** (and to a small extent in the mitochondria). **High-Yield Clinical Pearls for NEET-PG:** * **I-Cell Disease (Inclusion Cell Disease):** A lysosomal storage disorder caused by a failure of the Golgi to phosphorylate mannose residues (Mannose-6-Phosphate). Proteins are secreted extracellularly rather than being delivered to lysosomes. * **Cis vs. Trans:** The *Cis-face* receives vesicles from the RER; the *Trans-face* (Trans-Golgi Network) is the exit site for sorted cargo. * **Silver Staining:** The Golgi apparatus is best visualized using silver stains (e.g., Camillo Golgi’s method).

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Cell Membrane Structure and Function

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Membrane Transport Proteins

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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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