Membrane Structure and Organization — MCQs
In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?
Which of the following is not amphipathic?
Surfactant acts to maintain lung compliance by decreasing which factor?
Which component constitutes the highest percentage in the cell membrane?
Which of the following is a feature not typically associated with Hereditary Spherocytosis?
Which of the following is a characteristic finding in hereditary spherocytosis?
Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
Phospholipid associated with the mechanism of hormone action is
Maximum fluidity of the cell membrane is due to?
Proteoglycan present in the glomerular basement membrane is?
Membrane Structure and Organization Indian Medical PG Practice Questions and MCQs
Question 1: In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?
- A. Cardiolipin
- B. Sphingomyelin
- C. Phosphatidylinositol
- D. Lecithin (Correct Answer)
Explanation: ***Lecithin*** - **Lecithin** (also known as **phosphatidylcholine**) is the primary component of **surfactant** in the lungs, which reduces surface tension and prevents alveolar collapse. - In **preterm babies**, insufficient production of lecithin due to immature lung development leads to **respiratory distress syndrome (RDS)**. *Cardiolipin* - **Cardiolipin** is a major phospholipid found in the **inner mitochondrial membrane**, crucial for oxidative phosphorylation. - Deficiency is associated with mitochondrial disorders like **Barth syndrome**, not primary respiratory distress. *Sphingomyelin* - **Sphingomyelin** is a significant component of **cell membranes** and **myelin sheaths**, important for nerve insulation. - While present in the lungs, its primary role is not in surface tension reduction, and its deficiency is not directly linked to RDS. *Phosphatidylinositol* - **Phosphatidylinositol** is a precursor for various **signaling molecules** and plays a role in cell membrane structure. - While involved in cellular processes, it is not the critical surfactant component whose deficiency causes RDS.
Question 2: 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 3: Surfactant acts to maintain lung compliance by decreasing which factor?
- A. Surface tension (Correct Answer)
- B. Pleural fluid secretion
- C. Intrathoracic pressure
- D. Pleural pressure
Explanation: ***Surface tension*** - **Surfactant** directly reduces the **surface tension** at the air-liquid interface within the alveoli. - By lowering surface tension, surfactant prevents alveolar collapse, particularly at low lung volumes, and increases **lung compliance**. *Intrathoracic pressure* - **Intrathoracic pressure** (also known as pleural pressure) is the pressure within the chest cavity, which fluctuates with breathing. - While surfactant affects lung mechanics, it doesn't directly influence the overall intrathoracic pressure. *Pleural fluid secretion* - **Pleural fluid** lubricates the pleural surfaces and is secreted by the pleural membranes. - Surfactant's primary role is in the alveoli to reduce surface tension, not to regulate **pleural fluid secretion**. *Pleural pressure* - **Pleural pressure** is the pressure in the space between the parietal and visceral pleura. - Surfactant improves lung compliance, which indirectly affects how pressure changes during breathing, but it doesn't directly control the **pleural pressure** itself.
Question 4: 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 5: Which of the following is a feature not typically associated with Hereditary Spherocytosis?
- A. Gall stones
- B. Direct Coombs Positive (Correct Answer)
- C. Splenomegaly
- D. Increased Osmotic Fragility
Explanation: ***Direct Coomb's Positive*** - In Hereditary Spherocytosis, the **Coomb's test** is typically **negative**, indicating that hemolysis is not due to autoimmune factors. - Presence of **spherocytes** on the blood smear and increased fragility are hallmark findings, not antibodies against red cells [1]. *Splenomegaly* - **Splenomegaly** is common in Hereditary Spherocytosis as the spleen actively removes abnormal spherocytes from circulation [1]. - It can lead to **hypersplenism**, with resultant anemia and thrombocytopenia. *Increased Osmotic Fragility* - Increased osmotic fragility is a key feature of Hereditary Spherocytosis, as red blood cells are less able to withstand hypotonic solutions [1]. - This results from a defect in the red cell membrane, causing spherocyte shape and fragility. *Gall stones* - Patients may develop **gallstones** due to increased bilirubin from the breakdown of spherocytes, leading to **bilirubin stones** [1]. - Gallstones are a common complication due to chronic hemolysis. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 597-598.
Question 6: Which of the following is a characteristic finding in hereditary spherocytosis?
- A. Increased Reticulocyte Count
- B. Increased Osmotic Fragility (Correct Answer)
- C. Splenomegaly
- D. Negative Direct Coombs Test
Explanation: ***Increased Osmotic Fragility*** - **Hereditary spherocytosis** is characterized by defective red blood cell membrane proteins (spectrin, ankyrin, band 3, protein 4.2), leading to fragile, sphere-shaped red blood cells with reduced surface area-to-volume ratio [2]. - The **osmotic fragility test** measures the red blood cell's resistance to osmotic lysis in hypotonic solutions; in hereditary spherocytosis, the abnormally shaped spherocytes lyse more readily, showing **increased osmotic fragility** [1]. - This is the **diagnostic hallmark** and most characteristic laboratory finding of hereditary spherocytosis [1]. *Negative Direct Coombs Test* - The **Direct Coombs test (DCT)** detects antibodies or complement bound to red blood cell surfaces, which is characteristic of **autoimmune hemolytic anemias**. - Hereditary spherocytosis is a **hereditary intrinsic red blood cell membrane defect**, not an autoimmune process, so the DCT is **negative**. - While true, a negative test result helps differentiate HS from autoimmune hemolytic anemia but is not the primary diagnostic characteristic. *Increased Reticulocyte Count* - An **increased reticulocyte count** (reticulocytosis) is a common finding in various forms of hemolytic anemia, including hereditary spherocytosis, reflecting compensatory bone marrow response to accelerated red blood cell destruction [1]. - This is a **non-specific marker of hemolysis**, not pathognomonic for hereditary spherocytosis. *Splenomegaly* - **Splenomegaly** is a common clinical feature in hereditary spherocytosis due to the spleen's role in trapping and prematurely destroying the abnormal spherocytes through extravascular hemolysis [1], [2]. - This is a **clinical consequence** of the disease rather than the diagnostic laboratory hallmark related to the red blood cell's inherent membrane defect. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 597-598. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 640-641.
Question 7: Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
- A. Hydrated radius
- B. Electrical charge
- C. Lipid solubility (Correct Answer)
- D. Shape
Explanation: ***Lipid solubility*** - This property is crucial for substances that **readily diffuse directly through the lipid bilayer**. - Lipid-insoluble substances, by definition, **cannot diffuse through the lipid bilayer based on their lipid solubility**, requiring other mechanisms or factors like channels or carriers. *Hydrated radius* - The **size of a hydrated ion or molecule** is a critical determinant for its ability to pass through specific protein channels or pores in the cell membrane. - A larger hydrated radius impedes passage through narrow channels, directly affecting the diffusion of lipid-insoluble substances. *Electrical charge* - For **charged lipid-insoluble substances** (ions), their movement across the membrane is significantly influenced by the **transmembrane electrical potential difference**. - The electrical gradient can either facilitate or hinder the diffusion of these substances through channels or transporters. *Shape* - The **three-dimensional configuration** of a lipid-insoluble substance can affect its ability to bind to and pass through specific protein channels or carrier proteins. - A substance's shape must complement the architecture of the transport mechanism for efficient diffusion.
Question 8: Phospholipid associated with the mechanism of hormone action is
- A. Phosphatidylcholine
- B. Phosphatidylethanolamine
- C. Plasmalogen
- D. Phosphatidylinositol (Correct Answer)
Explanation: ***Phosphatidylinositol*** (Correct) - **Phosphatidylinositol (PI)** and its phosphorylated derivatives, particularly **PIP2 (phosphatidylinositol 4,5-bisphosphate)**, are critical in signal transduction pathways activated by many hormones. - Hormones binding to **G protein-coupled receptors** can activate phospholipase C, which cleaves PIP2 into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, leading to increased intracellular calcium and protein kinase C activation, respectively. *Phosphatidylcholine* (Incorrect) - **Phosphatidylcholine** is a major component of cell membranes and is involved in membrane structure and fluidity. - While it can be a source of signaling molecules like **lysophosphatidic acid**, it is not primarily associated with the initial intracellular signaling events of hormone action in the same way as phosphatidylinositol. *Phosphatidylethanolamine* (Incorrect) - **Phosphatidylethanolamine** is another abundant membrane phospholipid primarily involved in membrane structure and stability. - It can be a precursor for other lipids, but it does not directly participate in the **second messenger systems** triggered by most hormones as a primary signaling molecule. *Plasmalogen* (Incorrect) - **Plasmalogens** are a unique class of phospholipids containing an ether bond at the sn-1 position. - They are abundant in certain tissues, particularly nervous and cardiovascular tissues, and are thought to have antioxidant properties, but they are not directly involved in the initiating events of **hormone signaling pathways**.
Question 9: 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 10: 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.
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