Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 11: According to Fick's law, what factor increases the flux across a membrane?

A. Concentration gradient (Correct Answer)
B. Temperature
C. Increased size of the molecule
D. Molecular weight

Explanation: **Explanation:** Fick’s Law of Diffusion describes the rate at which a substance moves across a semi-permeable membrane. The law is mathematically expressed as: **$J = -D \cdot A \cdot \frac{\Delta C}{\Delta X}$** *(Where $J$ = Flux/Rate of diffusion, $D$ = Diffusion coefficient, $A$ = Surface area, $\Delta C$ = Concentration gradient, and $\Delta X$ = Membrane thickness).* **Why the Correct Answer is Right:** * **Concentration Gradient ($\Delta C$):** According to the formula, the rate of diffusion is **directly proportional** to the concentration gradient. A steeper difference in concentration between two compartments provides the driving force for molecules to move, thereby increasing the flux. **Why the Other Options are Wrong:** * **Temperature:** While increasing temperature generally increases molecular kinetic energy and diffusion in a physical system, it is not a primary variable defined in Fick’s Law regarding biological membrane flux. In a physiological context, body temperature is constant ($37^\circ C$); thus, it isn't a regulatory factor for flux. * **Increased Size/Molecular Weight:** According to **Graham’s Law**, the diffusion coefficient ($D$) is inversely proportional to the square root of the molecular weight. Therefore, larger or heavier molecules move more slowly, **decreasing** the flux. **High-Yield Clinical Pearls for NEET-PG:** * **Surface Area ($A$):** Flux is directly proportional to surface area. This explains why the small intestine has microvilli and the lungs have alveoli—to maximize nutrient and gas exchange. * **Membrane Thickness ($\Delta X$):** Flux is **inversely proportional** to thickness. In clinical conditions like **Pulmonary Fibrosis**, the increased thickness of the respiratory membrane reduces the flux of oxygen, leading to hypoxemia. * **Lipid Solubility:** For biological membranes, the permeability coefficient ($P$) is also determined by the oil-water partition coefficient; higher lipid solubility increases flux.

Question 12: Which of the following is co-transported with glucose?

A. Hydrogen ions
B. Potassium ions
C. Calcium ions
D. Sodium ions (Correct Answer)

Explanation: **Explanation:** The correct answer is **Sodium ions (D)**. This process is a classic example of **Secondary Active Transport**, specifically **Symport (Co-transport)**. Glucose is transported across the apical membrane of epithelial cells (primarily in the small intestine and proximal convoluted tubule of the kidney) via **SGLT (Sodium-Glucose Linked Transporters)**. This mechanism relies on the electrochemical gradient created by the Na⁺/K⁺ ATPase pump on the basolateral membrane. As sodium moves down its concentration gradient into the cell, it "drags" glucose molecules against their concentration gradient. **Analysis of Incorrect Options:** * **A. Hydrogen ions:** H⁺ is typically involved in **counter-transport (antiport)** with sodium (e.g., Na⁺-H⁺ exchanger in the kidneys) or co-transported with dipeptides/tripeptides (via PepT1). * **B. Potassium ions:** K⁺ is usually transported in exchange for sodium (Na⁺/K⁺ pump) or via specific leak channels. It is not co-transported with glucose. * **C. Calcium ions:** Ca²⁺ transport usually involves primary active transport (Ca²⁺ ATPase) or secondary active counter-transport (Na⁺-Ca²⁺ exchanger). **Clinical Pearls for NEET-PG:** * **SGLT-1:** Located in the **Small Intestine**; responsible for glucose absorption. This is the physiological basis for **Oral Rehydration Solution (ORS)**, where sodium and glucose are given together to enhance water absorption. * **SGLT-2:** Located in the **S1 segment of the PCT** in the kidney; responsible for 90% of renal glucose reabsorption. * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A major class of drugs used in Diabetes Mellitus to induce glucosuria. * **GLUT vs. SGLT:** Remember that SGLT is for *active* transport (apical), while **GLUT** (Glucose Transporters) are for *facilitated diffusion* (basolateral).

Question 13: Glucose transport across cell membranes occurs along with which ion?

A. K+
B. Na+ (Correct Answer)
C. Cl-
D. HCO3-

Explanation: **Explanation:** The correct answer is **Na+ (Sodium)**. This transport mechanism is a classic example of **Secondary Active Transport**, specifically **Symport (Cotransport)**. Glucose enters cells via two main mechanisms: 1. **SGLT (Sodium-Glucose Linked Transporters):** In the small intestine and renal proximal tubules, glucose is transported against its concentration gradient. This process is powered by the electrochemical gradient of Na+, which is maintained by the Na+-K+ ATPase pump. As Na+ moves down its gradient into the cell, it "drags" glucose along with it. 2. **GLUT (Glucose Transporters):** These facilitate passive diffusion and do not require ions, but the question specifically refers to the ion-dependent transport crucial for absorption and reabsorption. **Analysis of Incorrect Options:** * **A. K+ (Potassium):** While K+ is the primary intracellular cation, it is usually pumped *out* of the cell in exchange for Na+. It is not a co-transporter for glucose. * **C. Cl- (Chloride):** Chloride often follows Na+ to maintain electrical neutrality or is exchanged for bicarbonate (Chloride shift), but it does not drive glucose transport. * **D. HCO3- (Bicarbonate):** This is primarily involved in acid-base balance and CO2 transport via the Chloride-Bicarbonate exchanger (Band 3 protein). **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1:** Located in the **Small Intestine**; responsible for glucose absorption. This is the physiological basis for **ORS (Oral Rehydration Salt)**, where Na+ is added to enhance water and glucose uptake. * **SGLT-2:** Located in the **PCT of the Kidney**; responsible for 90% of glucose reabsorption. **SGLT-2 Inhibitors** (e.g., Dapagliflozin) are modern drugs used to treat Diabetes Mellitus. * **GLUT-4:** The only **insulin-dependent** glucose transporter, found in skeletal muscle and adipose tissue.

Question 14: Nitric oxide is produced from which of the following?

A. Endothelium (Correct Answer)
B. RBC
C. Platelets
D. Lymphocytes

Explanation: ### Explanation **Correct Option: A. Endothelium** Nitric Oxide (NO), formerly known as **Endothelium-Derived Relaxing Factor (EDRF)**, is synthesized primarily within vascular endothelial cells. The process involves the conversion of the amino acid **L-arginine** into L-citrulline and NO, catalyzed by the enzyme **Endothelial Nitric Oxide Synthase (eNOS)** in the presence of oxygen and NADPH. Once produced, NO diffuses into adjacent vascular smooth muscle cells, where it activates **soluble guanylyl cyclase**, increasing cGMP levels and leading to vasodilation. **Analysis of Incorrect Options:** * **B. RBC:** While Red Blood Cells do not produce NO, they play a critical role in its metabolism. Hemoglobin has a high affinity for NO, acting as a "sink" that inactivates it to form nitrates, thereby limiting the duration of its effect. * **C. Platelets:** Although platelets contain some NOS and NO can inhibit platelet aggregation, they are not the primary physiological source of NO production in the vascular system. * **D. Lymphocytes:** While certain immune cells (like macrophages) can produce NO via **inducible NOS (iNOS)** during inflammatory responses, the classic physiological production of NO for vascular tone regulation is attributed to the endothelium. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor:** L-arginine is the essential substrate for NO synthesis. * **Isoforms of NOS:** 1. **nNOS (NOS-1):** Neuronal (Neurotransmitter). 2. **iNOS (NOS-2):** Inducible (Immune response/Sepsis). 3. **eNOS (NOS-3):** Endothelial (Vasodilation). * **Mechanism:** NO → ↑ cGMP → Protein Kinase G → Dephosphorylation of Myosin Light Chain → **Vasodilation**. * **Potent Stimuli:** Shear stress (blood flow) and Acetylcholine (via M3 receptors on endothelium).

Question 15: Facilitated diffusion is characterized by all of the following except:

A. Vmax
B. Competitive inhibition
C. Requires energy (Correct Answer)
D. Specificity

Explanation: **Explanation:** The core concept tested here is the distinction between **Passive Transport** and **Active Transport**. **Why "Requires energy" is the correct answer:** Facilitated diffusion is a type of **passive transport**. It occurs along a concentration gradient (from high to low concentration) and, therefore, does **not** require metabolic energy (ATP). In contrast, active transport moves solutes against a gradient and requires energy. **Analysis of other options (Characteristics of Facilitated Diffusion):** Facilitated diffusion relies on specific **carrier proteins** (transporters). Because these proteins are biological molecules, they exhibit enzymatic properties: * **Vmax (Saturation):** Unlike simple diffusion, facilitated diffusion is "saturable." Once all available carrier proteins are occupied, the rate of transport reaches a maximum (Vmax). * **Specificity:** Carrier proteins are highly selective for specific molecules (e.g., GLUT4 is specific for glucose). * **Competitive Inhibition:** Since there are specific binding sites on the carriers, chemically similar molecules can compete for the same site, inhibiting the transport of the primary substance. **High-Yield NEET-PG Pearls:** 1. **GLUT Transporters:** The most classic example of facilitated diffusion is glucose transport via GLUT receptors (e.g., GLUT4 in muscle and adipose tissue). 2. **Simple vs. Facilitated:** Simple diffusion is the only transport mechanism that is **not** carrier-mediated and does not exhibit Vmax. 3. **Insulin Action:** Insulin increases glucose uptake in peripheral tissues by increasing the number of GLUT4 transporters in the membrane, thereby increasing the Vmax of facilitated diffusion.

Question 16: Nitric oxide (NO) is primarily released by which of the following cell types?

A. Neutrophils
B. Macrophages
C. Endothelial cells (Correct Answer)
D. Lymphocytes

Explanation: **Explanation:** **Correct Answer: C. Endothelial cells** Nitric Oxide (NO), formerly known as **Endothelium-Derived Relaxing Factor (EDRF)**, is primarily synthesized in vascular endothelial cells. It is produced from the amino acid **L-arginine** by the action of the enzyme **endothelial Nitric Oxide Synthase (eNOS)**. Once released, NO diffuses into adjacent vascular smooth muscle cells, where it activates **soluble guanylyl cyclase**, increasing intracellular **cGMP**. This leads to smooth muscle relaxation and subsequent vasodilation, playing a critical role in regulating blood pressure and regional blood flow. **Analysis of Incorrect Options:** * **A & B (Neutrophils and Macrophages):** While these cells can produce NO via **inducible Nitric Oxide Synthase (iNOS)** during inflammatory responses to kill pathogens, they are not the *primary* physiological source of NO in the body. eNOS in the endothelium provides the basal, continuous release required for vascular homeostasis. * **D (Lymphocytes):** These cells are primarily involved in adaptive immunity (antibody production and cell-mediated immunity) and are not a significant source of Nitric Oxide. **High-Yield Clinical Pearls for NEET-PG:** * **Isoforms of NOS:** There are three types: **nNOS** (Neuronal/Type I), **iNOS** (Inducible/Type II - found in macrophages), and **eNOS** (Endothelial/Type III). * **Mechanism:** NO → ↑ cGMP → Protein Kinase G → Dephosphorylation of Myosin Light Chain → **Vasodilation**. * **Potent Stimuli:** Shear stress (blood flow) and Acetylcholine trigger NO release from the endothelium. * **Therapeutic Link:** Nitroglycerin acts by being converted into NO, providing rapid relief in Angina Pectoris.

Question 17: Which of the following processes is NOT an function of the endoplasmic reticulum?

A. Protein synthesis
B. Steroid synthesis
C. DNA synthesis (Correct Answer)
D. Triglyceride synthesis

Explanation: **Explanation:** The **Endoplasmic Reticulum (ER)** is a multifunctional organelle divided into two types: Rough ER (RER) and Smooth ER (SER). **Why DNA Synthesis is the Correct Answer:** DNA synthesis (Replication) occurs exclusively in the **Nucleus** (for genomic DNA) and the **Mitochondria** (for mitochondrial DNA). The ER lacks the necessary enzymes, such as DNA polymerase, and the structural environment required for the replication of the genetic code. Therefore, it is not a function of the ER. **Analysis of Other Options:** * **Protein Synthesis (Option A):** This is a primary function of the **Rough ER**. The presence of ribosomes on its surface allows for the translation and translocation of proteins destined for secretion, membrane integration, or lysosomal enzymes. * **Steroid Synthesis (Option B):** This is a hallmark function of the **Smooth ER**. Cells specialized in steroid hormone production (e.g., adrenal cortex, Leydig cells) have an abundance of SER. * **Triglyceride Synthesis (Option D):** The **Smooth ER** is the major site for lipid biosynthesis, including phospholipids, cholesterol, and triglycerides. **High-Yield Clinical Pearls for NEET-PG:** * **Sarcoplasmic Reticulum:** A specialized form of SER in muscle cells that acts as the primary storage site for **Calcium ions ($Ca^{2+}$)**, crucial for muscle contraction. * **Detoxification:** The SER in hepatocytes contains the **Cytochrome P450** enzyme system, responsible for the detoxification of drugs and toxins. * **Nissl Bodies:** These are large granules of RER found in neurons; they are responsible for high-rate protein synthesis. * **ER Stress:** Accumulation of misfolded proteins in the ER triggers the "Unfolded Protein Response" (UPR), which is linked to neurodegenerative diseases.

Question 18: Transport of two substances in the same direction across a membrane is called as?

A. Symport (Correct Answer)
B. Antiport
C. Exocytosis
D. Pinocytosis

Explanation: **Explanation:** The transport of substances across cell membranes via carrier proteins is categorized based on the direction and number of molecules involved. **1. Why Symport is correct:** **Symport** (also known as **Cotransport**) is a type of secondary active transport where a carrier protein moves two different solutes in the **same direction** across the membrane. Typically, one substance (usually Sodium) moves down its electrochemical gradient, providing the energy to pull another substance (like glucose or amino acids) against its concentration gradient. **2. Why other options are incorrect:** * **Antiport (Counter-transport):** This involves the transport of two substances in **opposite directions**. A classic example is the Sodium-Calcium exchanger (NCX). * **Exocytosis:** This is a form of bulk transport where materials are exported out of the cell via secretory vesicles fusing with the plasma membrane. It does not involve specific carrier-mediated directional transport of individual solutes. * **Pinocytosis:** Known as "cell drinking," this is a form of endocytosis where the cell membrane invaginates to ingest extracellular fluid and small molecules. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 & SGLT-2:** These are the most clinically relevant examples of **Symport**. They transport Sodium and Glucose into cells (SGLT-1 in the small intestine; SGLT-2 in the proximal convoluted tubule of the kidney). * **SGLT-2 Inhibitors (e.g., Dapagliflozin):** A major class of drugs for Type 2 Diabetes that works by inhibiting this symporter, leading to glucosuria. * **Na⁺-K⁺-2Cl⁻ Symporter (NKCC2):** Located in the Thick Ascending Limb of Henle; it is the target of **Loop Diuretics** (Furosemide). * **Uniport:** Transport of a single substance across the membrane (e.g., GLUT-1 to GLUT-5).

Question 19: Nitric oxide acts by which second messenger?

A. cGMP (Correct Answer)
B. cAMP
C. Calcium
D. Kinase

Explanation: **Explanation:** Nitric Oxide (NO), also known as Endothelium-Derived Relaxing Factor (EDRF), is a soluble gas that acts as a potent vasodilator. **Why cGMP is correct:** NO diffuses across the cell membrane and binds to the heme group of **soluble Guanylyl Cyclase (sGC)**. This activation converts GTP into **cyclic Guanylate Monophosphate (cGMP)**. Increased levels of cGMP activate Protein Kinase G (PKG), which leads to the dephosphorylation of myosin light chains and the sequestration of intracellular calcium, resulting in smooth muscle relaxation. **Why other options are incorrect:** * **cAMP:** This is the second messenger for hormones like Glucagon, PTH, and Beta-adrenergic agonists via the Adenylyl Cyclase pathway. * **Calcium:** While calcium is a vital signaling molecule (often linked to the IP3/DAG pathway), NO actually works by *decreasing* cytosolic calcium to cause relaxation. * **Kinase:** While kinases (like PKG) are part of the downstream signaling cascade, they are enzymes, not the primary "second messenger" molecule itself. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Nitroglycerin:** It acts as a prodrug that is metabolized into NO, utilizing the cGMP pathway to relieve angina. * **Sildenafil (Viagra):** It inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme responsible for breaking down cGMP. By keeping cGMP levels high, it prolongs vasodilation. * **NO Synthase (NOS):** NO is synthesized from the amino acid **L-Arginine** by the enzyme NOS (isoforms: eNOS, nNOS, and iNOS). * **Inhaled NO:** Used clinically in neonates for persistent pulmonary hypertension.

Question 20: A permeable membrane separates two solutions. Side A has a urea concentration of 20, and Side B has a urea concentration of 40. If the concentration on Side A is increased to 40, what will happen to the rate of diffusion?

A. It will become double
B. It will become four times
C. It will be zero (Correct Answer)
D. It will be reduced four times

Explanation: ### Explanation **1. Why the correct answer is right:** The rate of net diffusion across a permeable membrane is governed by **Fick’s Law**, which states that the diffusion rate is directly proportional to the **concentration gradient ($\Delta C$)** between two compartments. * **Initial State:** Side A = 20, Side B = 40. The gradient ($\Delta C$) is $40 - 20 = 20$. * **Final State:** Side A is increased to 40. Now, Side A = 40 and Side B = 40. The gradient ($\Delta C$) becomes $40 - 40 = 0$. Since the concentration gradient is the driving force for net movement, when the concentrations on both sides are equal, the system reaches **chemical equilibrium**. While individual molecules continue to move randomly (Brownian motion), the **net diffusion rate becomes zero**. **2. Why the incorrect options are wrong:** * **Option A & B:** These would require the concentration gradient to increase (e.g., Side A becoming 0 or Side B becoming 80). Here, the gradient was abolished, not increased. * **Option D:** A reduction in rate occurs if the gradient narrows but remains positive. In this case, the gradient is completely eliminated, leading to a total halt in net flux, not just a fractional reduction. **3. NEET-PG High-Yield Pearls:** * **Fick’s Law Factors:** Net Diffusion is increased by a higher concentration gradient, larger surface area, and higher lipid solubility. It is decreased by increased membrane thickness and larger molecular weight. * **Urea Dynamics:** Urea is a small, polar molecule that moves via **facilitated diffusion** (using UT-A or UT-B transporters) in specific tissues like the renal collecting ducts, but it can also move slowly via simple diffusion. * **Osmotic vs. Effective Osmoles:** Urea is often considered an "ineffective osmole" because it crosses many cell membranes easily, meaning it does not create a long-term osmotic pressure gradient (unlike Sodium or Glucose).

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