Membrane Biochemistry — MCQs

Membrane Biochemistry Indian Medical PG Practice Questions and MCQs

Question 81: The approximate composition of proteins in a plasma cell membrane is

A. 75%
B. 55% (Correct Answer)
C. 25%
D. 13%

Explanation: ***55%*** - Proteins constitute approximately **50% of the mass** of most plasma membranes, making 55% the closest approximation among the given options. - The exact percentage varies slightly between different cell types, typically ranging from **45-55%**. - Membrane proteins are crucial for a variety of **membrane functions** including transport, enzymatic activity, and cell signaling. *75%* - While some specialized membranes, like the **inner mitochondrial membrane**, contain a higher percentage of protein (around 75-80%), this is not typical for the general plasma cell membrane. - A protein content of 75% would mean a significantly lower lipid content than is standard for a typical cell membrane. *25%* - This percentage is too low for the protein content of a typical plasma cell membrane. - This value is more characteristic of **myelin**, which is a specialized lipid-rich membrane. - Such low protein content would severely limit the membrane's ability to perform necessary functions like **active transport** and **receptor binding**. *13%* - A protein composition of 13% is significantly lower than that found in any functional plasma membrane. - This would result in a membrane almost entirely composed of **lipids**, lacking the crucial protein components for cell viability.

Question 82: Cell-matrix adhesions are mediated by?

A. Integrins (Correct Answer)
B. Selectins
C. Calmodulin
D. Cadherins

Explanation: ***Integrins*** - **Integrins** are transmembrane receptors that mediate cell adhesion to the **extracellular matrix (ECM)**, linking it to the cell's cytoskeleton. - They bind to various ECM components like **fibronectin**, **collagen**, and **laminin**. *Cadherins* - **Cadherins** are primarily involved in **cell-to-cell adhesion**, forming junctions like **adherens junctions** and **desmosomes**. - They are **calcium-dependent adhesion molecules** that do not directly bind to the extracellular matrix. *Selectins* - **Selectins** are cell adhesion molecules involved in **leukocyte rolling** and **adhesion to endothelial cells** during inflammation. - They mediate **transient cell-to-cell interactions**, not cell-matrix adhesion. *Calmodulin* - **Calmodulin** is a **calcium-binding protein** that acts as a signal transducer, regulating various intracellular processes. - It is involved in **calcium-dependent signaling pathways** and enzyme activation, not cell adhesion.

Question 83: Which of the following cytoplasmic proteins associates with caveolin to stabilize caveolae at the plasma membrane?

A. Desmin
B. Clathrin
C. Cavin (Correct Answer)
D. Dynamin

Explanation: ***Cavin*** - **Cavin proteins** (e.g., Cavin-1/PTRF, Cavin-2, Cavin-3) are **cytoplasmic coat proteins** that bind to the cytoplasmic face of caveolae and are essential for **caveolae formation and stabilization**. - They work together with **caveolin** (an integral membrane protein) to form functional caveolae, which are small **flask-shaped invaginations** of the plasma membrane involved in signal transduction, lipid regulation, and endocytosis. - Loss of cavin proteins leads to loss of caveolae, demonstrating their critical structural role. *Desmin* - **Desmin** is an intermediate filament protein primarily found in **muscle cells**, where it forms part of the cytoskeleton. - It plays a structural role in maintaining the integrity of myofibrils and connecting them to the sarcolemma, but it is **not involved in caveolae formation or stabilization**. *Dynamin* - **Dynamin** is a **GTPase** enzyme crucial for the **scission (pinching off)** of newly formed endocytic vesicles from the plasma membrane. - While it participates in caveolae-mediated endocytosis during vesicle budding, it does not serve as a structural component or stabilizer of caveolae. *Clathrin* - **Clathrin** forms a **cage-like lattice** around vesicles during **clathrin-mediated endocytosis**, a distinct endocytic pathway. - Caveolae are **clathrin-independent** structures that rely on the caveolin-cavin system rather than clathrin coating.

Question 84: Which of the following tissues contains keratan sulfate I and II?

A. Cornea
B. Cornea and cartilage (Correct Answer)
C. Loose connective tissue
D. Cartilage

Explanation: ***Cornea and cartilage*** - **Keratan sulfate I (KS-I)** is predominantly found in the **cornea**, where it is N-linked to asparagine residues and helps maintain corneal transparency and hydration. - **Keratan sulfate II (KS-II)** is predominantly found in **cartilage**, where it is O-linked to serine/threonine residues and contributes to the tissue's ability to withstand compressive forces. - Since the question asks for tissues containing **both KS-I and KS-II**, the correct answer must include **both cornea (for KS-I) and cartilage (for KS-II)**. *Cartilage* - **Cartilage** primarily contains **keratan sulfate II (KS-II)**, not KS-I. - While KS-II is abundant in cartilage, this option does not account for **KS-I**, which is found in the cornea. - This is an incomplete answer as it only covers one type of keratan sulfate. *Cornea* - The **cornea** is rich in **keratan sulfate I (KS-I)**, which plays a crucial role in maintaining its transparency and proper hydration. - However, the cornea does not contain significant amounts of **keratan sulfate II (KS-II)**. - This is an incomplete answer as it only covers one type of keratan sulfate. *Loose connective tissue* - **Loose connective tissue** contains various glycosaminoglycans (GAGs) like hyaluronic acid, dermatan sulfate, and heparan sulfate. - **Keratan sulfates (KS-I or KS-II)** are not typical or abundant components of loose connective tissue. - Its primary GAG profile does not include significant keratan sulfate content.

Question 85: Maximum fluidity of the cell membrane is due to?

A. palmitic acid
B. alpha-linolenic acid
C. arachidonic acid (Correct Answer)
D. linoleic acid (18:2)

Explanation: ***arachidonic acid*** - **Arachidonic acid** is a polyunsaturated fatty acid (PUFA) with 20 carbons and 4 double bonds, denoted as C20:4. The presence of multiple **double bonds** creates kinks in the fatty acid tails, preventing tight packing of phospholipids in the membrane and thus increasing fluidity. - Cell membrane fluidity is enhanced by **unsaturated fatty acids** due to the presence of C=C double bonds. The more double bonds a fatty acid has (i.e., higher degree of unsaturation), the greater its contribution to membrane fluidity. Since arachidonic acid has the most double bonds among the options, it confers the greatest fluidity. *alpha-linolenic acid* - **Alpha-linolenic acid** is an omega-3 fatty acid (C18:3). While it is a **polyunsaturated fatty acid** and contributes to fluidity, it has fewer double bonds (3) than arachidonic acid (4), making it less effective in maximizing fluidity compared to arachidonic acid. - Its presence increases membrane fluidity, but not to the same extent as fatty acids with a higher degree of unsaturation. *linoleic acid (18:2)* - **Linoleic acid** (C18:2) is an omega-6 fatty acid with two double bonds. It contributes to membrane fluidity because it is unsaturated, but **less so than fatty acids with more double bonds** such as alpha-linolenic acid or arachidonic acid. - The fewer double bonds mean the fatty acid tails can pack a bit more closely, offering less fluidity compared to highly unsaturated fatty acids. *palmitic acid* - **Palmitic acid** is a **saturated fatty acid** (C16:0), meaning it has no double bonds. Saturated fatty acids have straight hydrocarbon chains that can pack tightly together in the cell membrane. - This tight packing **reduces membrane fluidity** and makes the membrane more rigid, which is the opposite of what maximizes fluidity.

Question 86: Eukaryotic plasma membrane is made up of all the following components except:

A. Lecithin
B. Cholesterol
C. Carbohydrates
D. Triglycerides (Correct Answer)

Explanation: ***Triglycerides*** - **Triglycerides** are primarily energy storage molecules and are not a structural component of the eukaryotic plasma membrane. - While they can be transported in the blood, they do not form part of the **lipid bilayer** itself. *Carbohydrates* - **Carbohydrates** are present on the outer surface of the plasma membrane, forming the **glycocalyx**, and are involved in cell recognition and adhesion. - They are typically attached to proteins (**glycoproteins**) or lipids (**glycolipids**). *Lecithin* - **Lecithin** is a common name for phosphatidylcholine, which is a major **phospholipid** and a fundamental component of the lipid bilayer in eukaryotic plasma membranes. - It plays a crucial role in maintaining membrane fluidity and integrity. *Cholesterol* - **Cholesterol** is an essential component of eukaryotic plasma membranes, where it modulates the **fluidity and stability** of the lipid bilayer. - It inserts itself between phospholipids, preventing them from becoming too rigid or too fluid.

Question 87: Membrane proteins are synthesized in which part of the cell?

A. Nucleolus
B. Mitochondria
C. Cytosolic ribosome
D. Rough endoplasmic reticulum ribosome (Correct Answer)

Explanation: ***Rough endoplasmic reticulum ribosome*** - **Membrane proteins** destined for the cell membrane, lysosomes, or secretion are synthesized on **ribosomes attached to the rough endoplasmic reticulum (RER)**. - As these proteins are synthesized, they enter the **lumen of the RER** where they undergo folding, modification, and assembly, and are then directed to their final destination. *Cytosolic ribosome* - **Cytosolic ribosomes** synthesize proteins that function in the **cytosol** itself, such as enzymes involved in glycolysis. - They also synthesize proteins destined for the **nucleus**, peroxisomes, and mitochondria. *Nucleolus* - The **nucleolus** is primarily involved in the synthesis and assembly of **ribosomal RNA (rRNA)** and ribosomal subunits. - It does not directly synthesize membrane proteins. *Mitochondria* - **Mitochondria** have their own ribosomes and genetic material, and they synthesize a small number of their **own proteins** that are essential for their function. - However, the vast majority of mitochondrial proteins, and all other membrane proteins, are synthesized in the cytosol or on the RER.

Question 88: Which component constitutes the highest percentage in the cell membrane?

A. Cholesterol
B. Carbohydrates
C. Phospholipids
D. Proteins (Correct Answer)

Explanation: ***Proteins*** - **Proteins** constitute the largest percentage of the cell membrane **by mass**, typically around **50%** or more. - While phospholipids are more numerous as individual molecules, protein molecules are much larger and heavier, making them the dominant component by weight. - These proteins are crucial for various functions, including **transport**, **receptor activity**, **enzymatic reactions**, and **cell adhesion**. *Phospholipids* - **Phospholipids** are the most abundant molecules **by number** in the cell membrane and form the **lipid bilayer** structural framework. - However, they account for a smaller percentage of the total **mass** (~40-45%) compared to proteins because individual phospholipid molecules are much smaller than protein molecules. - Many phospholipid molecules are needed to equal the mass of relatively few large protein molecules. *Cholesterol* - **Cholesterol** is an important component of animal cell membranes, contributing to membrane **fluidity** and **stability**. - It makes up approximately **20-25%** of membrane lipids but a smaller percentage of total membrane mass compared to both proteins and phospholipids. *Carbohydrates* - **Carbohydrates** are found on the outer surface of the cell membrane as **glycoproteins** and **glycolipids**, forming the glycocalyx. - They are involved in **cell recognition** and **adhesion** but constitute the **smallest percentage** of the cell membrane's mass (~2-10%).

Question 89: Clathrin is used in:

A. Receptor-mediated endocytosis (Correct Answer)
B. Exocytosis
C. Cell to cell adhesion
D. Plasma membrane

Explanation: ***Receptor-mediated endocytosis*** - **Clathrin** forms a **triskelion structure** that assembles into a polyhedral cage, driving the budding of vesicles from the plasma membrane for specific uptake of extracellular molecules. - This is the **primary and classical function** of clathrin, crucial for the uptake of various molecules like **low-density lipoproteins (LDLs)**, transferrin, and specific hormones after binding to their respective receptors. - **Clathrin-mediated endocytosis (CME)** is one of the most important and well-characterized cellular uptake mechanisms. *Exocytosis* - Exocytosis is the process by which cells release substances to the outside, typically involving the fusion of vesicles with the plasma membrane. - While clathrin's **primary role is in endocytosis**, it can play a **secondary role** in retrieving membrane components after exocytosis, particularly in synaptic vesicle recycling. - However, exocytosis itself is primarily mediated by other proteins (SNAREs, synaptotagmin) that facilitate vesicle fusion, making this an incorrect answer for clathrin's main function. *Cell to cell adhesion* - **Cell-to-cell adhesion** is mediated by various cell adhesion molecules such as **cadherins**, integrins, and selectins, which form junctions between cells. - **Clathrin** is primarily involved in vesicle formation and intracellular trafficking, not in direct cell-to-cell attachment. *Plasma membrane* - The **plasma membrane** is the outer boundary of the cell, composed of a **lipid bilayer** and embedded proteins, regulating what enters and leaves the cell. - While clathrin acts at the plasma membrane during endocytosis, it is a **protein that assists in membrane budding**, not a structural component of the membrane itself.

Question 90: What is the main component of a bilayer cell membrane?

A. Cholesterol ester
B. Triacyl glycerol
C. Cholesterol
D. Phospholipids (Correct Answer)

Explanation: ***Correct: Phospholipids*** - **Phospholipids** are the primary structural components of cell membranes, forming a **bilayer** due to their amphipathic nature. - The **hydrophilic heads** face the aqueous environment, while the **hydrophobic tails** form the core of the membrane. *Incorrect: Cholesterol* - **Cholesterol** is an important component of animal cell membranes, contributing to fluidity and stability, but it is not the **main structural component**. - It inserts between phospholipids, modulating membrane fluidity by preventing the tight packing of fatty acid tails at lower temperatures and hindering excessive movement at higher temperatures. *Incorrect: Cholesterol ester* - **Cholesterol esters** are storage forms of cholesterol and are primarily found in intracellular lipid droplets or associated with lipoproteins in the bloodstream. - They are generally too **hydrophobic** to be significant structural components within the phospholipid bilayer itself. *Incorrect: Triacyl glycerol* - **Triacylglycerols** (triglycerides) are the primary form of **energy storage** in cells, found in lipid droplets within the cytoplasm. - They are highly **hydrophobic** and do not form a structural part of the cell membrane bilayer.

Practice by Chapter

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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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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