Cellular Physiology Practice Questions

Q: What is the function of cholesterol in the plasma membrane?

Maintains fluidity. **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**.

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

Transport of O2 across an artificial lipid bilayer. ***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.

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

Facilitated diffusion. **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.

Q: Which mechanism maintains the action potential across the cell membrane?

Na+/K+ pump. **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.

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

Replication of the genome. **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.

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

Nucleolus. **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.

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

Glycosylation. 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).

Q: What is the primary function of the Golgi apparatus?

Storing and packing of proteins. **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).

Question 1

What is the function of cholesterol in the plasma membrane?

Accepted Answer

Maintains fluidity

Answer

Helps in the transport of ions

Answer

Helps in exocytosis

Answer

Helps in the synthesis of bile acids

Question 2

Trace B best describes the kinetics of which of the following events?

Accepted Answer

Transport of O2 across an artificial lipid bilayer

Answer

Transport of Ca++ into the sarcoplasmic reticulum of a smooth muscle cell

Answer

Transport of K+ into a muscle cell

Answer

Transport of Na+ out of a nerve cell

Question 3

Which transport mechanism requires a carrier protein but no energy?

Accepted Answer

Facilitated diffusion

Answer

Simple diffusion

Answer

Osmosis

Answer

Active transport

Question 4

Which mechanism maintains the action potential across the cell membrane?

Accepted Answer

Na+/K+ pump

Answer

cAMP

Answer

Ca++

Answer

Phosphodiesterase

Question 5

Which of the following is NOT an example of ultrafiltration?

Accepted Answer

Secretion of bile by hepatocytes

Answer

Filtration at systemic capillaries

Answer

Formation of cerebrospinal fluid

Answer

Filtration at the glomerulus

Question 6

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

Accepted Answer

Replication of the genome

Answer

Condensation of the chromosomes

Answer

Fragmentation of the nuclear envelope

Answer

Alignment of the chromatids along the equatorial plate

Question 7

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

Accepted Answer

Na+

Answer

K+

Answer

Ca++

Answer

Cl-

Question 8

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

Accepted Answer

Nucleolus

Answer

Mitochondria

Answer

Nucleus

Answer

Endoplasmic reticulum

Question 9

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

Accepted Answer

Glycosylation

Answer

Na+

Answer

K+

Answer

ATP

Question 10

What is the primary function of the Golgi apparatus?

Accepted Answer

Storing and packing of proteins

Answer

Breaking of fatty acids

Answer

Steroid synthesis

Answer

DNA replication

Cellular Physiology — MCQs

Cellular Physiology — MCQs

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