Membrane Biochemistry Practice Questions

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

Saturated fatty acids. ### 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.

Q: Where is Cytochrome 450 primarily located?

Endoplasmic reticulum. **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.

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

GM2 Gangliosides. **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.

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

Anion exchange protein. **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**.

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

1:3. **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.

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

Rhodopsin. 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.

Q: Glycophorin is present in:

RBC. **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.

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

Van der Waals forces. ### 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.

Question 1

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

Accepted Answer

Saturated fatty acids

Answer

Cholesterol

Answer

Hydrocarbons

Answer

Unsaturated fatty acids

Question 2

Where is Cytochrome 450 primarily located?

Accepted Answer

Endoplasmic reticulum

Answer

Mitochondria

Answer

Cytoplasm

Answer

Golgi apparatus

Question 3

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

Accepted Answer

GM2 Gangliosides

Answer

Phosphoglyceride

Answer

Triacylglycerol

Answer

Sphingomyelin

Question 4

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

Accepted Answer

Anion exchange protein

Answer

Glycophorins A

Answer

Glycophorins B

Answer

Glycophorins C

Question 5

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

Accepted Answer

1:3

Answer

1:2

Answer

2:1

Answer

1:1

Question 6

Which of the following is not present in the basal lamina?

Accepted Answer

Rhodopsin

Answer

Entactin

Answer

Laminin

Answer

Integrin

Question 7

How do proteins enter peroxisomes?

Accepted Answer

Folded, using a C-terminal or internal signal sequence

Answer

Folded, using an N-terminal or internal signal sequence

Answer

Unfolded, using a C-terminal or internal signal sequence

Answer

Unfolded, using an N-terminal or internal signal sequence

Question 8

Glycophorin is present in:

Accepted Answer

RBC

Answer

Enterocyte

Answer

Hepatocyte

Answer

Lymphocyte

Question 9

In what form do proteins cross the mitochondrial membranes?

Accepted Answer

In unfolded extended form without chaperones

Answer

Bound to an importin protein with a signal sequence

Answer

In fully folded form

Answer

In unfolded extended form attached to Hsp 70 chaperones

Question 10

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

Accepted Answer

Van der Waals forces

Answer

Hydrophobic forces

Answer

Covalent forces

Answer

Electrostatic forces

Membrane Biochemistry — MCQs

Membrane Biochemistry — MCQs

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