Membrane Biochemistry — MCQs

Membrane Biochemistry Indian Medical PG Practice Questions and MCQs

Question 61: Which of the following actions is NOT performed by ionophores?

A. Abolish proton gradient
B. Inhibit ADP to ATP conversion
C. Hydrophilic in character (Correct Answer)
D. Abolish pH gradient

Explanation: **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)**.

Question 62: Several segments of the polypeptide chain of integral membrane proteins usually span the lipid bilayer. These segments frequently adopt which configuration?

A. An alpha-helical configuration (Correct Answer)
B. Hydrophilic amino acids
C. Covalent bonds with cholesterol
D. Unusually strong peptide bonds

Explanation: **Explanation:** **1. Why Alpha-Helical Configuration is Correct:** Integral membrane proteins must span the hydrophobic core of the lipid bilayer. The **alpha-helix** is the most common structural motif for these transmembrane segments because it allows for the maximal formation of **internal hydrogen bonds** between the carbonyl oxygen and the amide nitrogen of the peptide backbone. By satisfying these polar requirements internally, the protein can shield its hydrophilic backbone and expose only **hydrophobic side chains** to the lipid environment. A typical transmembrane helix consists of approximately 20–25 hydrophobic amino acids. **2. Why Other Options are Incorrect:** * **Option B (Hydrophilic amino acids):** The interior of the lipid bilayer is highly hydrophobic. Transmembrane segments must consist primarily of **non-polar (hydrophobic) amino acids** (e.g., Leucine, Valine, Phenylalanine) to be energetically stable within the fatty acid tails of phospholipids. * **Option C (Covalent bonds with cholesterol):** While cholesterol modulates membrane fluidity and interacts with proteins via Van der Waals forces, integral proteins do not typically form covalent bonds with cholesterol to span the membrane. * **Option D (Unusually strong peptide bonds):** The peptide bonds in membrane proteins are chemically identical to those in cytosolic proteins. Stability is derived from hydrophobic interactions and hydrogen bonding, not "stronger" covalent bonds. **3. High-Yield Facts for NEET-PG:** * **Hydropathy Plots:** Used to predict transmembrane segments by identifying sequences of 20+ hydrophobic amino acids. * **Glycophorin:** A classic example of a single-pass (Type I) transmembrane protein. * **G-Protein Coupled Receptors (GPCRs):** The most famous "serpentine" proteins, characterized by **7 transmembrane alpha-helices**. * **Porins:** An exception to the rule; these are found in the outer mitochondrial membrane and use **beta-barrel** configurations rather than alpha-helices.

Question 63: Which of the following phospholipids is predominantly found in the inner mitochondrial membrane?

A. Cardiolipin (Correct Answer)
B. Cephalin
C. Lecithin
D. Sphingomyelin

Explanation: **Explanation:** The correct answer is **Cardiolipin** (Diphosphatidylglycerol). **1. Why Cardiolipin is correct:** Cardiolipin is a unique phospholipid primarily localized in the **inner mitochondrial membrane (IMM)**, where it constitutes about 20% of the total lipid composition. Structurally, it consists of two phosphatidic acids joined by a glycerol bridge, giving it four fatty acid chains. Its primary physiological role is to stabilize the protein complexes of the **Electron Transport Chain (ETC)** and maintain the membrane's impermeability to protons, which is essential for oxidative phosphorylation. **2. Why the other options are incorrect:** * **Cephalin (Phosphatidylethanolamine):** While found in mitochondria, it is a major component of all biological membranes (especially nervous tissue) and is not specific to the IMM. * **Lecithin (Phosphatidylcholine):** This is the most abundant phospholipid in the eukaryotic cell membrane and the outer mitochondrial membrane, but it does not define the IMM. * **Sphingomyelin:** This is a major component of the **myelin sheath** and the plasma membrane (outer leaflet). It is notably absent or present in only trace amounts in mitochondrial membranes. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Barth Syndrome:** An X-linked genetic disorder caused by a mutation in the *TAZ* gene (encoding Tafazzin), leading to abnormal cardiolipin metabolism. It presents with cardiomyopathy, skeletal myopathy, and neutropenia. * **Antiphospholipid Antibody Syndrome (APS):** Cardiolipin is highly antigenic; anti-cardiolipin antibodies are a key diagnostic marker for APS, which is characterized by recurrent thrombosis and pregnancy loss. * **Origin:** The presence of cardiolipin in mitochondria (and its absence in other eukaryotic membranes) supports the **Endosymbiotic Theory**, as cardiolipin is also found in bacterial membranes.

Question 64: Which of the following is NOT an enzymatic marker of the plasma membrane?

A. 5'-nucleotidase
B. Adenylyl cyclase
C. Na+-K+-ATPase
D. Galactosyl transferase (Correct Answer)

Explanation: **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.

Question 65: What is ABC transporter?

A. P. glycoprotein (Correct Answer)
B. Membrane sparing
C. Channel
D. Adenylyl cyclase

Explanation: ### Explanation **ABC Transporters (ATP-Binding Cassette transporters)** represent one of the largest families of membrane transport proteins. They utilize the energy derived from **ATP hydrolysis** to move substrates (such as ions, lipids, and drugs) across cellular membranes against a concentration gradient. **Why P-glycoprotein is the correct answer:** **P-glycoprotein (P-gp)**, also known as **MDR1** (Multidrug Resistance Protein 1), is the most well-characterized member of the ABC transporter superfamily (ABCB1). It functions as an ATP-dependent efflux pump. In clinical medicine, it is notorious for pumping chemotherapeutic drugs out of cancer cells, leading to multidrug resistance. **Analysis of Incorrect Options:** * **Membrane sparing:** This is not a recognized functional classification of a transporter. It may be confused with "protein-sparing" actions in metabolism. * **Channel:** Channels (like aquaporins or ion channels) facilitate **passive transport** (down a gradient) and do not require ATP hydrolysis, whereas ABC transporters are primary **active transporters**. * **Adenylyl cyclase:** This is an enzyme (not a transporter) that converts ATP to cyclic AMP (cAMP) as part of G-protein signaling pathways. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cystic Fibrosis:** The **CFTR** (Cystic Fibrosis Transmembrane Conductance Regulator) is a unique ABC transporter that functions as a chloride channel. Mutations here cause Cystic Fibrosis. 2. **Tangier Disease:** Caused by a mutation in **ABCA1**, leading to a deficiency in cholesterol efflux and very low HDL levels. 3. **Dubin-Johnson Syndrome:** Caused by a mutation in **MRP2** (ABCC2), an ABC transporter responsible for secreting conjugated bilirubin into the bile. 4. **Adrenoleukodystrophy:** Linked to mutations in **ABCD1**, which transports very-long-chain fatty acids (VLCFA) into peroxisomes.

Question 66: Trimming and further modification of N-linked glycoproteins takes place at which cellular organelle?

A. Golgi apparatus (Correct Answer)
B. Endoplasmic reticulum
C. Peroxisomes
D. Lysosomes

Explanation: **Explanation:** The synthesis of N-linked glycoproteins is a highly coordinated process occurring in two distinct stages across the Endomembrane system. **1. Why Golgi Apparatus is Correct:** While the initial core glycosylation begins in the Rough Endoplasmic Reticulum (RER), the **Golgi apparatus** is the primary site for the **trimming** of mannose residues and the **further modification** (addition of galactose, sialic acid, and fucose). This "post-translational processing" occurs as proteins move from the *cis* to the *trans* Golgi cisternae, resulting in complex or hybrid oligosaccharide chains. **2. Analysis of Incorrect Options:** * **Endoplasmic Reticulum (ER):** This is the site of **initial** N-glycosylation. A pre-formed 14-sugar precursor (GlcNAc, mannose, and glucose) is transferred from a **Dolichol** lipid carrier to the Asparagine residue of the protein. Only minor trimming of glucose occurs here. * **Peroxisomes:** These are involved in long-chain fatty acid oxidation (VLCFA) and hydrogen peroxide metabolism, not protein glycosylation. * **Lysosomes:** These organelles are responsible for the **degradation** of glycoproteins via acid hydrolases, not their synthesis or modification. **3. NEET-PG High-Yield Pearls:** * **N-linked glycosylation** occurs on the **Asparagine** residue (Asn-X-Ser/Thr motif). * **O-linked glycosylation** occurs exclusively in the **Golgi** on Serine or Threonine residues. * **I-Cell Disease:** A high-yield clinical correlate where a deficiency in *phosphotransferase* in the Golgi fails to phosphorylate mannose residues. This prevents proteins from being targeted to lysosomes, leading to inclusion bodies and skeletal deformities. * **Tunicamycin** is an antibiotic that inhibits N-linked glycosylation by blocking the synthesis of the Dolichol-PP-GlcNAc intermediate.

Question 67: Specific cell surface proteins, such as alkaline phosphatase and lipoprotein lipase, are anchored to the cell membrane by covalent binding through an oligosaccharide bridge to a component of the cell membrane. What is this component?

A. Sphingomyelin
B. Phosphatidic acid
C. Phosphatidylserine
D. Phosphatidylinositol (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Phosphatidylinositol** The proteins mentioned (alkaline phosphatase and lipoprotein lipase) are examples of **GPI-anchored proteins**. These proteins are not transmembrane; instead, they are tethered to the outer leaflet of the plasma membrane via a **Glycosylphosphatidylinositol (GPI) anchor**. The structure of this anchor consists of a phosphoethanolamine unit linked to a core oligosaccharide, which is in turn covalently bonded to **Phosphatidylinositol (PI)**. The fatty acid chains of the phosphatidylinositol embed into the lipid bilayer, effectively "anchoring" the protein to the cell surface. This mechanism allows for high mobility of the protein and rapid release from the membrane via specific phospholipases (like Phospholipase C). **Analysis of Incorrect Options:** * **A. Sphingomyelin:** A major structural lipid in the myelin sheath and plasma membranes, but it does not form covalent bridges to anchor surface proteins. * **B. Phosphatidic acid:** A precursor for the synthesis of many glycerophospholipids. It lacks the complex carbohydrate structure required for protein anchoring. * **C. Phosphatidylserine:** Primarily located on the inner leaflet of the plasma membrane. Its translocation to the outer leaflet is a hallmark signal for **apoptosis** (programmed cell death). **High-Yield Clinical Pearls for NEET-PG:** * **Paroxysmal Nocturnal Hemoglobinuria (PNH):** This clinical condition is caused by a deficiency in the **PIGA gene**, which is essential for synthesizing the GPI anchor. Without the anchor, cells lack protective proteins like **CD55 (DAF)** and **CD59 (MIRL)**, leading to complement-mediated hemolysis. * **Marker Enzyme:** Alkaline phosphatase is a classic marker for the plasma membrane due to its GPI-anchored nature. * **Cleavage:** GPI-anchored proteins can be specifically released by **Phospholipase C (PLC)**.

Question 68: Which sequence is responsible for retaining proteins in the membrane?

A. Translocon
B. Sec 61 complex
C. Docking protein
D. Halt signal (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Halt signal** The **Halt signal** (also known as the **Stop-Transfer Anchor Sequence**) is a stretch of approximately 20 hydrophobic amino acids within a nascent polypeptide chain. During protein synthesis on the Rough Endoplasmic Reticulum (RER), this sequence is recognized by the translocon. Instead of passing through into the ER lumen, the hydrophobic nature of the halt signal causes it to get "stuck" or anchored within the phospholipid bilayer. This mechanism effectively stops further translocation and ensures the protein remains embedded as a **transmembrane protein**. **Analysis of Incorrect Options:** * **A. Translocon:** This is the protein-lined channel (aqueous pore) in the ER membrane through which the polypeptide chain passes. It is the *structure* that facilitates transport, not the specific *sequence* that triggers retention. * **B. Sec 61 complex:** This is the major structural component of the eukaryotic translocon. While it forms the "gate" through which proteins move, it is a protein complex, not a signaling sequence within the polypeptide. * **C. Docking protein:** Also known as the **SRP receptor**, this protein is located on the ER membrane. Its role is to bind the Signal Recognition Particle (SRP)-ribosome complex to the ER surface. It initiates the process but does not determine membrane retention. **High-Yield Clinical Pearls for NEET-PG:** * **Signal Hypothesis:** Proposed by Günter Blobel (Nobel Prize), stating that proteins have intrinsic signals for targeting. * **Signal Peptidase:** The enzyme that cleaves the N-terminal signal sequence once the protein enters the ER lumen. * **KDEL Sequence:** A specific C-terminal sequence (Lys-Asp-Glu-Leu) responsible for the **retrieval** of proteins back to the ER from the Golgi, preventing their secretion. * **I-Cell Disease:** A clinical correlation where a deficiency in the targeting signal (Mannose-6-Phosphate) leads to lysosomal enzymes being secreted extracellularly instead of being directed to lysosomes.

Question 69: A bilayer cell membrane contains which of the following components?

A. Cholesterol (Correct Answer)
B. Triacylglycerol
C. Cholesterol ester
D. Glycerol

Explanation: ### 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**.

Question 70: What are the transmembrane adhesive molecules present in hemidesmosomes that specifically bind to the basal lamina glycoprotein laminin?

A. integrin α6-β4 (Correct Answer)
B. the catenins desmoplakin
C. cadherins desmoglein
D. α and β catenin

Explanation: **Explanation:** **1. Why Option A is Correct:** Hemidesmosomes are specialized junctional complexes that anchor the basal surface of epithelial cells to the underlying basement membrane (basal lamina). The primary transmembrane proteins in hemidesmosomes are **integrins**, specifically **integrin α6β4**. Unlike other integrins that bind to the extracellular matrix via actin filaments, α6β4 connects intracellularly to **keratin intermediate filaments** (via plectin and BP180) and extracellularly to **laminin-332** in the basal lamina. This linkage is crucial for maintaining dermo-epidermal stability. **2. Why Other Options are Incorrect:** * **Option B (Desmoplakin):** This is an intracellular **plaque protein** found in desmosomes (not hemidesmosomes) that links cadherins to intermediate filaments. It is not a transmembrane molecule. * **Option C (Desmoglein):** This is a transmembrane protein belonging to the **cadherin** family. It is found in **desmosomes** (cell-to-cell junctions), not hemidesmosomes, and mediates adhesion between adjacent keratinocytes. * **Option D (α and β catenin):** These are intracellular signaling and anchoring proteins found in **Adherens junctions** (Zonula adherens). They link transmembrane E-cadherin to the **actin cytoskeleton**, not the basal lamina. **3. Clinical Pearls for NEET-PG:** * **Bullous Pemphigoid:** An autoimmune blistering disease where antibodies target **BP180 (Type XVII Collagen)** or **BP230** within the hemidesmosomes. It results in subepidermal blisters. * **Pemphigus Vulgaris:** Antibodies target **Desmoglein 3** (desmosomes), leading to intraepidermal blisters (acantholysis). * **Mnemonic:** **H**emidesmosomes help cells stick to the **H**ypodermis (basal lamina); **D**esmosomes stick **D**ouble cells together.

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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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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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