Membrane Biochemistry Practice Questions

Q: Which of the following intracellular structures does NOT have a cell membrane?

Nucleolus. **Explanation:** The correct answer is **C. Nucleolus**. **1. Why Nucleolus is the correct answer:** In cellular physiology, organelles are classified as membrane-bound or non-membrane-bound. The **nucleolus** is a dense, spherical structure located within the nucleus. It is essentially a large aggregate of macromolecules (RNA, proteins, and chromatin) and is **not enclosed by a lipid bilayer membrane**. It functions as the site for ribosomal RNA (rRNA) synthesis and ribosome subunit assembly. Because it lacks a membrane, its contents are in direct contact with the nucleoplasm. **2. Why the other options are incorrect:** * **Mitochondria (A):** These are double-membrane-bound organelles. They possess an outer membrane and a folded inner membrane (cristae), essential for the electron transport chain. * **Nucleus (B):** The nucleus is enclosed by the **nuclear envelope**, which consists of two concentric lipid bilayer membranes perforated by nuclear pores. * **Endoplasmic Reticulum (D):** The ER is an extensive network of membrane-enclosed sacs (cisternae) and tubules. It is a single-membrane-bound organelle continuous with the outer nuclear membrane. **3. NEET-PG High-Yield Facts:** * **Non-membrane bound structures:** Nucleolus, Ribosomes, Centrioles/Centrosomes, Cytoskeleton (microtubules, microfilaments), and Inclusion bodies. * **Double-membrane bound structures:** Nucleus, Mitochondria, and Chloroplasts (in plants). * **Single-membrane bound structures:** ER, Golgi apparatus, Lysosomes, and Peroxisomes. * **Clinical Pearl:** The size and number of nucleoli increase in cells with high protein synthesis requirements (e.g., cancer cells or plasma cells), a feature often used by pathologists to grade malignancy.

Q: What are residual bodies?

Lysosomes. **Explanation:** **Residual bodies** are the final stage of the lysosomal digestive process. When lysosomes (specifically secondary lysosomes or phagolysosomes) break down cellular debris or foreign material via hydrolytic enzymes, some substances remain undigested. These membrane-bound vesicles containing undigested debris are termed **residual bodies**. In most cells, these are eliminated via exocytosis; however, in long-lived cells like neurons and cardiac muscles, they persist as **lipofuscin granules** (the "wear-and-tear" pigment). **Why other options are incorrect:** * **Mitochondria (A):** These are the "powerhouses" of the cell, primarily involved in ATP production through oxidative phosphorylation, not waste storage or digestion. * **Nucleus (B):** This is the genetic control center containing DNA. It does not participate in the cytoplasmic degradation of macromolecules. * **Golgi Apparatus (C):** This organelle is responsible for modifying, sorting, and packaging proteins. While it produces the primary lysosomes, it does not function as a residual body itself. **NEET-PG High-Yield Pearls:** * **Lipofuscin:** A brown-yellow pigment found in the residual bodies of aging cells. It is a hallmark of lipid peroxidation and cellular aging. * **Primary Lysosome:** A newly formed vesicle from the Golgi containing inactive enzymes. * **Secondary Lysosome:** Formed by the fusion of a primary lysosome with a phagosome/endosome; this is where active digestion occurs. * **Autophagy:** The process where lysosomes digest the cell's own damaged organelles, a critical mechanism during starvation and cellular remodeling.

Q: Metastasis of cancer has its roots in structural abnormalities of which of the following?

Glycoproteins on cell surface. **Explanation:** The correct answer is **B. Glycoproteins on cell surface.** **Why it is correct:** Metastasis is a complex process involving the detachment of cancer cells from the primary tumor, their migration through the extracellular matrix (ECM), and colonization of distant sites. This process is heavily dependent on **cell-surface glycoproteins**, which act as cell adhesion molecules (CAMs). 1. **Cadherins:** A loss of E-cadherin (a glycoprotein) reduces cell-to-cell adhesion, allowing cells to detach. 2. **Integrins:** These glycoproteins mediate the attachment of cells to the ECM; alterations in integrin expression facilitate migration and "homing" to distant organs. 3. **Selectins:** These glycoproteins are involved in the "docking" of circulating tumor cells to the vascular endothelium during extravasation. **Why incorrect options are wrong:** * **A. Glycolipids in nervous tissue:** While glycolipids (like gangliosides) are vital for signal transduction and myelin stability in the nervous system, they are not the primary structural drivers of systemic cancer metastasis. * **C. Lipoproteins in blood:** Lipoproteins (HDL, LDL, VLDL) function as transport vehicles for lipids and cholesterol. While they may play a minor role in providing energy to cancer cells, they do not form the structural basis for metastatic spread. **High-Yield Clinical Pearls for NEET-PG:** * **E-Cadherin:** Often referred to as a "metastasis suppressor." Its downregulation is a hallmark of **Epithelial-Mesenchymal Transition (EMT)**. * **MMPs (Matrix Metalloproteinases):** These are zinc-dependent enzymes that degrade the basement membrane (collagen type IV), working alongside glycoprotein changes to facilitate invasion. * **CEA (Carcinoembryonic Antigen):** A well-known clinical tumor marker that is itself a cell-surface glycoprotein.

Q: Protein to lipid ratio is LEAST in which of the following biological membranes?

Myelin sheath. **Explanation:** The protein-to-lipid ratio of a biological membrane is directly proportional to its **metabolic activity**. Proteins serve as enzymes, transporters, and receptors; therefore, membranes involved in complex biochemical processes have high protein content, while membranes serving primarily as insulators have high lipid content. **Why Myelin Sheath is Correct:** The **Myelin sheath** functions as an electrical insulator for axons to facilitate saltatory conduction. To minimize ion leakage and provide maximum insulation, it is composed of approximately **80% lipids** and only **20% proteins**. This results in a protein-to-lipid ratio of roughly **0.25:1**, the lowest among all biological membranes. **Analysis of Incorrect Options:** * **Inner Mitochondrial Membrane (IMM):** This is the most metabolically active membrane, housing the Electron Transport Chain (ETC) and ATP synthase. It has the **highest** protein content (~75% protein), with a ratio of approximately **3:1**. * **Outer Mitochondrial Membrane (OMM):** Contains various enzymes and porins. Its protein-to-lipid ratio is roughly **1:1**, significantly higher than myelin. * **Red Blood Cell (RBC) Membrane:** Contains numerous structural proteins (spectrin, ankyrin) and transporters (GLUT-1, Anion exchanger). Its ratio is approximately **1.1:1**. **High-Yield Clinical Pearls for NEET-PG:** * **Highest Protein Content:** Inner Mitochondrial Membrane (due to ETC). * **Highest Lipid Content:** Myelin Sheath (due to insulating function). * **Unique Lipid:** **Cardiolipin** is found exclusively in the Inner Mitochondrial Membrane and is essential for the function of ETC complexes. * **Major Myelin Lipids:** Sphingomyelin and Cerebrosides (Galactosylceramide). * **Clinical Correlation:** Demyelinating diseases like **Multiple Sclerosis** (CNS) and **Guillain-Barré Syndrome** (PNS) involve the destruction of these lipid-rich layers.

Q: Decreased protein to lipid ratio is characteristic of which membrane?

Myelin sheath membrane. **Explanation:** The protein-to-lipid ratio of a biological membrane is determined by its physiological function. Membranes involved in high metabolic activity or signal transduction (like ion pumping and electron transport) have a high protein content, while membranes acting primarily as electrical insulators have a high lipid content. **Why Myelin Sheath is Correct:** The **myelin sheath** serves as an electrical insulator for axons to facilitate saltatory conduction. To minimize ion leakage and capacitance, it is composed of approximately **80% lipids** and only **20% proteins**. This results in a very **low protein-to-lipid ratio (0.25:1)**, the lowest among biological membranes. **Analysis of Incorrect Options:** * **Inner Mitochondrial Membrane:** This membrane is the site of the Electron Transport Chain (ETC) and ATP synthesis. It is packed with enzymes and carrier proteins, giving it the **highest protein-to-lipid ratio (approx. 3:1 or 75% protein)**. * **Outer Mitochondrial Membrane:** While less protein-dense than the inner membrane, it contains numerous porins and enzymes, maintaining a ratio of roughly 1:1. * **Sarcoplasmic Reticulum:** This membrane is specialized for active calcium transport (via Ca²⁺-ATPase pumps). It has a high protein content (approx. 65%) to support its role in muscle contraction/relaxation. **High-Yield Facts for NEET-PG:** * **Standard Plasma Membrane:** Typically has a 1:1 protein-to-lipid ratio. * **Myelin Composition:** High in **sphingomyelin** and cholesterol. * **Clinical Correlation:** Demyelinating diseases like **Multiple Sclerosis** (CNS) and **Guillain-Barré Syndrome** (PNS) involve the destruction of these lipid-rich layers, leading to slowed nerve conduction.

Q: Which of the following is an example of a multipass membrane protein?

Anion exchange protein. **Explanation:** Membrane proteins are classified based on how they associate with the lipid bilayer. **Multipass membrane proteins** (also known as polytopic proteins) are integral proteins where the polypeptide chain crosses the lipid bilayer multiple times. **Why the Correct Answer is Right:** * **Anion Exchange Protein (Band 3):** This is a classic example of a multipass membrane protein found in the erythrocyte membrane. It spans the membrane **14 times**. Its primary function is to facilitate the "Chloride Shift" (exchanging $HCO_3^-$ for $Cl^-$), which is essential for $CO_2$ transport in the blood. Because it weaves through the membrane multiple times, it forms a channel-like structure necessary for ion transport. **Why the Other Options are Wrong:** * **Glycophorins (A, B, and C):** These are **single-pass (bitopic) transmembrane proteins**. They span the lipid bilayer only once, with their N-terminal domain (rich in carbohydrates) exposed on the external surface and the C-terminal domain in the cytosol. * **Glycophorin A** is the most abundant and carries the M and N blood group antigens. * **Glycophorin B and C** are structurally similar single-pass proteins that contribute to the RBC's negative surface charge (zeta potential). **High-Yield Clinical Pearls for NEET-PG:** * **Band 3 Protein:** Apart from ion exchange, it serves as an anchor for the RBC cytoskeleton (via ankyrin), maintaining the biconcave shape. * **GLUT-1:** Another high-yield multipass protein in RBCs (spans the membrane 12 times). * **G-Protein Coupled Receptors (GPCRs):** These are the most famous multipass proteins, spanning the membrane exactly **7 times** (serpentine receptors). * **Clinical Correlation:** Mutations in Band 3 protein are associated with **Hereditary Spherocytosis** and **Hereditary Stomatocytosis**.

Q: What is the role of phosphatidylserine?

Apoptosis. ### Explanation **Phosphatidylserine (PS)** is a phospholipid that is normally restricted to the **inner leaflet** of the plasma membrane's lipid bilayer [1]. This asymmetrical distribution is maintained by the enzyme **flippase**. **1. Why Apoptosis is Correct:** During the early stages of **apoptosis** (programmed cell death), the enzyme flippase is inactivated, and an enzyme called **scramblase** is activated. This causes phosphatidylserine to "flip" from the inner leaflet to the **outer leaflet** of the cell membrane. Once exposed on the external surface, PS acts as an **"eat-me" signal**, marking the cell for recognition and phagocytosis by macrophages without triggering an inflammatory response. **2. Why the Other Options are Incorrect:** * **B. Surfactant:** The primary component of pulmonary surfactant is **Dipalmitoylphosphatidylcholine (DPPC)**, also known as Lecithin. It reduces surface tension in the alveoli. * **C. Second Messenger:** Common second messengers include **Inositol triphosphate (IP3)**, **Diacylglycerol (DAG)**, and **cAMP**. While phosphatidylinositol is a precursor for second messengers, phosphatidylserine is not typically classified as one. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Annexin V Assay:** In laboratory medicine, Annexin V is a protein that binds specifically to phosphatidylserine. It is used as a marker in flow cytometry to detect and quantify apoptotic cells. * **Coagulation:** On the surface of activated platelets, exposed phosphatidylserine provides a scaffold for the assembly of coagulation factor complexes (Tenase and Prothrombinase complexes). * **Membrane Asymmetry:** The maintenance of PS on the inner leaflet is an ATP-dependent process [1]. Loss of this asymmetry is one of the earliest detectable signs of apoptosis.

Q: Mutations in connexin can cause which of the following diseases?

Charcot Marie Tooth disease. **Explanation:** **Connexins** are the structural protein subunits that form **gap junctions**, which are specialized intercellular channels allowing the passage of ions and small molecules between adjacent cells. Six connexins assemble to form a hemichannel called a **connexon**. **1. Why Option A is Correct:** **Charcot-Marie-Tooth (CMT) disease**, specifically the X-linked form (**CMTX1**), is caused by mutations in the **GJB1 gene**, which encodes the protein **Connexin 32 (Cx32)**. In the peripheral nervous system, Cx32 is expressed in Schwann cells and is located in the Schmidt-Lanterman incisures and paranodal regions. Mutations disrupt the gap junction-mediated communication between the layers of the myelin sheath, leading to progressive demyelination and peripheral neuropathy. **2. Why Incorrect Options are Wrong:** * **Huntington’s Chorea (B):** This is an autosomal dominant neurodegenerative disorder caused by a **CAG trinucleotide repeat expansion** in the *HTT* gene on chromosome 4, leading to the accumulation of the huntingtin protein. * **Friedreich’s Ataxia (C):** This is an autosomal recessive condition caused by a **GAA trinucleotide repeat expansion** in the *FXN* gene, which leads to a deficiency of the mitochondrial protein **frataxin**. **High-Yield Clinical Pearls for NEET-PG:** * **Connexin 26 (Cx26):** Mutations are the most common cause of non-syndromic **congenital sensorineural deafness**. * **Connexin 46 & 50:** Mutations are associated with **congenital cataracts** (expressed in the lens). * **Connexin 43:** The primary connexin in the **ventricular myocardium**; alterations are linked to cardiac arrhythmias. * **Gap Junctions** are the only type of cell junction that allows direct metabolic and electrical coupling.

Q: Which of the following is not a function of the smooth endoplasmic reticulum?

Protein synthesis. The Smooth Endoplasmic Reticulum (SER) is a multifunctional organelle characterized by the absence of ribosomes on its surface, which dictates its physiological roles. **Why Protein Synthesis is the Correct Answer:** Protein synthesis is the primary function of the **Rough Endoplasmic Reticulum (RER)**. The RER is studded with ribosomes, which are the sites of translation. Proteins synthesized here are typically destined for secretion, incorporation into the cell membrane, or storage in lysosomes. Since the SER lacks ribosomes, it cannot perform protein synthesis. **Analysis of Incorrect Options:** * **Lipid Synthesis:** The SER contains enzymes necessary for the synthesis of phospholipids, cholesterol, and steroid hormones (e.g., testosterone, estrogen, and cortisol). * **Metabolism of Drugs:** The SER in hepatocytes contains the **Cytochrome P450 system**, which is essential for the detoxification of drugs (like phenobarbital) and various toxins. * **Supply of Calcium:** In muscle cells, the SER is specialized as the **Sarcoplasmic Reticulum**, which stores and releases calcium ions ($Ca^{2+}$) to trigger muscle contraction. **High-Yield NEET-PG Pearls:** * **Nissl Bodies:** These are large granules of RER found in neurons; they are responsible for high-rate protein synthesis. * **Sarcoplasmic Reticulum:** A specialized SER in myocytes that regulates excitation-contraction coupling via calcium sequestration. * **Hypertrophy of SER:** Chronic use of certain drugs (e.g., barbiturates) can lead to the hypertrophy of the SER in liver cells due to enzyme induction, contributing to drug tolerance.

Q: What is the main constituent of the plasma membrane?

Protein. **Explanation:** The plasma membrane is a selectively permeable lipid bilayer that defines the cell boundary. According to the **Fluid Mosaic Model** (Singer and Nicolson), the membrane is composed of proteins, lipids, and carbohydrates. **Why Protein is the Correct Answer:** While the "bilayer" is made of lipids, **proteins are the main constituent by weight** in most cell membranes. In a typical human cell membrane (like the erythrocyte membrane), the composition is approximately **52% Proteins**, 40% Lipids, and 8% Carbohydrates. Proteins are responsible for the membrane's functional diversity, acting as transporters, receptors, enzymes, and structural anchors. **Analysis of Incorrect Options:** * **B & C. Lipid/Phospholipid:** Although phospholipids form the structural backbone (the "matrix") of the membrane and are numerically more abundant, they account for only about 40% of the total mass. Phospholipids provide fluidity, but proteins provide the bulk of the weight. * **A. Carbohydrate:** These are the least abundant components (approx. 2-10%). They are found only on the outer surface, forming the **glycocalyx**, which is crucial for cell-to-cell recognition and immune response. **High-Yield NEET-PG Pearls:** * **Exception to the Rule:** The **Myelin sheath** is a notable exception where lipids (80%) outweigh proteins (20%) to provide electrical insulation. * **Mitochondrial Membrane:** The inner mitochondrial membrane has the highest protein-to-lipid ratio (approx. 3:1 or 75% protein) due to the presence of the electron transport chain. * **Cholesterol:** It is present in eukaryotic membranes to regulate fluidity; it is notably absent in prokaryotic membranes.

Question 1

Which of the following intracellular structures does NOT have a cell membrane?

Accepted Answer

Nucleolus

Answer

Mitochondria

Answer

Nucleus

Answer

Endoplasmic reticulum

Question 2

What are residual bodies?

Accepted Answer

Lysosomes

Answer

Mitochondria

Answer

Nucleus

Answer

Golgi apparatus

Question 3

Metastasis of cancer has its roots in structural abnormalities of which of the following?

Accepted Answer

Glycoproteins on cell surface

Answer

Glycolipids in nervous tissue

Answer

Lipoproteins in blood

Answer

None of the above

Question 4

Protein to lipid ratio is LEAST in which of the following biological membranes?

Accepted Answer

Myelin sheath

Answer

Inner mitochondrial membrane

Answer

Outer mitochondrial membrane

Answer

Red blood cell membrane

Question 5

Decreased protein to lipid ratio is characteristic of which membrane?

Accepted Answer

Myelin sheath membrane

Answer

Inner mitochondrial membrane

Answer

Outer mitochondrial membrane

Answer

Sarcoplasmic reticulum

Question 6

Which of the following is an example of a multipass membrane protein?

Accepted Answer

Anion exchange protein

Answer

Glycophorins A

Answer

Glycophorins B

Answer

Glycophorins C

Question 7

What is the role of phosphatidylserine?

Accepted Answer

Apoptosis

Answer

Surfactant

Answer

Second messenger

Answer

All of the above

Question 8

Mutations in connexin can cause which of the following diseases?

Accepted Answer

Charcot Marie Tooth disease

Answer

Huntington's chorea

Answer

Friedreich's ataxia

Answer

None of the above

Question 9

Which of the following is not a function of the smooth endoplasmic reticulum?

Accepted Answer

Protein synthesis

Answer

Lipid synthesis

Answer

Metabolism of drugs

Answer

Supply of calcium for cellular functions

Question 10

What is the main constituent of the plasma membrane?

Accepted Answer

Protein

Answer

Carbohydrate

Answer

Lipid

Answer

Phospholipid

Membrane Biochemistry — MCQs

Membrane Biochemistry — MCQs

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