Membrane Biochemistry Practice Questions

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

A stretch of hydrophobic amino acids. **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.

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

Cholesterol. ### 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.

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

Steroid receptors. **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.

Q: Which membrane phospholipid does not contain glycerol?

Sphingomyelin. **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).

Q: What is the function of cholesterol in the plasma membrane?

Decrease the fluidity of the lipid bilayer. **Explanation** **1. Why Option B is Correct:** Cholesterol is a crucial component of the eukaryotic plasma membrane, acting as a **"fluidity buffer."** At physiological temperatures (body temperature), cholesterol molecules intercalate between the fatty acid tails of phospholipids [1]. Its rigid steroid ring structure restricts the lateral movement of these phospholipids, thereby **decreasing the fluidity** and increasing the mechanical stability of the membrane [2]. This makes the membrane less permeable to small, water-soluble molecules. **2. Why the Other Options are Incorrect:** * **Option A:** While cholesterol prevents the membrane from becoming too rigid at *low* temperatures (by preventing tight packing), its primary role at body temperature is to stabilize the membrane and reduce excessive fluidity. * **Option C:** Ion diffusion is primarily regulated by **integral membrane proteins** (channels and pumps), not by cholesterol. In fact, by packing the lipid bilayer more tightly, cholesterol acts as a barrier to the free diffusion of polar substances. * **Option D:** Hormone transport (especially steroid hormones) occurs via simple diffusion through the lipid bilayer or via specific transporters. Cholesterol does not "assist" this transport; it is a precursor for steroid hormones but remains a structural component within the membrane itself [1]. **High-Yield NEET-PG Pearls:** * **Lipid Rafts:** Cholesterol is a key component of "lipid rafts," which are specialized microdomains that organize signaling proteins [2]. * **Ratio:** The plasma membrane typically has a 1:1 ratio of cholesterol to phospholipids. * **Prokaryotes:** Unlike eukaryotes, most bacterial membranes **lack cholesterol** (except *Mycoplasma*). * **Amphipathic Nature:** Cholesterol is amphipathic; its hydroxyl (-OH) group aligns with the phospholipid head groups, while the steroid body aligns with the tails.

Q: Membrane fluidity is increased by:

Linoleic acid. **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.

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

Acetyl CoA. 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 1

Which of the following statements accurately describes the effect of essential fatty acids on cell membranes?

Accepted Answer

All of the above

Answer

They increase the transition temperature of the lipid bilayer.

Answer

They decrease the fluidity of the lipid bilayer.

Answer

They decrease the transition temperature of the lipid bilayer.

Question 2

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

Accepted Answer

A stretch of hydrophobic amino acids

Answer

A stretch of hydrophilic amino acids

Answer

A disulphide loop

Answer

Alternating hydrophilic and hydrophobic amino acids

Question 3

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

Accepted Answer

Cholesterol

Answer

Linoleic acid

Answer

Proteins

Answer

None of the above

Question 4

The biological cell membrane contains all of the following except?

Accepted Answer

Steroid receptors

Answer

Prostaglandins

Answer

Cholesterol

Answer

Cytochrome

Question 5

Which membrane phospholipid does not contain glycerol?

Accepted Answer

Sphingomyelin

Answer

Lecithin

Answer

Cardiolipin

Answer

Ceramide

Question 6

What is the function of cholesterol in the plasma membrane?

Accepted Answer

Decrease the fluidity of the lipid bilayer

Answer

Increase the fluidity of the lipid bilayer

Answer

Facilitate the diffusion of ions into the lipid bilayer

Answer

Assist in the transport of hormones across the lipid bilayer

Question 7

Membrane fluidity is increased by:

Accepted Answer

Linoleic acid

Answer

Stearic acid

Answer

Palmitic acid

Answer

Cholesterol

Question 8

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

Accepted Answer

Acetyl CoA

Answer

ATP

Answer

Inorganic phosphate

Answer

Pyruvate

Question 9

What maintains the membrane integrity of red blood cells?

Accepted Answer

Spectrin

Answer

G-protein

Answer

Haemoglobin

Answer

Ankyrin

Question 10

All of the following are true about cell membrane lipids except?

Accepted Answer

Exhibit symmetrical arrangement

Answer

Amphipathic in nature

Answer

Exhibit lateral diffusion along with proteins

Answer

Form bilayer

Membrane Biochemistry — MCQs

Membrane Biochemistry — MCQs

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