Membrane Biochemistry — MCQs

Membrane Biochemistry Indian Medical PG Practice Questions and MCQs

Question 31: The transmembrane region of a protein is likely to have which characteristic?

A. A stretch of hydrophilic amino acids
B. A stretch of hydrophobic amino acids (Correct Answer)
C. A disulphide loop
D. Alternating hydrophilic and hydrophobic amino acids

Explanation: **Explanation:** The biological membrane is a lipid bilayer with a central core composed of non-polar fatty acid tails. For a protein to span this membrane (transmembrane proteins), the portion embedded within the bilayer must be energetically compatible with this non-polar environment. **1. Why Option B is Correct:** The transmembrane domain consists of a **stretch of hydrophobic (non-polar) amino acids** (e.g., Leucine, Isoleucine, Valine, Phenylalanine). These residues interact favorably with the hydrophobic lipid tails via van der Waals forces. Most commonly, these segments form an **alpha-helix**, where approximately 20–25 hydrophobic amino acids are required to span the 30Å thickness of the lipid bilayer. **2. Why Other Options are Incorrect:** * **Option A:** A stretch of hydrophilic amino acids would be energetically unfavorable within the lipid core; these are typically found in the extra-cytoplasmic or cytosolic domains. * **Option C:** Disulphide loops (cysteine-cysteine bonds) are primarily found in the extracellular domains of proteins to stabilize their 3D structure; the reducing environment of the cytosol generally prevents their formation. * **Option D:** Alternating residues are characteristic of amphipathic structures (like beta-barrels in porins), but a continuous hydrophobic stretch is the classic hallmark of a standard transmembrane alpha-helix. **High-Yield Clinical Pearls for NEET-PG:** * **Hydropathy Plots:** These are used to predict transmembrane segments by identifying long sequences of hydrophobic amino acids. * **Glycophorin:** A classic example of a single-pass transmembrane protein found in RBCs. * **GPCRs:** These are "serpentine" receptors with **seven** transmembrane hydrophobic alpha-helices. * **Stop-Transfer Sequence:** This is the hydrophobic signal that halts translocation through the ER membrane, anchoring the protein as a transmembrane entity.

Question 32: Which of the following is a marker of the plasma membrane?

A. Galactosyl transferase
B. 5-Nucleotidase (Correct Answer)
C. Adenylyl cyclase
D. ATP synthase

Explanation: **Explanation:** Cellular organelles possess specific enzymes known as **marker enzymes**, which are uniquely localized to their respective membranes or matrices. These markers are essential in biochemistry for identifying and assessing the purity of isolated subcellular fractions during centrifugation. **Why 5-Nucleotidase is the correct answer:** **5-Nucleotidase** is a classic marker enzyme for the **plasma membrane**. It is an ecto-enzyme (located on the outer surface) that catalyzes the hydrolysis of nucleoside 5′-monophosphates (like AMP) into nucleosides and inorganic phosphate. Along with **Na⁺-K⁺ ATPase** and **Alkaline Phosphatase**, it is frequently used to identify plasma membrane fractions. **Analysis of Incorrect Options:** * **A. Galactosyl transferase:** This is the specific marker for the **Golgi apparatus**. It plays a critical role in the glycosylation of proteins. * **C. Adenylyl cyclase:** While found in the plasma membrane, it is not considered a "standard" diagnostic marker because its activity is highly regulated by hormones and G-proteins, making it less reliable for quantifying membrane yield compared to 5-Nucleotidase. * **D. ATP synthase (Complex V):** This is the marker for the **Inner Mitochondrial Membrane**. It is responsible for oxidative phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondria:** Marker for Matrix is *Isocitrate Dehydrogenase*; Inner Membrane is *ATP synthase/Cytochrome oxidase*. * **Lysosomes:** Marker is *Acid Phosphatase*. * **Peroxisomes:** Marker is *Catalase* or *Urate oxidase*. * **Endoplasmic Reticulum:** Marker is *Glucose-6-Phosphatase* (Smooth ER) or *Cytochrome P450*. * **Clinical Correlation:** Serum levels of 5-Nucleotidase are elevated in **cholestatic liver diseases** (similar to Alkaline Phosphatase), helping to differentiate hepatobiliary disease from bone disease.

Question 33: Cell membrane rafts are enriched in which of the following components?

A. Cholesterol (Correct Answer)
B. Linoleic acid
C. Proteins
D. None of the above

Explanation: ### Explanation **Concept Overview:** Lipid rafts are specialized, highly ordered microdomains within the cell membrane that act as platforms for signal transduction and protein trafficking. Unlike the surrounding "fluid" mosaic membrane, these rafts are tightly packed and less fluid. **Why Cholesterol is Correct:** Cholesterol is a fundamental structural component of lipid rafts. It acts as a "dynamic glue" that fills the gaps between the long, saturated fatty acid chains of **sphingolipids** (specifically sphingomyelin and glycosphingolipids). This interaction creates a "liquid-ordered" state. Without high concentrations of cholesterol, these microdomains would lose their structural integrity and fail to organize signaling proteins. **Why Other Options are Incorrect:** * **Linoleic Acid:** This is a polyunsaturated fatty acid (PUFA). PUFAs have "kinks" in their hydrocarbon chains that increase membrane fluidity and prevent tight packing. Therefore, they are typically **excluded** from lipid rafts, which favor saturated fatty acids. * **Proteins:** While specific proteins (like GPI-anchored proteins) are *present* in rafts, the rafts are primarily defined by their unique **lipid composition** (cholesterol and sphingolipids). Proteins are passengers or functional components, but cholesterol is the primary enriching structural molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Lipid rafts are enriched in **Cholesterol**, **Sphingolipids**, and **GPI-anchored proteins**. * **Function:** They serve as hubs for **cell signaling** (e.g., T-cell receptor signaling) and entry points for certain pathogens (e.g., HIV, Cholera toxin). * **Detergent Resistance:** Because of their tight packing, lipid rafts are also known as **Detergent-Resistant Membranes (DRMs)** when treated with non-ionic detergents like Triton X-100 at low temperatures.

Question 34: The biological cell membrane contains all of the following except?

A. Steroid receptors (Correct Answer)
B. Prostaglandins
C. Cholesterol
D. Cytochrome

Explanation: **Explanation:** The correct answer is **Steroid receptors**. **1. Why Steroid Receptors are the correct answer:** Steroid hormones (such as cortisol, estrogen, and testosterone) are lipophilic molecules derived from cholesterol. Due to their lipid-soluble nature, they easily diffuse across the phospholipid bilayer of the cell membrane. Consequently, their receptors are located **intracellularly**—either in the **cytosol** or the **nucleus**—rather than on the cell membrane. Once the hormone binds to its receptor, the complex acts as a transcription factor to regulate gene expression. **2. Analysis of Incorrect Options:** * **Prostaglandins:** These are eicosanoids derived from arachidonic acid found in membrane phospholipids. They are synthesized at the membrane level and act via G-protein coupled receptors (GPCRs) located on the cell membrane. * **Cholesterol:** A fundamental structural component of eukaryotic cell membranes. It regulates membrane fluidity and stability, especially within "lipid rafts." * **Cytochrome:** Various cytochromes are integral components of biological membranes. For example, **Cytochrome P450** enzymes are found in the Smooth Endoplasmic Reticulum (SER) membrane, and **Cytochrome c oxidase** is located in the inner mitochondrial membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Exceptions to the Rule:** While most lipid-soluble hormones have intracellular receptors, **Thyroid hormones (T3/T4)** are unique; they are lipophilic but require specific membrane transporters to enter the cell. * **Membrane Proteins:** Remember that peripheral proteins are attached by electrostatic interactions, while integral proteins (like Cytochromes) are embedded via hydrophobic interactions. * **Fluid Mosaic Model:** Proposed by Singer and Nicolson, it defines the membrane as a fluid lipid bilayer with a "mosaic" of proteins.

Question 35: Which membrane phospholipid does not contain glycerol?

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

Explanation: **Explanation:** The fundamental classification of membrane lipids depends on their **alcohol backbone**. Most membrane phospholipids are **Glycerophospholipids**, meaning they use **glycerol** as the structural backbone. **Why Sphingomyelin is the Correct Answer:** Sphingomyelin is a **Sphingophospholipid**. Instead of glycerol, it contains **Sphingosine**, an 18-carbon amino alcohol. Structurally, it consists of a sphingosine backbone attached to a fatty acid (forming Ceramide) and a phosphorylcholine head group. It is the only significant membrane phospholipid that lacks a glycerol backbone. It is a major component of the **myelin sheath** in nervous tissue. **Analysis of Incorrect Options:** * **A. Lecithin (Phosphatidylcholine):** This is the most abundant glycerophospholipid in cell membranes. It consists of a glycerol backbone, two fatty acids, and a phosphate-choline group. * **C. Cardiolipin (Diphosphatidylglycerol):** Found exclusively in the **inner mitochondrial membrane**, it contains two molecules of phosphatidic acid linked by a glycerol bridge. It essentially has a "triple glycerol" structure. * **D. Ceramide:** While Ceramide also lacks glycerol (it is Sphingosine + Fatty acid), it is **not a phospholipid** because it lacks a phosphate group. It serves as the structural precursor for sphingomyelin and glycosphingolipids. **High-Yield NEET-PG Pearls:** * **Niemann-Pick Disease:** Caused by a deficiency of **Sphingomyelinase**, leading to the accumulation of sphingomyelin in reticuloendothelial cells (look for "Foamy histiocytes" and "Cherry-red spot" on the macula). * **L/S Ratio:** The ratio of Lecithin to Sphingomyelin in amniotic fluid is used to assess **fetal lung maturity** (Normal > 2). * **Cardiolipin:** It is the antigen used in the **VDRL test** for Syphilis. Anti-cardiolipin antibodies are a hallmark of Antiphospholipid Antibody Syndrome (APLA).

Question 36: Membrane fluidity is increased by:

A. Stearic acid
B. Palmitic acid
C. Cholesterol
D. Linoleic acid (Correct Answer)

Explanation: **Explanation:** Membrane fluidity is primarily determined by the fatty acid composition of the phospholipid bilayer. The correct answer is **Linoleic acid** because it is a polyunsaturated fatty acid (PUFA). **1. Why Linoleic Acid is Correct:** The fluidity of the cell membrane depends on how tightly the fatty acid tails are packed. Saturated fatty acids have straight chains that pack closely together, increasing membrane rigidity. In contrast, unsaturated fatty acids (like Linoleic acid) contain **"kinks"** caused by double bonds in the *cis* configuration. These kinks prevent tight packing, increasing the space between molecules and thereby **increasing membrane fluidity**. **2. Why the Other Options are Incorrect:** * **Stearic acid (18:0) & Palmitic acid (16:0):** These are long-chain saturated fatty acids. They lack double bonds, allowing them to pack tightly via Van der Waals forces, which **decreases** fluidity and increases the melting point of the membrane. * **Cholesterol:** In the context of the plasma membrane, cholesterol generally acts as a bidirectional regulator. However, at physiological temperatures, it inserts itself between phospholipids, restricting their movement and **decreasing** fluidity (stabilizing the membrane). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Length vs. Saturation:** Fluidity is **increased** by shorter chain lengths and a higher degree of unsaturation (more double bonds). * **Temperature Effect:** As temperature decreases, membranes become more rigid. Cells compensate by increasing the proportion of unsaturated fatty acids to maintain fluidity (Homeoviscous Adaptation). * **Clinical Correlation:** Altered membrane fluidity is seen in various pathologies, such as **Acanthocytosis** (spur cell anemia), where increased cholesterol-to-phospholipid ratios in RBC membranes lead to decreased fluidity and premature destruction.

Question 37: The fluid mosaic model of membrane structure was proposed by?

A. Watson and Crick
B. Edward Angle
C. G.V. Black
D. Singer and Nicolson (Correct Answer)

Explanation: **Explanation:** The **Fluid Mosaic Model**, proposed by **S.J. Singer and G.L. Nicolson in 1972**, is the most widely accepted model describing the structure of the plasma membrane. According to this model, the membrane is a "quasifluid" structure where proteins (the "mosaic") are embedded in or attached to a lipid bilayer. The "fluidity" allows for the lateral movement of proteins and lipids, which is essential for processes like cell signaling, membrane fusion, and molecular transport. **Analysis of Options:** * **Singer and Nicolson (Correct):** They revolutionized cell biology by describing the membrane as a dynamic, fluid bilayer of phospholipids with globular integral and peripheral proteins. * **Watson and Crick:** They are famous for discovering the **double-helix structure of DNA** in 1953, not cell membrane architecture. * **Edward Angle:** Known as the "Father of Modern Orthodontics," he developed classifications for malocclusion. * **G.V. Black:** Known as the "Father of Operative Dentistry," he established principles for cavity preparation. **High-Yield Facts for NEET-PG:** * **Membrane Fluidity:** Regulated by **cholesterol** (acts as a temperature buffer) and the ratio of unsaturated to saturated fatty acids. * **Asymmetry:** The membrane is asymmetrical; for example, **Phosphatidylserine** is normally restricted to the inner leaflet. Its appearance on the outer leaflet is a clinical marker for **apoptosis**. * **Integral Proteins:** These span the entire bilayer (e.g., Glycophorin, G-protein coupled receptors). * **Carbohydrates:** Always located on the **extracellular surface**, forming the glycocalyx.

Question 38: The fluid mosaic model was proposed by whom?

A. Singer and Nicolson (Correct Answer)
B. David Robeson
C. Meselson and Sahli
D. Robe Darwin

Explanation: **Explanation:** The **Fluid Mosaic Model**, proposed by **S.J. Singer and G.L. Nicolson in 1972**, is the most widely accepted model describing the structure of the plasma membrane. According to this model, the membrane is a "quasifluid" structure where proteins (the "mosaic") are embedded in or attached to a lipid bilayer (the "fluid"). This fluidity allows for the lateral movement of proteins and lipids, which is essential for functions like cell signaling, membrane fusion, and molecular transport. **Analysis of Options:** * **Singer and Nicolson (Correct):** They revolutionized cell biology by describing the membrane as a dynamic, fluid lipid bilayer with integral and peripheral proteins, replacing the older "sandwich" models. * **David Robeson:** This name is not associated with any major discovery in biochemistry or cell biology. * **Meselson and Stahl:** They are famous for the **Meselson-Stahl experiment (1958)**, which proved the **semi-conservative nature of DNA replication**. * **Robe Darwin:** Likely a distractor combining names; Charles Darwin is known for the theory of evolution, not membrane structure. **High-Yield Clinical Pearls for NEET-PG:** * **Membrane Fluidity:** Regulated by **cholesterol** (acts as a fluidity buffer) and the ratio of saturated to unsaturated fatty acids. * **Asymmetry:** The membrane is asymmetrical; **Phosphatidylserine** is normally restricted to the inner leaflet. Its appearance on the outer leaflet is a clinical marker for **apoptosis** (recognized by macrophages). * **Lipid Rafts:** Specialized microdomains rich in cholesterol and sphingolipids that serve as platforms for cell signaling.

Question 39: All of the following molecules, without further metabolism, can cross the inner membrane of functioning mitochondria, EXCEPT:

A. ATP
B. Acetyl CoA (Correct Answer)
C. Inorganic phosphate
D. Pyruvate

Explanation: The inner mitochondrial membrane (IMM) is highly selective and impermeable to most ions and polar molecules. This impermeability is essential for maintaining the electrochemical gradient required for oxidative phosphorylation. **Why Acetyl CoA is the correct answer:** Acetyl CoA is a large, polar molecule that **lacks a specific transporter** in the inner mitochondrial membrane. While it is produced inside the mitochondria (via PDH complex), it cannot cross into the cytosol directly for processes like fatty acid synthesis. Instead, it must first condense with oxaloacetate to form **Citrate**, which is then transported out via the tricarboxylate transporter. This is a high-yield concept often tested as the "Citrate Shuttle." **Explanation of incorrect options:** * **ATP:** Crosses the IMM via the **Adenine Nucleotide Translocase (ANT)**, which exports ATP to the cytosol in exchange for ADP. * **Inorganic Phosphate (Pi):** Enters the matrix via the **Phosphate Translocase** (a symporter that brings in Pi along with a proton). * **Pyruvate:** Crosses the IMM through the **Mitochondrial Pyruvate Carrier (MPC)** to enter the matrix for conversion into Acetyl CoA or Oxaloacetate. **High-Yield Clinical Pearls for NEET-PG:** * **The "Shuttle" Rule:** If a molecule cannot cross the IMM, it needs a shuttle. Examples: **Malate-Aspartate shuttle** (for NADH), **Carnitine shuttle** (for long-chain fatty acids), and **Citrate shuttle** (for Acetyl CoA). * **Oxaloacetate** is another key molecule that **cannot** cross the IMM directly; it must be converted to Malate or Aspartate first. * **Cardiolipin:** A unique phospholipid found in the IMM that decreases its permeability to small ions.

Question 40: Where is cardiolipin found?

A. Outer membrane of mitochondria
B. Inner membrane of mitochondria (Correct Answer)
C. Lysosomes
D. Rough endoplasmic reticulum

Explanation: **Explanation:** **Cardiolipin (Diphosphatidylglycerol)** is a unique phospholipid that contains four fatty acid chains and two phosphate groups. It is synthesized and localized almost exclusively in the **inner mitochondrial membrane (IMM)**, where it constitutes about 20% of the total lipid composition. **Why Option B is Correct:** Cardiolipin is essential for the structural integrity and optimal function of the **Electron Transport Chain (ETC)**. It acts as a "molecular glue" that stabilizes respiratory chain complexes (Complex I, III, and IV) into supercomplexes. It is also vital for the activity of ATP synthase and the ADP/ATP carrier protein. **Why Other Options are Incorrect:** * **Option A (Outer Mitochondrial Membrane):** While mitochondria contain cardiolipin, it is highly sequestered in the inner membrane. The outer membrane has a lipid profile more similar to the ER (rich in phosphatidylcholine). * **Option C (Lysosomes):** Lysosomes are characterized by a unique lipid called **bis(monoacylglycero)phosphate (BMP)**, which helps in the degradation of lipids, but they do not contain cardiolipin. * **Option D (Rough ER):** The ER is the primary site for the synthesis of most phospholipids (like Lecithin and Cephalin), but cardiolipin synthesis is unique because it occurs within the mitochondria itself. **High-Yield Clinical Pearls for NEET-PG:** 1. **Barth Syndrome:** An X-linked genetic disorder caused by a mutation in the *TAZ* gene (encoding Tafazzin), leading to abnormal cardiolipin metabolism. Clinical features include cardiomyopathy, skeletal myopathy, and neutropenia. 2. **Antiphospholipid Antibody Syndrome (APS):** Cardiolipin is highly antigenic. Anti-cardiolipin antibodies are a hallmark of APS, leading to recurrent thrombosis and pregnancy loss. 3. **Apoptosis:** During the early stages of apoptosis, cardiolipin translocates from the inner to the outer mitochondrial membrane, facilitating the release of Cytochrome C into the cytosol.

Practice by Chapter

Membrane Structure and Organization

Practice Questions

Membrane Lipids and Fluidity

Practice Questions

Membrane Proteins: Integral and Peripheral

Practice Questions

Transport Across Membranes

Practice Questions

Ion Channels and Transporters

Practice Questions

Sodium-Potassium ATPase

Practice Questions

Calcium Transport and Calcium ATPase

Practice Questions

Glucose Transporters

Practice Questions

Membrane Receptors and Signal Transduction

Practice Questions

Lipid Rafts and Caveolae

Practice Questions

Membrane Disorders and Diseases

Practice Questions

Biochemistry of Endocytosis and Exocytosis

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free