Membrane Biochemistry Practice Questions

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: Which of the following actions is NOT performed by ionophores?

Hydrophilic in character. **Explanation:** Ionophores are lipid-soluble molecules that facilitate the transport of ions across biological membranes. To understand why **Option C** is the correct answer, we must look at their structural requirements. **1. Why "Hydrophilic in character" is the correct answer (The "NOT" action):** Ionophores must be **hydrophobic (lipophilic)** in nature. To transport ions across the non-polar lipid bilayer of the mitochondrial membrane, the ionophore must be able to dissolve into and diffuse through the lipid phase. They typically have a hydrophilic center to bind the ion and a hydrophobic exterior to interact with the membrane lipids. Therefore, being hydrophilic would prevent them from crossing the membrane, making Option C the false statement. **2. Analysis of Incorrect Options:** * **A & D (Abolish proton/pH gradient):** Ionophores like **2,4-Dinitrophenol (DNP)** or **CCCP** act as mobile carriers that pick up protons ($H^+$) from the intermembrane space and release them into the matrix. This "leaks" protons back across the membrane, effectively collapsing both the electrical and pH gradients. * **B (Inhibit ADP to ATP conversion):** By dissipating the proton motive force (the energy source for ATP synthase), ionophores decouple oxidation from phosphorylation. While the Electron Transport Chain (ETC) continues or even accelerates, the synthesis of ATP from ADP is inhibited. **Clinical Pearls & High-Yield Facts:** * **Valinomycin:** A classic example of a mobile carrier ionophore specific for $K^+$ ions. * **Gramicidin:** A channel-forming ionophore that allows the passage of monovalent cations ($H^+, Na^+, K^+$). * **Uncoupling Effect:** Ionophores act as "uncouplers." In the presence of uncouplers, energy is dissipated as **heat** rather than being captured as ATP. This is the physiological basis of thermogenesis in brown adipose tissue via the protein **Thermogenin (UCP1)**.

Q: Which of the following is NOT an enzymatic marker of the plasma membrane?

Galactosyl transferase. **Explanation:** In cell biology, specific enzymes are localized to particular organelles, serving as "biochemical markers" to identify or assess the purity of subcellular fractions. **Why Galactosyl transferase is the correct answer:** **Galactosyl transferase** is the classic enzymatic marker for the **Golgi apparatus**. It is involved in the post-translational modification of proteins (glycosylation). Since it is localized to the Golgi cisternae and not the plasma membrane, it is the correct "NOT" option. **Analysis of Incorrect Options (Plasma Membrane Markers):** * **5'-nucleotidase:** A widely used marker for the plasma membrane, particularly in liver cells (canaliculi). It catalyzes the conversion of nucleoside 5'-monophosphates to nucleosides. * **Adenylyl cyclase:** This enzyme is bound to the inner surface of the plasma membrane. It is crucial for signal transduction, converting ATP to cAMP in response to hormonal stimuli (via G-proteins). * **Na⁺-K⁺-ATPase:** This is the gold-standard marker for the plasma membrane. It is an integral membrane protein responsible for maintaining the resting membrane potential and osmotic balance in almost all animal cells. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondria Markers:** ATP synthase (Inner membrane), Monoamine oxidase (Outer membrane). * **Lysosome Marker:** Acid phosphatase. * **Peroxisome Marker:** Catalase. * **Endoplasmic Reticulum Marker:** Glucose-6-phosphatase. * **Cytosol Marker:** Lactate dehydrogenase (LDH). * **Clinical Correlation:** Elevated serum levels of **5'-nucleotidase** are used clinically to differentiate between hepatobiliary disease and bone disease when Alkaline Phosphatase (ALP) is raised.

Q: The fluidity of the plasma membrane is increased by

Arachidonic acid. **Explanation:** The fluidity of the plasma membrane is primarily determined by the **degree of saturation** of the fatty acid tails in the phospholipid bilayer and the presence of cholesterol. **1. Why Arachidonic Acid is Correct:** Arachidonic acid is a **polyunsaturated fatty acid (PUFA)** containing 20 carbons and **four double bonds**. In the plasma membrane, double bonds in fatty acid chains create "kinks" or bends. These kinks prevent the phospholipid molecules from packing tightly together, increasing the lateral movement of lipids and proteins, thereby **increasing membrane fluidity**. **2. Why the Other Options are Incorrect:** * **Palmitic Acid (C16) and Stearic Acid (C18):** These are **saturated fatty acids** (no double bonds). Their straight-chain structure allows them to pack tightly together via van der Waals forces, which stabilizes the membrane and **decreases fluidity** (making it more rigid). * **Cholesterol:** At physiological temperatures, cholesterol acts as a "fluidity buffer." It inserts itself between the fatty acid tails, restricting their movement and **decreasing fluidity**. (Note: At very low temperatures, it prevents the membrane from freezing, but in the context of standard human physiology, it generally stabilizes/stiffens the membrane). **High-Yield Clinical Pearls for NEET-PG:** * **Fluid Mosaic Model:** Proposed by Singer and Nicolson (1972); emphasizes that the membrane is a dynamic, fluid structure. * **Temperature Effect:** Fluidity increases with higher temperatures and decreases with lower temperatures. * **Chain Length:** Shorter fatty acid chains increase fluidity because they have less surface area to undergo van der Waals interactions. * **Ratio:** A high **Unsaturated:Saturated** fatty acid ratio is a hallmark of high membrane fluidity.

Q: A bilayer cell membrane contains which of the following components?

Cholesterol. ### Explanation **Correct Option: A. Cholesterol** The eukaryotic cell membrane is a fluid mosaic of **phospholipids, proteins, and cholesterol**. Cholesterol is an essential structural component of the lipid bilayer. It is an **amphipathic molecule**, meaning it has both a hydrophilic part (the hydroxyl group at C3) and a hydrophobic part (the steroid nucleus and hydrocarbon tail). * **Function:** It acts as a "fluidity buffer." At high temperatures, it stabilizes the membrane by restricting the movement of phospholipid fatty acid chains. At low temperatures, it prevents the membrane from freezing by disrupting the tight packing of phospholipids. **Why Incorrect Options are Wrong:** * **B. Triacylglycerol (TAG):** These are highly hydrophobic storage lipids found in **adipocytes (fat droplets)**, not in the structural framework of cell membranes. * **C. Cholesterol Ester:** This is the storage form of cholesterol (cholesterol + fatty acid). Because it lacks the free hydroxyl group, it is entirely non-polar and resides in the **interior of lipoprotein particles** or cytosolic lipid droplets, not in the bilayer. * **D. Glycerol:** While glycerol forms the backbone of phospholipids (glycerophospholipids), free glycerol is a water-soluble intermediate of metabolism and is not a structural component of the membrane. **High-Yield Clinical Pearls for NEET-PG:** 1. **Lipid Rafts:** These are specialized microdomains in the plasma membrane enriched in **cholesterol and glycosphingolipids** that facilitate cell signaling. 2. **Acanthocytosis (Spur Cell Anemia):** Seen in liver disease; an increase in membrane cholesterol leads to decreased fluidity and the formation of "thorny" RBCs (Acanthocytes). 3. **Ratio:** In most mammalian plasma membranes, the molar ratio of cholesterol to phospholipids is approximately **1:1**.

Q: Which type of ion channel is affected by mutations in the CFTR gene?

Chloride. ***Chloride***- The **CFTR** (Cystic Fibrosis Transmembrane Conductance Regulator) gene encodes an **ATP-gated chloride channel** that controls the movement of chloride ions across epithelial cell membranes.- Mutations in the CFTR gene cause defective chloride secretion and increased sodium and water reabsorption, leading to the dehydrated, thick mucus characteristic of **Cystic Fibrosis (CF)**.*Sodium*- While the lack of functional CFTR leads to excessive **sodium reabsorption** via the epithelial sodium channel (ENaC) in airways, CFTR itself is a chloride channel, not a sodium channel.- ENaC hyperactivity is a *secondary* effect resulting from the failure of CFTR to inhibit ENaC activity appropriately.*Potassium*- The primary role of the CFTR protein is not the regulation of potassium ions; potassium channels are distinct proteins involved in maintaining resting membrane potential and cell volume.- CFTR activity and the resultant disease phenotype are directly linked to chloride imbalance, not potassium transport defects.*Calcium*- CFTR does not function as a calcium channel; calcium channels are separate entities crucial for many cellular processes, including neurotransmitter release and muscle contraction.- Although intracellular calcium levels can sometimes modulate CFTR activity through signaling pathways, the channel protein itself transports chloride.

Question 1

Glycophorin is present in:

Accepted Answer

RBC

Answer

Enterocyte

Answer

Hepatocyte

Answer

Lymphocyte

Question 2

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 3

What is the correct sequence of vesicle transport?

Accepted Answer

ER to Cis-Golgi Network to Trans-Golgi Network to Cell Membrane

Answer

ER to Trans-Golgi Network to Lysosome

Answer

ER to Lysosome to Trans-Golgi Network to Cis-Golgi Network

Answer

Cis-Golgi Network to ER to Trans-Golgi Network to Cell Membrane

Question 4

Which of the following actions is NOT performed by ionophores?

Accepted Answer

Hydrophilic in character

Answer

Abolish proton gradient

Answer

Inhibit ADP to ATP conversion

Answer

Abolish pH gradient

Question 5

Several segments of the polypeptide chain of integral membrane proteins usually span the lipid bilayer. These segments frequently adopt which configuration?

Accepted Answer

An alpha-helical configuration

Answer

Hydrophilic amino acids

Answer

Covalent bonds with cholesterol

Answer

Unusually strong peptide bonds

Question 6

Which of the following is NOT an enzymatic marker of the plasma membrane?

Accepted Answer

Galactosyl transferase

Answer

5'-nucleotidase

Answer

Adenylyl cyclase

Answer

Na+-K+-ATPase

Question 7

The fluidity of the plasma membrane is increased by

Accepted Answer

Arachidonic acid

Answer

Cholesterol

Answer

Palmitic acid

Answer

Stearic acid

Question 8

Which sequence is responsible for retaining proteins in the membrane?

Accepted Answer

Halt signal

Answer

Translocon

Answer

Sec 61 complex

Answer

Docking protein

Question 9

A bilayer cell membrane contains which of the following components?

Accepted Answer

Cholesterol

Answer

Triacylglycerol

Answer

Cholesterol ester

Answer

Glycerol

Question 10

Which type of ion channel is affected by mutations in the CFTR gene?

Accepted Answer

Chloride

Answer

Calcium

Answer

Potassium

Answer

Sodium

Membrane Biochemistry — MCQs

Membrane Biochemistry — MCQs

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