Membrane Biochemistry — MCQs

Membrane Biochemistry Indian Medical PG Practice Questions and MCQs

Question 41: Transport of ADP into and ATP out of mitochondria is inhibited by:

A. Rotenone
B. Oligomycin
C. Atractyloside (Correct Answer)
D. Antimycin-A

Explanation: ### Explanation The correct answer is **Atractyloside**. **1. Mechanism of the Correct Answer:** The transport of ADP into the mitochondrial matrix and ATP out into the cytosol is mediated by a specific transport protein called **Adenine Nucleotide Translocase (ANT)**, located in the inner mitochondrial membrane. **Atractyloside**, a plant toxin derived from the Mediterranean thistle (*Atractylis gummifera*), competitively inhibits this translocase. By blocking the entry of ADP, it depletes the substrate required for ATP synthase, effectively halting oxidative phosphorylation. Another inhibitor of this translocase is **Bongkrekic acid**. **2. Analysis of Incorrect Options:** * **Rotenone (Option A):** This is a specific inhibitor of **Complex I** (NADH-Q oxidoreductase) of the Electron Transport Chain (ETC). It prevents the transfer of electrons from NADH to Coenzyme Q. * **Oligomycin (Option B):** This is an inhibitor of **ATP Synthase (Complex V)**. It binds to the $F_0$ subunit and blocks the proton channel, preventing the phosphorylation of ADP to ATP. While it stops ATP production, it does not directly inhibit the transporter protein itself. * **Antimycin-A (Option D):** This is an inhibitor of **Complex III** (Cytochrome $bc_1$ complex). It blocks electron flow from Cytochrome $b$ to Cytochrome $c_1$. **3. High-Yield Clinical Pearls for NEET-PG:** * **Uncouplers vs. Inhibitors:** Inhibitors (like those above) stop both electron flow and ATP synthesis. Uncouplers (like 2,4-DNP or Thermogenin) allow electron flow to continue but dissipate the proton gradient as heat, stopping ATP synthesis. * **Bongkrekic Acid:** Often tested alongside Atractyloside; it inhibits ANT by binding to the matrix side, whereas Atractyloside binds to the cytosolic side. * **Cyanide/CO:** These are lethal inhibitors of **Complex IV** (Cytochrome $c$ oxidase).

Question 42: Perlecan is a:

A. Sacolemmal protein
B. Proteoglycan (Correct Answer)
C. Glycoprotein
D. Integral protein

Explanation: **Explanation:** **Perlecan** is a large, multidomain **heparan sulfate proteoglycan (HSPG)**. It is a primary structural component of all basement membranes (basal laminae), where it plays a critical role in maintaining the endothelial barrier, promoting cell adhesion, and sequestering growth factors. 1. **Why Proteoglycan is correct:** Perlecan consists of a large core protein (approx. 470 kDa) covalently linked to three long glycosaminoglycan (GAG) chains, typically heparan sulfate. This structure—a protein core with attached GAGs—is the biochemical definition of a proteoglycan. In the glomerular basement membrane, its negative charge contributes significantly to the charge-selective filtration barrier. 2. **Why other options are incorrect:** * **Sarcolemmal protein:** While Perlecan interacts with the sarcolemma (muscle cell membrane) via dystroglycan, it is an extracellular matrix (ECM) protein, not a structural protein of the sarcolemma itself (like dystrophin). * **Glycoprotein:** Although proteoglycans are technically glycosylated, the term "glycoprotein" usually refers to proteins with shorter, branched carbohydrate chains without repeating disaccharide units. Perlecan is specifically categorized as a proteoglycan due to its long GAG chains. * **Integral protein:** Perlecan is secreted into the extracellular matrix; it does not span the phospholipid bilayer. **High-Yield Facts for NEET-PG:** * **Location:** Found in all basement membranes and cartilage. * **Function:** Acts as a "biological sieve" in the kidneys; loss of its negative charge leads to **albuminuria**. * **Clinical Correlation:** Mutations in the *HSPG2* gene (encoding Perlecan) lead to **Schwartz-Jampel syndrome** (myotonia and skeletal dysplasia) or **Dyssegmental dysplasia, Silverman-Handmaker type** (lethal neonatal dwarfism).

Question 43: How do lipids and proteins primarily interact within a cell membrane?

A. Hydrophobic interactions (Correct Answer)
B. Both hydrophobic and covalent interactions
C. Covalent bonds
D. Hydrogen bonds

Explanation: **Explanation:** The cell membrane is organized according to the **Fluid Mosaic Model**. The primary interaction between lipids and proteins is non-covalent, specifically driven by **hydrophobic interactions**. **Why Hydrophobic Interactions are Correct:** The core of the lipid bilayer consists of non-polar fatty acid tails. Integral membrane proteins possess "transmembrane domains" rich in non-polar amino acids (like Valine, Leucine, and Isoleucine). These hydrophobic regions of the protein sequester themselves away from water by embedding into the lipid bilayer's fatty acid core. This thermodynamic drive—minimizing the disruption of water's hydrogen-bonded structure—is the primary force stabilizing the membrane structure. **Analysis of Incorrect Options:** * **Covalent Bonds:** These involve the sharing of electrons and are very strong. While some proteins are "lipid-anchored" via covalent bonds (e.g., GPI anchors), this is a specific modification rather than the *primary* mode of interaction for the bulk of membrane proteins. * **Hydrogen Bonds:** These occur mainly between polar head groups and the aqueous environment or within the secondary structures (alpha-helices) of the proteins themselves, but they do not provide the main anchoring force within the hydrophobic core. * **Both Hydrophobic and Covalent:** Incorrect because covalent interactions are the exception, not the rule, for the majority of the "mosaic" proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Amphipathic Nature:** Both membrane lipids (phospholipids) and integral proteins are amphipathic, containing both hydrophilic and hydrophobic regions. * **Detergents:** In the lab, integral membrane proteins can only be removed by using detergents (like SDS) which disrupt these **hydrophobic interactions**. * **Peripheral Proteins:** Unlike integral proteins, peripheral proteins interact with the membrane primarily through **electrostatic (ionic) interactions** and can be removed by changing pH or salt concentration.

Question 44: What maintains the membrane integrity of red blood cells?

A. G-protein
B. Haemoglobin
C. Spectrin (Correct Answer)
D. Ankyrin

Explanation: ### Explanation **Correct Answer: C. Spectrin** The red blood cell (RBC) must be highly deformable to navigate through narrow splenic sinusoids and capillaries. This structural integrity and flexibility are maintained by the **cytoskeleton**, located just beneath the lipid bilayer. **Spectrin** is the primary structural protein of this cytoskeleton. It exists as a long, flexible heterodimer ($\alpha$ and $\beta$ chains) that forms a hexagonal meshwork. This meshwork acts as a "molecular spring," allowing the RBC to deform under pressure and recoil to its original biconcave shape, thus maintaining membrane integrity. **Analysis of Incorrect Options:** * **A. G-protein:** These are membrane-associated proteins involved in signal transduction (linking receptors to second messengers) and do not play a structural role in the RBC cytoskeleton. * **B. Haemoglobin:** This is the functional cytoplasmic protein responsible for oxygen transport. While its concentration affects cell rheology (viscosity), it is not a structural component of the membrane. * **D. Ankyrin:** While Ankyrin is crucial, it serves as an **adapter protein**. It anchors the spectrin meshwork to the plasma membrane by binding to the Band 3 anion transport protein. While its deficiency causes spherocytosis, Spectrin is considered the "primary" scaffold maintaining the integrity of the network itself. **High-Yield Clinical Pearls for NEET-PG:** * **Hereditary Spherocytosis:** Most commonly caused by a deficiency in **Ankyrin** (approx. 50% of cases), followed by mutations in Spectrin or Band 3. This leads to loss of membrane surface area, resulting in spherical, fragile cells. * **Hereditary Elliptocytosis:** Most commonly due to a defect in **Spectrin** (specifically interfering with the formation of spectrin tetramers) or Protein 4.1. * **Vertical vs. Horizontal Interactions:** Defects in vertical interactions (Ankyrin/Band 3) lead to Spherocytosis; defects in horizontal interactions (Spectrin dimers) lead to Elliptocytosis.

Question 45: Which of the following membranes has the highest protein content per gram of tissue?

A. Inner mitochondrial membrane (Correct Answer)
B. Outer mitochondrial membrane
C. Plasma membrane
D. Myelin sheath

Explanation: The protein-to-lipid ratio of a biological membrane is directly proportional to its metabolic activity. Membranes involved in complex biochemical processes like electron transport or active transport require a higher density of proteins. ### **1. Why the Inner Mitochondrial Membrane (IMM) is Correct** The IMM is the most protein-rich membrane in the human body, consisting of approximately **75-80% protein** and 20-25% lipid. This high protein content is necessary to house the components of the **Electron Transport Chain (ETC)**, ATP synthase complexes, and numerous specific transport proteins (e.g., translocases). The high protein density is also why the IMM is highly impermeable to most ions and polar molecules. ### **2. Analysis of Incorrect Options** * **Outer Mitochondrial Membrane (OMM):** Contains about 50% protein. It is more permeable than the IMM due to the presence of **porins** (VDACs), but it lacks the dense machinery of the respiratory chain. * **Plasma Membrane:** Typically has a balanced ratio of roughly 50% protein and 50% lipid (though this varies by cell type). It serves structural and signaling roles rather than intensive metabolic catalysis. * **Myelin Sheath:** This is the "least" protein-rich membrane (~20% protein, 80% lipid). Its primary function is **electrical insulation**, which requires a high lipid content (sphingomyelin and cholesterol) to prevent ion leakage. ### **High-Yield Clinical Pearls for NEET-PG** * **Cardiolipin:** A unique phospholipid found almost exclusively in the IMM; it is essential for the optimal function of ETC enzymes. * **Marker Enzymes:** * Inner Mitochondrial Membrane: **ATP Synthase / Succinate Dehydrogenase.** * Outer Mitochondrial Membrane: **Monoamine Oxidase (MAO).** * Plasma Membrane: **Na⁺-K⁺ ATPase / 5'-Nucleotidase.** * **Rule of Thumb:** More metabolic "work" = More protein. More "insulation" = More lipid.

Question 46: Protein to lipid ratio is LEAST in which of the following biological membranes?

A. Inner mitochondrial membrane
B. Outer mitochondrial membrane
C. Red blood cell membrane
D. Myelin sheath (Correct Answer)

Explanation: **Explanation:** The protein-to-lipid ratio of a biological membrane is directly proportional to its **metabolic activity**. Proteins serve as enzymes, transporters, and receptors; therefore, membranes involved in complex biochemical processes have high protein content, while membranes serving primarily as insulators have high lipid content. **Why Myelin Sheath is Correct:** The **Myelin sheath** functions as an electrical insulator for axons to facilitate saltatory conduction. To minimize ion leakage and provide maximum insulation, it is composed of approximately **80% lipids** and only **20% proteins**. This results in a protein-to-lipid ratio of roughly **0.25:1**, the lowest among all biological membranes. **Analysis of Incorrect Options:** * **Inner Mitochondrial Membrane (IMM):** This is the most metabolically active membrane, housing the Electron Transport Chain (ETC) and ATP synthase. It has the **highest** protein content (~75% protein), with a ratio of approximately **3:1**. * **Outer Mitochondrial Membrane (OMM):** Contains various enzymes and porins. Its protein-to-lipid ratio is roughly **1:1**, significantly higher than myelin. * **Red Blood Cell (RBC) Membrane:** Contains numerous structural proteins (spectrin, ankyrin) and transporters (GLUT-1, Anion exchanger). Its ratio is approximately **1.1:1**. **High-Yield Clinical Pearls for NEET-PG:** * **Highest Protein Content:** Inner Mitochondrial Membrane (due to ETC). * **Highest Lipid Content:** Myelin Sheath (due to insulating function). * **Unique Lipid:** **Cardiolipin** is found exclusively in the Inner Mitochondrial Membrane and is essential for the function of ETC complexes. * **Major Myelin Lipids:** Sphingomyelin and Cerebrosides (Galactosylceramide). * **Clinical Correlation:** Demyelinating diseases like **Multiple Sclerosis** (CNS) and **Guillain-Barré Syndrome** (PNS) involve the destruction of these lipid-rich layers.

Question 47: Mitochondrial membrane proteins are involved in the transport of which molecule?

A. NADH (Correct Answer)
B. Acetyl CoA
C. NADPH
D. ATP

Explanation: **Explanation:** The mitochondrial inner membrane is highly selective and impermeable to most polar molecules. To overcome this, specific **shuttle systems** involving mitochondrial membrane proteins are required to transport reducing equivalents. **Why NADH is correct:** NADH generated during glycolysis in the cytosol cannot directly cross the inner mitochondrial membrane to enter the Electron Transport Chain (ETC). It requires specific membrane protein-mediated shuttles: 1. **Malate-Aspartate Shuttle:** Predominant in the heart, liver, and kidneys (yields ~2.5 ATP). 2. **Glycerol-3-Phosphate Shuttle:** Predominant in brown adipose tissue and muscle (yields ~1.5 ATP). These systems use membrane transporters (like the Malate-alpha-ketoglutarate transporter) to effectively "move" NADH equivalents into the matrix. **Why other options are incorrect:** * **Acetyl CoA:** The membrane is impermeable to Acetyl CoA. It must first condense with oxaloacetate to form **Citrate**, which is then transported out via the tricarboxylate transporter (Citrate Shuttle). * **NADPH:** Primarily generated in the cytosol via the HMP shunt; it does not have a dedicated mitochondrial transport shuttle like NADH for the ETC. * **ATP:** While ATP is transported via the ATP/ADP translocase, the question specifically tests the concept of **shuttle systems** for metabolic intermediates. In the context of "transporting the molecule" (meaning the reducing power), NADH is the classic biochemical focus. **High-Yield Clinical Pearls for NEET-PG:** * **Malate-Aspartate Shuttle** is more energy-efficient than the Glycerol-3-Phosphate shuttle. * **Arsenite poisoning** inhibits the Pyruvate Dehydrogenase complex, preventing the formation of mitochondrial Acetyl CoA. * **Brown Fat Thermogenesis:** Uses the Glycerol-3-Phosphate shuttle to rapidly deliver electrons to the ETC for heat production via UCP-1 (Thermogenin).

Question 48: Which protein maintains the integrity of the RBC membrane?

A. Spectrin (Correct Answer)
B. Laminin
C. Collagen
D. Elastin

Explanation: **Explanation:** The correct answer is **Spectrin**. **1. Why Spectrin is Correct:** Spectrin is the primary structural protein of the Red Blood Cell (RBC) membrane skeleton. It is a long, flexible heterodimer (composed of $\alpha$ and $\beta$ chains) that forms a hexagonal meshwork underneath the lipid bilayer. This "cytoskeletal scaffold" is crucial for maintaining the **structural integrity, shape, and extreme deformability** of the RBC, allowing it to squeeze through narrow splenic sinusoids and capillaries without rupturing. Spectrin is anchored to the membrane via proteins like **Ankyrin** (linking it to Band 3) and **Protein 4.1** (linking it to Glycophorin). **2. Why Other Options are Incorrect:** * **Laminin:** A major glycoprotein of the **basal lamina** (extracellular matrix), involved in cell adhesion and tissue structure, not RBC stability. * **Collagen:** The most abundant protein in the body, providing tensile strength to connective tissues (skin, bone, tendons). It is an **extracellular** protein. * **Elastin:** Provides elasticity to tissues like the aorta, lungs, and skin. While it allows recoil, it is not a component of the RBC cytoskeleton. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hereditary Spherocytosis:** Most commonly caused by a deficiency or defect in **Ankyrin** (most common) or **Spectrin**. This leads to a loss of membrane surface area, resulting in spherical, fragile RBCs that are destroyed in the spleen. * **Hereditary Elliptocytosis:** Primarily associated with defects in **Spectrin self-association** or Protein 4.1. * **Key Marker:** Spectrin is a peripheral membrane protein, whereas Band 3 and Glycophorins are integral membrane proteins.

Question 49: Which of the following minerals is a major component of biological membranes?

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

Explanation: **Explanation:** The correct answer is **Phosphorus (Option D)**. Biological membranes are primarily composed of a **phospholipid bilayer**. Phospholipids are amphipathic molecules consisting of a glycerol or sphingosine backbone, two fatty acid tails, and a **phosphate group** in the hydrophilic head. This phosphate group is the structural cornerstone of the membrane, providing the negative charge and polar nature required to form the barrier between the intra- and extracellular compartments. Additionally, phosphorus is vital for membrane-associated signaling molecules like Phosphatidylinositol 4,5-bisphosphate ($PIP_2$). **Why other options are incorrect:** * **Potassium (A) and Sodium (B):** These are the primary intracellular and extracellular **electrolytes**, respectively. While they interact with the membrane via ion channels and the $Na^+/K^+$-ATPase pump to maintain membrane potential, they are not structural components of the membrane itself. * **Calcium (C):** Calcium acts as a secondary messenger and is essential for membrane stability and fusion (exocytosis). However, it is not a primary building block of the lipid bilayer structure. **High-Yield Clinical Pearls for NEET-PG:** * **Fluid Mosaic Model:** Proposed by Singer and Nicolson, it describes the membrane as a fluid lipid bilayer with embedded proteins. * **Phosphatidylcholine (Lecithin):** The most abundant phospholipid in the mammalian plasma membrane. * **Sphingomyelin:** A phosphorus-containing lipid found abundantly in the myelin sheath of nerve fibers. * **Cardiolipin:** A unique phospholipid containing two phosphate groups, found exclusively in the **inner mitochondrial membrane**; its deficiency is seen in Barth Syndrome.

Question 50: On a weight basis, what is the approximate percentage of proteins in a cell membrane?

A. 70%
B. 30%
C. 50% (Correct Answer)
D. 80%

Explanation: ### Explanation **1. Why the Correct Answer (C) is Right:** The biochemical composition of a typical mammalian cell membrane (like the plasma membrane) is approximately **50% protein**, **40% lipid**, and **10% carbohydrate** by weight. While lipids (phospholipids and cholesterol) provide the structural fluid matrix, proteins are responsible for the membrane's functional diversity, acting as transporters, receptors, enzymes, and adhesion molecules. Because proteins are much larger and heavier molecules than individual lipids, they contribute significantly to the total mass, even if they are outnumbered by lipid molecules. **2. Why the Incorrect Options are Wrong:** * **Option A (70%) & D (80%):** These values are too high for a standard plasma membrane. However, specialized membranes like the **inner mitochondrial membrane** are protein-rich (approx. 75%) to accommodate the electron transport chain components. * **Option B (30%):** This value is too low for most membranes. An exception is the **myelin sheath**, which is lipid-rich (approx. 80% lipid, 20% protein) to provide electrical insulation for nerve impulses. **3. High-Yield Clinical Pearls for NEET-PG:** * **Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972), it describes the membrane as a "sea of lipids" with "icebergs of proteins." * **Protein-to-Lipid Ratio:** This ratio varies by function. Myelin = 0.23 (low protein); Human Erythrocyte = 1.1; Inner Mitochondrial Membrane = 3.2 (high protein). * **Carbohydrates:** Always located on the **extracellular surface** (forming the glycocalyx), never on the cytosolic face. * **Peripheral vs. Integral Proteins:** Integral proteins span the bilayer (e.g., Glycophorin), while peripheral proteins are loosely attached (e.g., Spectrin in RBCs).

Practice by Chapter

Membrane Structure and Organization

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Membrane Lipids and Fluidity

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Membrane Proteins: Integral and Peripheral

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Transport Across Membranes

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Ion Channels and Transporters

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Sodium-Potassium ATPase

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Calcium Transport and Calcium ATPase

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Glucose Transporters

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Membrane Receptors and Signal Transduction

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Lipid Rafts and Caveolae

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Membrane Disorders and Diseases

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Biochemistry of Endocytosis and Exocytosis

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