Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 231: Transport of neutral substances across the cell membrane occurs by which mechanism?

A. Porins
B. Ionophores
C. Lipopolysaccharides
D. Diffusion (Correct Answer)

Explanation: **Explanation:** **Mechanism of Transport for Neutral Substances:** The cell membrane is a semi-permeable lipid bilayer. **Diffusion** (specifically simple diffusion) is the primary mechanism for the transport of **neutral, non-polar, and lipid-soluble substances** (e.g., $O_2$, $CO_2$, $N_2$, and steroid hormones). These substances dissolve in the hydrophobic lipid bilayer and move down their concentration gradient without the need for carrier proteins or energy. While water is polar, it is small enough to also move via simple diffusion (osmosis) through the lipid matrix. **Analysis of Incorrect Options:** * **A. Porins:** These are large transmembrane proteins found in the outer membranes of Gram-negative bacteria and mitochondria. They form aqueous channels that allow the passage of hydrophilic molecules, not neutral lipid-soluble substances. * **B. Ionophores:** These are lipid-soluble molecules (often antibiotics like Valinomycin) that facilitate the transport of specific **ions** (charged particles) across the membrane, not neutral substances. * **C. Lipopolysaccharides (LPS):** These are structural components of the outer membrane of Gram-negative bacteria (endotoxins). They are not transport mechanisms. **High-Yield Clinical Pearls for NEET-PG:** * **Fick’s Law of Diffusion:** The rate of net diffusion is proportional to the concentration gradient, surface area, and membrane permeability, but inversely proportional to the membrane thickness. * **Facilitated Diffusion:** Unlike simple diffusion, this requires a carrier protein (e.g., **GLUT-4** for glucose) and exhibits **stereospecificity** and **saturation kinetics** ($V_{max}$). * **Gases:** $O_2$ and $CO_2$ transport across the alveolar-capillary membrane is the classic physiological example of simple diffusion.

Question 232: ATP synthetase is a marker of which organelle?

A. Golgi apparatus
B. Mitochondria (Correct Answer)
C. Cytosol
D. Endoplasmic reticulum

Explanation: **Explanation:** **ATP synthetase** (also known as Complex V or $F_oF_1$-ATPase) is the enzyme responsible for synthesizing Adenosine Triphosphate (ATP) from ADP and inorganic phosphate. This process occurs via **oxidative phosphorylation** on the **inner mitochondrial membrane**. The enzyme utilizes the proton gradient generated by the electron transport chain to drive the rotation of its subunits, making it the definitive functional marker for **Mitochondria**. **Analysis of Incorrect Options:** * **Golgi Apparatus:** Its primary markers are enzymes involved in protein modification, such as **Galactosyltransferase**. It is responsible for packaging and sorting proteins. * **Cytosol:** The primary markers for the cytosol are enzymes of the glycolytic pathway, such as **Lactate Dehydrogenase (LDH)**. * **Endoplasmic Reticulum (ER):** The marker for the Smooth ER is **Glucose-6-Phosphatase**, while the Rough ER is characterized by the presence of ribosomes and **Ribophorins**. **High-Yield NEET-PG Pearls:** * **Mitochondrial Markers:** * Outer Membrane: Monoamine Oxidase (MAO). * Inner Membrane: ATP synthetase, Succinate dehydrogenase, Cytochrome oxidase. * Matrix: Enzymes of the TCA cycle (except Succinate dehydrogenase) and Pyruvate dehydrogenase. * **Clinical Correlation:** Inhibitors of ATP synthetase include **Oligomycin**, which binds to the $F_o$ subunit and prevents the flow of protons, effectively halting ATP production. * **Mitochondrial DNA:** Mitochondria are semi-autonomous organelles containing their own circular DNA, inherited exclusively from the mother.

Question 233: Which ion is most diffusable in excitable tissue?

A. Na+
B. K+ (Correct Answer)
C. PO4-
D. Cl-

Explanation: ### Explanation The correct answer is **B. K+**. **Why K+ is the correct answer:** The permeability of a cell membrane to a specific ion depends on the number of open leak channels. In excitable tissues (like neurons and muscle cells) at rest, the membrane is significantly more permeable to **Potassium (K+)** than to any other ion. This is because the cell membrane contains a high density of **non-gated K+ leak channels** that remain open at resting membrane potential. According to the Goldman-Hodgkin-Katz equation, the resting membrane potential (approx. -70 to -90 mV) is closest to the equilibrium potential of K+ (-94 mV) precisely because K+ is the most diffusible ion. **Analysis of Incorrect Options:** * **A. Na+:** At rest, the membrane is nearly impermeable to Sodium. Na+ channels are primarily voltage-gated and remain closed until a stimulus reaches the threshold. The resting permeability of Na+ is roughly 50–100 times less than that of K+. * **C. PO4-:** Phosphate ions are large, negatively charged intracellular proteins and organic anions. They are essentially **non-diffusible** and remain trapped inside the cell, contributing to the negative internal charge. * **D. Cl-:** While Chloride does have some permeability, it is much lower than K+ in most excitable tissues. Its movement is often passive and follows the electrochemical gradient established by K+ and Na+. **High-Yield NEET-PG Pearls:** * **Relative Permeability Ratio:** At rest, the permeability ratio of **K+ : Cl- : Na+** is approximately **1 : 0.4 : 0.01**. * **Resting Membrane Potential (RMP):** Is primarily determined by K+ efflux. * **Gibbs-Donnan Effect:** Explains the behavior of non-diffusible ions (like PO4- and proteins) which influence the distribution of diffusible ions across the membrane. * **Na+-K+ ATPase:** This pump maintains the concentration gradient but only contributes about -4 to -5 mV directly to the RMP; the rest is due to K+ diffusion.

Question 234: Synthesis of protein occurs on:

A. Mitochondria
B. Poly ribosomes (Correct Answer)
C. Nucleus
D. Golgi bodies

Explanation: **Explanation:** **Why Polyribosomes is the correct answer:** Protein synthesis (translation) occurs on **ribosomes**. When multiple ribosomes attach to a single strand of messenger RNA (mRNA) to translate it simultaneously, the complex is called a **polyribosome** (or polysome). This arrangement allows the cell to produce multiple copies of the same polypeptide efficiently from a single mRNA molecule. Ribosomes can either be free in the cytosol (synthesizing proteins for internal use) or attached to the Rough Endoplasmic Reticulum (synthesizing proteins for secretion or membranes). **Why other options are incorrect:** * **Mitochondria:** While mitochondria contain their own DNA and some ribosomes (mitoribosomes) to synthesize a few organelle-specific proteins, they are primarily the site of ATP production (oxidative phosphorylation), not the primary site for general cellular protein synthesis. * **Nucleus:** This is the site of **transcription** (DNA to mRNA) and DNA replication. Protein synthesis does not occur here because ribosomes are located in the cytoplasm. * **Golgi bodies:** These are responsible for the **post-translational modification**, sorting, and packaging of proteins received from the Endoplasmic Reticulum. They do not synthesize proteins themselves. **High-Yield Facts for NEET-PG:** * **Ribosome Subunits:** Eukaryotes have 80S ribosomes (40S + 60S); Prokaryotes have 70S (30S + 50S). * **Clinical Correlation:** Many antibiotics target protein synthesis by binding to specific ribosomal subunits (e.g., **A**minoglycosides/Tetracyclines act on **30S**; **C**hloramphenicol/Erythromycin act on **50S**—Mnemonic: **"Buy AT 30, CELL at 50"**). * **Signal Sequence:** Proteins destined for secretion have a "signal peptide" that directs the ribosome to the Rough ER.

Question 235: The plasma membrane of a cell is bounded to the cytoskeleton by which of the following molecules?

A. Spectrin
B. Ankyrin (Correct Answer)
C. Tubulin
D. Laminin

Explanation: **Explanation:** The plasma membrane is anchored to the underlying cytoskeleton through a specialized protein network that provides structural integrity and maintains cell shape. **Why Ankyrin is correct:** **Ankyrin** acts as a primary "adapter" or "bridge" protein. It possesses specific binding sites that allow it to attach integral membrane proteins (such as the **Band 3 anion exchanger** or Na⁺-K⁺ ATPase) to the underlying **spectrin** cytoskeleton. By tethering the lipid bilayer to the meshwork of the cytoskeleton, ankyrin ensures the membrane remains stable under mechanical stress. **Analysis of Incorrect Options:** * **Spectrin (Option A):** While spectrin is a major component of the membrane skeleton, it forms the horizontal meshwork *underneath* the membrane. It does not bind directly to the lipid bilayer; it requires ankyrin to facilitate that connection. * **Tubulin (Option B):** Tubulin is the protein subunit of **microtubules**. Microtubules are involved in intracellular transport and cell division (mitotic spindle) but are not the primary molecules anchoring the plasma membrane to the cortical cytoskeleton. * **Laminin (Option D):** Laminin is a major glycoprotein of the **extracellular matrix (basal lamina)**. It is involved in cell adhesion to external structures, not the internal anchoring of the membrane to the cytoskeleton. **Clinical Pearls for NEET-PG:** * **Hereditary Spherocytosis:** A deficiency or mutation in **Ankyrin** (most common cause, ~50-60% of cases) or Spectrin leads to a loss of membrane surface area. This results in spherical, fragile RBCs that are prematurely destroyed in the spleen. * **Band 3 Protein:** This is the specific integral membrane protein that Ankyrin binds to in the erythrocyte membrane. * **Mnemonic:** **A**nkyrin **A**nchors the membrane.

Question 236: Rough endoplasmic reticulum is the site of synthesis for which of the following?

A. Proteins (Correct Answer)
B. Cholesterol
C. Carbohydrates
D. Fats

Explanation: **Explanation:** The **Rough Endoplasmic Reticulum (RER)** is characterized by the presence of **ribosomes** attached to its outer surface, giving it a "rough" appearance under electron microscopy. These ribosomes are the primary machinery for protein synthesis. Specifically, the RER is responsible for synthesizing proteins destined for **secretion** (e.g., hormones, digestive enzymes), incorporation into the **cell membrane**, or storage within **lysosomes**. Once synthesized, these proteins undergo post-translational modifications (like N-linked glycosylation) within the RER lumen. **Analysis of Incorrect Options:** * **B & D (Cholesterol and Fats):** These are lipids. The synthesis of lipids, including cholesterol, phospholipids, and steroid hormones (e.g., testosterone, estrogen), occurs in the **Smooth Endoplasmic Reticulum (SER)**, which lacks ribosomes. * **C (Carbohydrates):** While some carbohydrate attachment (glycosylation) occurs in the ER, the primary site for complex carbohydrate synthesis and final "packaging" of glycoproteins is the **Golgi Apparatus**. **High-Yield Clinical Pearls for NEET-PG:** * **Nissl Bodies:** In neurons, the RER is found in large aggregates called Nissl bodies, which are essential for synthesizing neurotransmitter proteins. * **Sarcoplasmic Reticulum:** A specialized form of SER in muscle cells that acts as the primary storage site for **Calcium ions ($Ca^{2+}$)**. * **Detoxification:** The SER in hepatocytes contains the **Cytochrome P450** enzyme system, responsible for the detoxification of drugs and toxins. * **Protein Folding:** The RER contains "chaperone proteins" (like BiP) that ensure proteins are folded correctly; misfolded proteins lead to "ER stress."

Question 237: Which ion has the maximum equilibrium potential?

A. Sodium (Na+)
B. Potassium (K+) (Correct Answer)
C. Chloride (Cl-)
D. None of the above

Explanation: ### Explanation The equilibrium potential ($E_{ion}$) of an ion is the membrane potential at which the electrical gradient exactly balances the chemical concentration gradient, resulting in no net movement of that ion across the membrane. This is calculated using the **Nernst Equation**. #### Why Potassium (K+) is Correct Potassium has the highest concentration gradient across the cell membrane (approx. 140 mEq/L intracellular vs. 4 mEq/L extracellular). According to the Nernst equation, a larger concentration ratio results in a higher absolute value for the equilibrium potential. For a typical mammalian neuron, **$E_{K^+}$ is approximately -94 mV**. In the context of NEET-PG questions, when comparing absolute values (magnitude), Potassium's potential (~90-94 mV) is greater than that of Sodium or Chloride. #### Why Other Options are Incorrect * **Sodium (Na+):** The concentration gradient for Sodium (142 mEq/L extracellular vs. 14 mEq/L intracellular) is smaller than that of Potassium. Its equilibrium potential (**$E_{Na^+}$**) is approximately **+61 mV**. * **Chloride (Cl-):** Chloride ions are primarily extracellular. Its equilibrium potential (**$E_{Cl^-}$**) is approximately **-70 to -80 mV**, which is close to the Resting Membrane Potential (RMP) but lower in magnitude than Potassium. #### High-Yield Clinical Pearls for NEET-PG * **Resting Membrane Potential (RMP):** In most excitable cells, the RMP is approximately **-70 to -90 mV**. * **The "Goldman" Rule:** The RMP is closest to the equilibrium potential of the ion with the **highest permeability**. Since the resting membrane is 50–100 times more permeable to $K^+$ than $Na^+$, the RMP stays very close to $E_{K^+}$. * **Na+/K+ ATPase:** This pump is electrogenic; it contributes about **-4 to -5 mV** directly to the RMP by pumping 3 $Na^+$ out and 2 $K^+$ in. * **Hypokalemia:** Increases the concentration gradient, making $E_{K^+}$ more negative (hyperpolarization), which makes cells less excitable.

Question 238: Which of the following statements is true regarding secondary active transport?

A. It occurs only in the same direction as the coupled solute.
B. It occurs only in the opposite direction of the coupled solute.
C. It directly uses an ATP molecule for transport. (Correct Answer)
D. It does not require a carrier protein.

Explanation: ### Explanation **Correct Answer: C. It directly uses an ATP molecule for transport.** **Underlying Medical Concept:** Secondary active transport (also known as coupled transport) is a process where the transport of a solute against its electrochemical gradient is powered by the energy stored in the electrochemical gradient of another solute (usually $Na^+$). While it does not hydrolyze ATP directly, it **indirectly** relies on ATP because the primary active transporter (like the $Na^+$-$K^+$ ATPase pump) must use ATP to establish the initial concentration gradient that drives the secondary process. **Analysis of Options:** * **Option A & B (Incorrect):** Secondary active transport can occur in two forms: **Symport (Cotransport)**, where both solutes move in the same direction (e.g., SGLT-1 in the intestine), or **Antiport (Counter-transport)**, where solutes move in opposite directions (e.g., $Na^+$-$Ca^{2+}$ exchanger). It is not restricted to one direction. * **Option D (Incorrect):** All forms of active transport (primary and secondary) are **carrier-mediated**. They require specific transmembrane proteins to facilitate the movement of solutes across the lipid bilayer. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 & SGLT-2:** Classic examples of secondary active transport (Symport) used in glucose reabsorption in the kidneys and absorption in the gut. * **Digitalis Mechanism:** Digoxin inhibits the $Na^+$-$K^+$ ATPase (Primary). This leads to an increase in intracellular $Na^+$, which subsequently inhibits the $Na^+$-$Ca^{2+}$ exchanger (Secondary), increasing intracellular $Ca^{2+}$ and cardiac contractility. * **Oral Rehydration Therapy (ORT):** Works on the principle of $Na^+$-Glucose cotransport (SGLT-1); $Na^+$ absorption enhances water reabsorption.

Question 239: CAMP acts through which mechanism?

A. Activation of protein kinase (Correct Answer)
B. Activation of adenylate cyclase
C. Ca2+ release
D. All of the above

Explanation: **Explanation:** The correct answer is **A. Activation of protein kinase.** **Mechanism of Action:** Cyclic Adenosine Monophosphate (cAMP) is a classic **second messenger**. When a hormone (first messenger) binds to a G-protein coupled receptor (GPCR), it activates the enzyme **Adenylate Cyclase**, which converts ATP into cAMP. The primary downstream effect of cAMP is the activation of **Protein Kinase A (PKA)**. PKA then phosphorylates specific target proteins (enzymes or transcription factors), leading to the physiological response within the cell. **Why other options are incorrect:** * **B. Activation of adenylate cyclase:** This is the enzyme responsible for *producing* cAMP, not the mechanism through which cAMP exerts its effects. Adenylate cyclase is upstream of cAMP. * **C. Ca2+ release:** This is typically the mechanism for the **IP3/DAG** second messenger pathway. In that pathway, Phospholipase C generates IP3, which triggers the release of Calcium from the sarcoplasmic/endoplasmic reticulum. * **D. All of the above:** Incorrect because cAMP specifically targets protein kinases to initiate its signaling cascade. **High-Yield Clinical Pearls for NEET-PG:** * **Termination:** cAMP signaling is terminated by the enzyme **Phosphodiesterase (PDE)**, which breaks cAMP down into 5'-AMP. Drugs like Theophylline and Sildenafil work by inhibiting different isoforms of PDE. * **Vibrio cholerae:** Cholera toxin causes permanent activation of Gs proteins, leading to constitutive activation of Adenylate Cyclase and high cAMP levels, resulting in secretory diarrhea. * **Hormones using cAMP:** FLAT ChAMP (FSH, LH, ACTH, TSH, CRH, hCG, ADH (V2 receptor), MSH, PTH) and Glucagon.

Question 240: Glucose symport occurs with which ion?

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

Explanation: **Explanation:** Glucose transport across cell membranes occurs via two primary mechanisms: facilitated diffusion (GLUT transporters) and **secondary active transport** (SGLT transporters). **Why Na+ is correct:** Glucose symport (cotransport) specifically refers to the **SGLT (Sodium-Glucose Linked Transporter)**. This process utilizes the electrochemical gradient of **Sodium (Na+)** created by the Na+-K+ ATPase pump. As Na+ moves down its concentration gradient into the cell, it "drags" glucose against its concentration gradient. This occurs primarily in the proximal convoluted tubule (PCT) of the kidney and the enterocytes of the small intestine. **Why other options are incorrect:** * **Ca++:** Calcium is typically involved in secondary active transport via **antiport** mechanisms (e.g., Na+-Ca++ exchanger) to maintain low intracellular calcium levels, not glucose transport. * **K+:** Potassium is the primary intracellular cation. While it is moved by the Na+-K+ pump, it is not the driving ion for glucose symport. * **Cl-:** Chloride often follows Na+ passively to maintain electrical neutrality or is exchanged via antiports (e.g., Chloride-Bicarbonate shift), but it does not drive glucose symport. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1:** Located in the **Small Intestine** (and late PCT); responsible for glucose absorption from the diet. * **SGLT-2:** Located in the **early PCT (S1 segment)** of the kidney; responsible for 90% of renal glucose reabsorption. * **Clinical Correlation:** **SGLT-2 Inhibitors** (e.g., Dapagliflozin) are used in Type 2 Diabetes to induce glucosuria and lower blood sugar. * **Oral Rehydration Therapy (ORT):** Works on the principle of Na+-Glucose symport; the presence of glucose enhances Na+ (and subsequently water) absorption.

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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