Membrane Biochemistry — MCQs

Membrane Biochemistry Indian Medical PG Practice Questions and MCQs

Question 71: Which of the following is NOT a component of the transport system?

A. Receptor activation (Correct Answer)
B. Selective gate
C. Non-selective channel
D. Energy coupling system

Explanation: ### Explanation The transport system refers to the mechanisms by which solutes (ions, molecules) move across the biological membrane. A functional transport system requires a pathway and, often, a driving force. **Why "Receptor Activation" is the correct answer:** Receptor activation is a component of **Signal Transduction**, not the transport system itself. While a ligand binding to a receptor may *trigger* the opening of a channel (e.g., ligand-gated ion channels), the act of "activation" is a signaling event. The transport system specifically involves the physical translocation of substances, whereas receptors transmit information across the membrane. **Analysis of Incorrect Options:** * **Selective Gate:** This is a hallmark of **facilitated diffusion** and **active transport**. Selectivity filters ensure that only specific ions (like $K^+$ or $Na^+$) pass through, maintaining cellular homeostasis. * **Non-selective Channel:** These allow the passage of molecules based primarily on size or charge without high specificity (e.g., **Porins** in the outer mitochondrial membrane or certain gap junctions). They are fundamental components of passive transport. * **Energy Coupling System:** This is essential for **Active Transport**. It couples the movement of a solute against its concentration gradient with an energy source, such as ATP hydrolysis (Primary Active Transport) or an electrochemical gradient (Secondary Active Transport). **High-Yield Clinical Pearls for NEET-PG:** * **GLUT Transporters:** Examples of facilitated diffusion (Uniport) that use selective gates but no energy coupling. * **SGLT-1/2:** Examples of Secondary Active Transport (Symport) that utilize energy coupling via the $Na^+$ gradient. * **ABC Transporters:** (e.g., CFTR protein) utilize ATP-binding cassettes; mutations in CFTR lead to **Cystic Fibrosis**, a classic transport system defect. * **Ionophores:** Substances like Valinomycin that increase membrane permeability to specific ions, disrupting the transport system’s selectivity.

Question 72: The signal sequence in a type 2 membrane protein with N-terminal facing cytoplasm is located in?

A. C-terminal
B. Middle of protein (Correct Answer)
C. Both N and C terminal
D. N-terminal

Explanation: ***Middle of protein*** - For a type 2 membrane protein with the **N-terminal facing the cytoplasm**, the **start-transfer sequence** (which acts as a signal sequence) is found in the **middle of the protein**. - This **internal signal sequence** allows for membrane integration with the correct orientation, often functioning as a **transmembrane domain**. *C-terminal* - A C-terminal signal is typically seen in **Type I membrane proteins** where the **N-terminus is in the ER lumen** and the C-terminus is in the cytoplasm. - This orientation requires a **stop-transfer sequence** in the middle of the protein. *Both N and C terminal* - While some proteins may have multiple signal sequences, a single protein typically utilizes **one dominant signal** for its initial targeting and membrane insertion. - Having both an N-terminal and C-terminal primary signal for integration would lead to **conflicting targeting signals** for this specific type of protein. *N-terminal* - An N-terminal signal sequence is characteristic of **Type I membrane proteins** or **secreted proteins**, guiding them to the ER and often being cleaved. - In a **type 2 protein**, with the **N-terminus in the cytoplasm**, an N-terminal signal would result in the N-terminus entering the ER lumen, contradicting the given orientation.

Question 73: Which membrane channel is mainly affected in Cystic fibrosis?

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

Explanation: ***Chloride*** - Cystic fibrosis is caused by a mutation in the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)** gene, which encodes a chloride channel. - Dysfunction of this **chloride channel** leads to impaired transport of chloride ions, mainly affecting epithelial cells in various organs. *Sodium* - While sodium transport is indirectly affected in cystic fibrosis, the primary defect is not in a sodium channel itself but rather in the **chloride channel**, which influences water and sodium movement. - The abnormal **chloride transport** leads to a compensatory but ineffective increase in sodium absorption in some tissues like the airway. *Calcium* - Calcium channels are not primarily implicated in the pathophysiology of **cystic fibrosis**. - **Calcium dysregulation** can occur secondarily in some CF-related processes, but it is not the main affected membrane channel. *Potassium* - **Potassium channels** are not the main membrane channels affected in cystic fibrosis. - While potassium transport is vital for cellular function, it is not the primary defect underlying the disease's respiratory and gastrointestinal manifestations.

Question 74: Which of the following is not amphipathic?

A. Triglycerides (Correct Answer)
B. Sphingolipids
C. Glycolipids
D. Phosphoglycerol

Explanation: ***Triglycerides*** - Triglycerides are composed of a **glycerol backbone** esterified to three fatty acids, making them entirely **hydrophobic** and thus not amphipathic. - They serve primarily as **energy storage** molecules and do not form membranes because they lack a polar head group. *Sphingolipids* - Sphingolipids are amphipathic because they contain a **hydrophilic polar head group** (e.g., phosphocholine or a sugar) and two **hydrophobic tails** derived from a fatty acid and the sphingosine backbone. - This dual nature allows them to be fundamental components of **cell membranes**. *Glycolipids* - Glycolipids are characterized by a **carbohydrate head group** attached to a lipid moiety, rendering them amphipathic. - The sugar portion is **hydrophilic**, while the lipid portion (e.g., ceramide) is **hydrophobic**, enabling them to participate in cell recognition and membrane stability. *Phosphoglycerol* - Phosphoglycerol (more commonly referred to as **glycerophospholipids**) are amphipathic, consisting of a **glycerol backbone**, two fatty acid tails, and a **phosphate group** often linked to an alcohol. - The **phosphate and alcohol group** form the hydrophilic head, and the **fatty acid tails** form the hydrophobic region, making them crucial for lipid bilayers.

Question 75: An aggregate of surfactant molecules or ions in solution is known as?

A. Micelle (Correct Answer)
B. Atom
C. Basion
D. Chilon

Explanation: ***Micelle*** - A **micelle** is an aggregate of **surfactant** molecules dispersed in a liquid colloid. - In an aqueous solution, the **hydrophilic "head" regions** of the surfactant molecules spontaneously arrange to face the surrounding solvent, while the **hydrophobic "tail" regions** cluster together in the micelle's center. *Atom* - An **atom** is the smallest unit of ordinary matter that forms a chemical element. - It consists of a nucleus of **protons and neutrons** surrounded by a cloud of electrons. *Basion* - The **basion** is a term used in anatomy, referring to the midpoint of the anterior margin of the **foramen magnum** of the skull. - It is a craniometric landmark used in various **cephalometric measurements**. *Chilon* - **Chilon** is not a recognized scientific term or concept related to molecular aggregates or biological structures. - It does not describe an aggregate of **surfactant molecules or ions in solution**.

Question 76: Degradation of a substance takes place in which part of the cell:

A. ER
B. Lysosome (Correct Answer)
C. Mitochondria
D. Nucleus

Explanation: ***Lysosome*** - **Lysosomes** are membrane-bound organelles that contain a variety of **hydrolytic enzymes**, such as proteases, lipases, nucleases, and glycosidases. - These enzymes are responsible for the **breakdown and degradation** of waste materials, cellular debris, and foreign substances within the cell. *ER* - The **endoplasmic reticulum (ER)** is involved in protein synthesis and folding, lipid metabolism, and detoxification, but not primary degradation. - While it has enzymes for certain metabolic transformations, it is not the main site for general substance degradation. *Mitochondria* - **Mitochondria** are primarily known for their role in **cellular respiration** and **ATP production**, serving as the powerhouse of the cell. - They also play a role in apoptosis and calcium homeostasis, but not directly in the degradation of cellular waste. *Nucleus* - The **nucleus** houses the cell's genetic material (**DNA**) and is involved in replication, transcription, and regulation of gene expression. - It does not contain the necessary enzymes for the degradation of general cellular substances.

Question 77: What is the primary role of cholesterol in the plasma membranes of animal cells?

A. Energy storage
B. Structural integrity and fluidity regulation (Correct Answer)
C. Influencing membrane signaling pathways
D. Facilitating protein synthesis

Explanation: ***Structural integrity and fluidity regulation*** - **Cholesterol** is amphipathic and inserts into the **lipid bilayer**, where it modulates membrane fluidity by preventing the tight packing of phospholipids at low temperatures and restricting excessive movement at high temperatures. - This dual role helps maintain the structural integrity and optimal function of the **plasma membrane** across various physiological conditions. *Energy storage* - While cholesterol is a lipid, its primary role in the plasma membrane is not **energy storage**. - **Triglycerides** are the main form of energy storage in animal cells. *Influencing membrane signaling pathways* - Although cholesterol can indirectly affect **membrane signaling** by influencing the organization of **lipid rafts** where signaling molecules often reside, its fundamental role is structural. - These signaling effects are secondary to its primary function in regulating membrane dynamics. *Facilitating protein synthesis* - **Protein synthesis** primarily occurs on **ribosomes** in the cytoplasm and endoplasmic reticulum, with the plasma membrane having no direct role in this process. - Cholesterol is not involved in the machinery of protein synthesis.

Question 78: Where are lipid rafts primarily located?

A. Nucleus
B. Mitochondrial membrane
C. Golgi apparatus
D. Plasma membrane (Correct Answer)

Explanation: ***Plasma membrane*** - **Lipid rafts** are dynamic microdomains within the **plasma membrane** that are enriched in cholesterol and sphingolipids. - They play crucial roles in **cell signaling**, **membrane trafficking**, and **endocytosis**. *Nucleus* - The **nucleus** contains the cell's genetic material and is involved in gene expression, not in forming lipid rafts. - Lipid rafts are components of cell membranes, not internal organelles like the nucleus. *Mitochondrial membrane* - The **mitochondrial membrane** is involved in cellular respiration and ATP production. - It has a distinct lipid composition and does not form lipid rafts in the same way as the plasma membrane. *Golgi apparatus* - The **Golgi apparatus** is involved in modifying, sorting, and packaging proteins and lipids. - While it processes membrane components, lipid rafts are primarily functional structures on the **plasma membrane**.

Question 79: Proteoglycan present in the glomerular basement membrane is?

A. Keratan sulfate 1
B. Keratan sulfate 2
C. Heparan sulfate (Correct Answer)
D. Chondroitin sulfate

Explanation: ***Heparan sulphate*** - Heparan sulphate is a key component of the **glomerular basement membrane** (GBM), crucial for its **negative charge and filtration function** [1][2]. - It plays a significant role in **filtration barrier** properties and affects the permeability of the GBM to proteins [1]. *Keratan sulphate 1* - Predominantly found in **cartilage** and **corneal tissue**, not associated with the glomerular basement membrane. - It contributes to **mechanical support** but lacks the essential role in renal filtration. *Keratan sulphate 2* - Similar to Keratan sulphate 1, this variant is involved in **cartilage** but not in the structure of the glomerular basement membrane. - Has distinct functions related to **tissue hydration** rather than the filtration dynamics of the GBM. *Chondroitin sulphate* - Commonly located in **cartilage** and connective tissues, it does not play a significant role in the structure of the glomerular basement membrane. - While it assists in **cell signaling** and regeneration, it does not influence the GBM permeability like heparan sulphate. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 905-907. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 34-35.

Question 80: In the context of cell membrane composition, what is the typical weight ratio of protein to lipid?

A. 1 : 2
B. 4 : 1
C. 1 : 1 (Correct Answer)
D. 2 : 1

Explanation: ***1 : 1*** - The **typical weight ratio of protein to lipid** in most cell membranes is approximately **1:1** (equal by weight). - While the **number of lipid molecules** far exceeds the number of protein molecules, proteins are much larger and heavier, resulting in roughly equal weight contributions. - This **1:1 ratio represents an average** for typical plasma membranes, though it can vary significantly depending on membrane type and function. *1 : 2* - This protein:lipid ratio would indicate **lipids contribute twice as much by weight** as proteins. - This is characteristic of **myelin membranes**, which are specialized for insulation and have exceptionally high lipid content. - However, this is **not typical** of most cell membranes. *2 : 1* - This ratio would suggest **proteins contribute twice as much by weight** as lipids. - While some protein-rich membranes exist, this is **higher than the average** for typical cell membranes. - The typical ratio is closer to 1:1 rather than being protein-dominant at 2:1. *4 : 1* - A 4:1 protein:lipid ratio represents an **extremely protein-rich membrane**. - This is characteristic of the **inner mitochondrial membrane**, which is packed with electron transport chain proteins. - This is a **specialized membrane**, not representative of typical cell membranes.

Practice by Chapter

Membrane Structure and Organization

Practice Questions

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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