Membrane Biochemistry — MCQs

Membrane Biochemistry Indian Medical PG Practice Questions and MCQs

Question 51: The transition temperature of lipid bilayers of the cell membrane is increased by?

A. Cholesterol
B. Saturated fatty acids (Correct Answer)
C. Hydrocarbons
D. Unsaturated fatty acids

Explanation: ### Explanation **Core Concept: Membrane Fluidity and Transition Temperature ($T_m$)** The transition temperature ($T_m$) is the temperature at which a lipid bilayer changes from a rigid, ordered "gel" state to a fluid, disordered "liquid-crystalline" state. Factors that make the membrane more rigid or tightly packed **increase** the $T_m$, as more thermal energy is required to achieve fluidity. **Why Saturated Fatty Acids (Option B) is Correct:** Saturated fatty acids have straight, linear hydrocarbon chains with no double bonds. This structure allows them to pack closely together through maximal van der Waals interactions. This tight packing stabilizes the gel state, making the membrane more rigid and significantly **increasing the transition temperature**. **Analysis of Incorrect Options:** * **Unsaturated Fatty Acids (Option D):** These contain "kinks" (cis-double bonds) that prevent tight packing. This increases membrane fluidity and **decreases** the $T_m$. * **Cholesterol (Option A):** Cholesterol acts as a "fluidity buffer." While it can interfere with the movement of fatty acid chains, its primary role is to blur the transition rather than simply increasing the $T_m$. It prevents the membrane from becoming too rigid at low temperatures and too fluid at high temperatures. * **Hydrocarbons (Option C):** Short-chain hydrocarbons generally disrupt packing and increase fluidity, thereby lowering the $T_m$. **High-Yield Clinical Pearls for NEET-PG:** * **Chain Length:** Longer fatty acid chains increase $T_m$ due to increased surface area for van der Waals forces. * **Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972); emphasizes that the membrane is a dynamic, fluid structure. * **Clinical Correlation:** The fluidity of the RBC membrane is crucial; in **Abetalipoproteinemia**, altered lipid composition leads to "Acanthocytes" (spur cells), which are less flexible and prone to hemolysis.

Question 52: Where is Cytochrome 450 primarily located?

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

Explanation: **Explanation:** Cytochrome P450 (CYP450) enzymes are a superfamily of heme-containing proteins primarily involved in the oxidative metabolism of drugs, toxins, and endogenous compounds like steroids. **Why Endoplasmic Reticulum (ER) is correct:** The vast majority of CYP450 enzymes are integral membrane proteins located in the **Smooth Endoplasmic Reticulum (SER)**, particularly in hepatocytes. These are often referred to as "microsomal" enzymes because, during cell fractionation, the ER fragments into vesicles called microsomes. They play a critical role in Phase I metabolism (hydroxylation, oxidation) to make lipophilic compounds more water-soluble for excretion. **Why other options are incorrect:** * **Mitochondria:** While a specific subset of CYP450 enzymes exists in the inner mitochondrial membrane (involved in steroidogenesis and Vitamin D activation), the **primary** and most abundant location discussed in general pharmacology and biochemistry is the ER. * **Cytoplasm:** CYP450 enzymes are membrane-bound to facilitate the electron transport chain required for their catalytic cycle; they do not function as soluble proteins in the cytosol. * **Golgi apparatus:** This organelle is involved in protein modification and trafficking, not the oxidative metabolism characteristic of the CYP450 system. **High-Yield NEET-PG Pearls:** * **Inducers vs. Inhibitors:** Knowledge of CYP450 inducers (e.g., Rifampicin, Phenytoin, Carbamazepine) and inhibitors (e.g., Ketoconazole, Erythromycin, Cimetidine) is high-yield for both Biochemistry and Pharmacology. * **CYP3A4:** This is the most abundant isoform in the liver and is responsible for metabolizing nearly 50% of all clinical drugs. * **Requirement:** The CYP450 system requires **NADPH** and **Molecular Oxygen (O₂)** to function.

Question 53: Which important component of the cell wall has a carbohydrate moiety?

A. Phosphoglyceride
B. Triacylglycerol
C. Sphingomyelin
D. GM2 Gangliosides (Correct Answer)

Explanation: **Explanation:** The question asks for a cell membrane component containing a **carbohydrate moiety**. **1. Why GM2 Ganglioside is Correct:** Gangliosides are a subtype of **acidic glycosphingolipids**. They are composed of a ceramide backbone (sphingosine + fatty acid) attached to an oligosaccharide chain that contains at least one residue of **N-acetylneuraminic acid (NANA or Sialic acid)**. This complex carbohydrate head group projects from the outer leaflet of the plasma membrane, serving as a receptor for hormones and bacterial toxins (e.g., Cholera toxin). **2. Why the Other Options are Incorrect:** * **Phosphoglycerides (A):** These are the most abundant membrane lipids, consisting of glycerol, two fatty acids, and a phosphate group (often attached to an alcohol like choline or ethanolamine). They do not contain carbohydrates. * **Triacylglycerol (B):** These are storage lipids (neutral fats) found in adipocytes. They consist of glycerol and three fatty acids. They are not structural components of the cell membrane and lack carbohydrates. * **Sphingomyelin (C):** While it is a sphingolipid, it is a **phospholipid**, not a glycolipid. Its polar head group is either phosphorylcholine or phosphorylethanolamine. It contains no carbohydrate moiety. **Clinical Pearls for NEET-PG:** * **Tay-Sachs Disease:** Caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of **GM2 Gangliosides** in lysosomes. Key findings: Cherry-red spot on the macula and onion-skin lysosomes. * **Guillain-Barré Syndrome (GBS):** Often involves molecular mimicry where antibodies against *C. jejuni* cross-react with **gangliosides** (like GM1) in peripheral myelin. * **Cholera Toxin:** Specifically binds to the **GM1 ganglioside** receptor on intestinal mucosal cells.

Question 54: Which of the following is an example of a multipass membrane protein?

A. Glycophorins A
B. Glycophorins B
C. Glycophorins C
D. Anion exchange protein (Correct Answer)

Explanation: **Explanation:** Membrane proteins are classified based on how they associate with the lipid bilayer. **Multipass membrane proteins** (also known as polytopic proteins) are integral proteins where the polypeptide chain crosses the lipid bilayer multiple times. **Why the Correct Answer is Right:** * **Anion Exchange Protein (Band 3):** This is a classic example of a multipass membrane protein found in the erythrocyte membrane. It spans the membrane **14 times**. Its primary function is to facilitate the "Chloride Shift" (exchanging $HCO_3^-$ for $Cl^-$), which is essential for $CO_2$ transport in the blood. Because it weaves through the membrane multiple times, it forms a channel-like structure necessary for ion transport. **Why the Other Options are Wrong:** * **Glycophorins (A, B, and C):** These are **single-pass (bitopic) transmembrane proteins**. They span the lipid bilayer only once, with their N-terminal domain (rich in carbohydrates) exposed on the external surface and the C-terminal domain in the cytosol. * **Glycophorin A** is the most abundant and carries the M and N blood group antigens. * **Glycophorin B and C** are structurally similar single-pass proteins that contribute to the RBC's negative surface charge (zeta potential). **High-Yield Clinical Pearls for NEET-PG:** * **Band 3 Protein:** Apart from ion exchange, it serves as an anchor for the RBC cytoskeleton (via ankyrin), maintaining the biconcave shape. * **GLUT-1:** Another high-yield multipass protein in RBCs (spans the membrane 12 times). * **G-Protein Coupled Receptors (GPCRs):** These are the most famous multipass proteins, spanning the membrane exactly **7 times** (serpentine receptors). * **Clinical Correlation:** Mutations in Band 3 protein are associated with **Hereditary Spherocytosis** and **Hereditary Stomatocytosis**.

Question 55: What is the lipid to protein ratio of the inner mitochondrial membrane?

A. 1:2
B. 2:1
C. 1:1
D. 1:3 (Correct Answer)

Explanation: **Explanation:** The composition of biological membranes varies significantly based on their physiological function. The **Inner Mitochondrial Membrane (IMM)** is unique because it is the site of the Electron Transport Chain (ETC) and ATP synthesis. **1. Why 1:3 is correct:** The IMM has the highest protein concentration of any membrane in the cell. Approximately **75-80% of its mass is protein**, while only 20-25% is lipid. This results in a **lipid-to-protein ratio of roughly 1:3**. This high protein density is necessary to accommodate the numerous complexes of the respiratory chain (Complexes I-IV), ATP synthase, and specific transport proteins (e.g., ADP/ATP translocase). **2. Analysis of Incorrect Options:** * **Option A (1:2):** While this indicates a high protein content, it underestimates the actual density found in the IMM. * **Option B (2:1):** This represents a lipid-rich membrane. An example is **Myelin**, which acts as an insulator and contains ~80% lipid and ~20% protein. * **Option C (1:1):** This is the typical ratio for a standard **Plasma Membrane** (e.g., Human Erythrocyte), where the mass is distributed roughly equally between lipids and proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiolipin:** The IMM is rich in this unusual phospholipid (diphosphatidylglycerol), which makes the membrane impermeable to ions (especially H+) to maintain the electrochemical gradient. * **Surface Area:** The IMM is folded into **cristae** to increase the surface area available for oxidative phosphorylation. * **Mitochondrial DNA:** Unlike the outer membrane, the IMM is the barrier protecting the mitochondrial matrix where mtDNA and ribosomes reside.

Question 56: Which of the following is not present in the basal lamina?

A. Rhodopsin (Correct Answer)
B. Entactin
C. Laminin
D. Integrin

Explanation: The **Basal Lamina** is a specialized form of extracellular matrix (ECM) that underlies all epithelial and endothelial sheets. It provides structural support and acts as a selective barrier. ### Why Rhodopsin is the Correct Answer **Rhodopsin** is a light-sensitive G-protein coupled receptor (GPCR) found exclusively in the **photoreceptor cells (rods) of the retina**. It is an integral membrane protein located within the disc membranes of the outer segments, not a component of the extracellular matrix or basal lamina. ### Explanation of Other Options * **Laminin:** This is the primary organizing component of the basal lamina. It is a large, heterotrimeric glycoprotein that binds to cell surface receptors and other ECM components. * **Entactin (Nidogen):** This is a rod-like glycoprotein that functions as a "molecular bridge." It non-covalently links laminin and Type IV collagen, stabilizing the basal lamina structure. * **Integrins:** These are transmembrane receptors that facilitate cell-extracellular matrix adhesion. They anchor the cell cytoskeleton to the basal lamina by binding to laminin and fibronectin. ### High-Yield NEET-PG Pearls * **Major Components of Basal Lamina:** Type IV Collagen (structural framework), Laminin (organizer), Entactin/Nidogen (linker), and Perlecan (heparan sulfate proteoglycan). * **Type IV Collagen:** Specifically found in basement membranes; defects lead to **Alport Syndrome** (nephritis and sensorineural deafness). * **Laminin vs. Luminal:** Do not confuse Laminin (ECM protein) with **Lamins** (intermediate filaments that support the nuclear envelope). * **Goodpasture Syndrome:** Characterized by autoantibodies against the non-collagenous (NC1) domain of Type IV collagen in the glomerular basement membrane.

Question 57: How do proteins enter peroxisomes?

A. Folded, using a C-terminal or internal signal sequence (Correct Answer)
B. Folded, using an N-terminal or internal signal sequence
C. Unfolded, using a C-terminal or internal signal sequence
D. Unfolded, using an N-terminal or internal signal sequence

Explanation: ### Explanation **1. Why Option A is Correct:** Peroxisomal protein import is unique because proteins are imported in their **fully folded, native state** (and sometimes even as oligomeric complexes). This is facilitated by specific **Peroxisomal Targeting Signals (PTS)**: * **PTS1 (Most common):** A tripeptide sequence (Serine-Lysine-Leucine or **SKL**) located at the extreme **C-terminus**. * **PTS2 (Less common):** Located near the **N-terminus**. The import process involves cytosolic receptors (**Pex5** for PTS1) that escort the folded protein through a dynamic translocon pore in the peroxisomal membrane. **2. Why Other Options are Incorrect:** * **Options C & D (Unfolded):** Proteins enter the **Mitochondria** and **Endoplasmic Reticulum (ER)** in an **unfolded** state (requiring chaperones like HSP70). Peroxisomes and the Nucleus are the primary organelles that import fully folded proteins. * **Options B & D (N-terminal only):** While PTS2 is N-terminal, the vast majority of peroxisomal proteins use the **C-terminal** PTS1 signal. Therefore, "C-terminal or internal" is the more characteristic descriptor for peroxisomal import compared to the strictly N-terminal "leader sequences" used for ER or mitochondrial targeting. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Zellweger Syndrome:** The most severe peroxisomal biogenesis disorder. It is caused by mutations in **PEX genes** (commonly *PEX5*), leading to "empty" peroxisomes (ghosts) because proteins cannot be imported. Clinical features: Hypotonia, seizures, hepatomegaly, and early death. * **X-linked Adrenoleukodystrophy (X-ALD):** Defect in the **ABCD1** transporter; leads to the accumulation of **Very Long Chain Fatty Acids (VLCFA)**, causing demyelination. * **Key Function:** Peroxisomes are the exclusive site for **alpha-oxidation** (Phytanic acid metabolism) and the initial steps of **plasmalogen synthesis** (essential for myelin).

Question 58: Glycophorin is present in:

A. Enterocyte
B. Hepatocyte
C. RBC (Correct Answer)
D. Lymphocyte

Explanation: **Explanation:** **Glycophorin** is a major sialoglycoprotein found specifically in the plasma membrane of **Red Blood Cells (RBCs)**. It is a classic example of a single-pass transmembrane protein. **Why RBC is the correct answer:** Glycophorin (specifically Glycophorin A) is the most abundant surface protein on erythrocytes after Band 3. Its primary function is to provide a heavy coat of **Sialic acid**, which gives the RBC membrane a strong **negative charge** (zeta potential). This negative charge causes RBCs to repel each other, preventing spontaneous aggregation and ensuring smooth blood flow through microvasculature. **Why the other options are incorrect:** * **Enterocytes, Hepatocytes, and Lymphocytes:** While these cells possess various glycoproteins (like GLUT transporters or MHC molecules), they do not contain Glycophorin. Glycophorin is considered a lineage-specific marker for erythroid cells. **High-Yield Clinical Pearls for NEET-PG:** 1. **Malaria Link:** Glycophorins A and B serve as the primary receptors for the attachment of *Plasmodium falciparum* (via the EBA-175 protein) to the RBC surface. 2. **Blood Groups:** Glycophorins A and B carry the antigenic determinants for the **MNS blood group system**. 3. **Structure:** It spans the lipid bilayer once (Type I transmembrane protein) with its N-terminal (glycosylated end) outside the cell and C-terminal inside. 4. **Cytoskeleton:** The cytoplasmic tail of Glycophorin C interacts with **Protein 4.1**, helping to anchor the underlying spectrin-actin cytoskeleton to the membrane, maintaining the biconcave shape.

Question 59: In what form do proteins cross the mitochondrial membranes?

A. Bound to an importin protein with a signal sequence
B. In fully folded form
C. In unfolded extended form attached to Hsp 70 chaperones
D. In unfolded extended form without chaperones (Correct Answer)

Explanation: ### Explanation **Core Concept: Mitochondrial Protein Import** Most mitochondrial proteins are synthesized on cytosolic ribosomes as precursors. To pass through the narrow translocation channels—the **TOM complex** (Outer Membrane) and **TIM complex** (Inner Membrane)—proteins must be in an **unfolded, extended polypeptide state**. **Why Option D is Correct:** While cytosolic chaperones (like Hsp70) are essential for *maintaining* the protein in an unfolded state and delivering it to the mitochondria, the actual translocation through the membrane pores occurs in an **unfolded, extended form**. Once the protein enters the pore, it is stripped of its cytosolic chaperones. The translocation is driven by the electrochemical gradient and ATP hydrolysis, not by the chaperones themselves crossing the membrane with the protein. **Analysis of Incorrect Options:** * **Option A:** **Importins** are transport receptors used for **Nuclear transport**, not mitochondrial transport. Mitochondrial transport uses the TOM/TIM machinery. * **Option B:** Proteins cannot cross mitochondrial membranes in a **fully folded form** because the translocation pores (TOM/TIM) are too narrow. Folded protein transport is characteristic of **Peroxisomes** (via the PTS pathway). * **Option C:** While **Hsp70 chaperones** are involved in the cytosolic journey, they do not cross the membrane *attached* to the protein; they release the protein at the outer membrane so it can thread through the channel. **High-Yield Clinical Pearls for NEET-PG:** * **Signal Sequence:** Mitochondrial proteins have an N-terminal **Presequence** (rich in basic/amphipathic amino acids) which is cleaved by Mitochondrial Processing Peptidase (MPP) after entry. * **Energy Requirement:** Translocation requires **ATP** (for chaperone release) and **Proton Motive Force** (across the inner membrane). * **Zellweger Syndrome:** Contrast this with mitochondrial transport; Zellweger is a defect in importing proteins into **peroxisomes** (where proteins *can* be folded).

Question 60: What are the forces arising from attractions between transient dipoles generated by the rapid movement of electrons of all neutral atoms?

A. Hydrophobic forces
B. Covalent forces
C. Van der Waals forces (Correct Answer)
D. Electrostatic forces

Explanation: ### Explanation **Correct Option: C. Van der Waals forces** Van der Waals forces are weak, short-range attractions that occur between all atoms and molecules, regardless of whether they are polar or non-polar. They arise from **transient (temporary) dipoles**. At any given instant, the rapid movement of electrons around a nucleus may result in an asymmetrical distribution, creating a momentary partial charge. This transient dipole induces a complementary dipole in a neighboring atom, leading to a weak attraction. In biochemistry, while individually weak, the summation of these forces is critical for stabilizing the interior of proteins and the packing of lipid bilayers in cell membranes. **Why other options are incorrect:** * **A. Hydrophobic forces:** These are not "attractions" between atoms but rather the tendency of non-polar molecules to aggregate in water to minimize their contact with the aqueous environment, thereby increasing the entropy of water. * **B. Covalent forces:** These involve the **sharing of electron pairs** between atoms. They are strong, permanent chemical bonds (e.g., peptide bonds) rather than transient attractions. * **D. Electrostatic forces:** Also known as ionic bonds or salt bridges, these occur between **permanently charged** groups (e.g., a carboxylate group $COO^-$ and an amino group $NH_3^+$). They do not rely on transient electron movement. **High-Yield NEET-PG Pearls:** * **Strength Hierarchy:** Covalent > Ionic > Hydrogen bonds > Van der Waals. * **Biological Significance:** Van der Waals forces are essential for the **binding of substrates to enzyme active sites** and the specific "lock and key" fit of antibodies to antigens. * **Membrane Fluidity:** In the lipid bilayer, the length and saturation of fatty acid tails determine the strength of Van der Waals interactions, directly influencing the membrane's melting point and fluidity.

Practice by Chapter

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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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