Chapter 117 – MCQs for Lab (Biochemistry)

Explanation: Slow reacting substance of anaphylaxis (SRS-A) is a mixture of leukotrienes (specifically LTC4, LTD4, and LTE4), which are potent mediators of inflammation and bronchoconstriction.

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Why other options are incorrect:

• • Prostaglandins: While they are eicosanoids involved in inflammation, they are distinct from the specific leukotriene mixture that constitutes SRS-A.
• • Prostacyclins: Primarily function in vasodilation and the inhibition of platelet aggregation.
• • Thromboxanes: Primarily involved in platelet aggregation and vasoconstriction, not the primary component of SRS-A.

Explanation: Dipalmitoyl lecithin, also known as dipalmitoylphosphatidylcholine (DPPC), is the primary surface-active component of lung surfactant. It reduces surface tension in the alveoli, preventing lung collapse during expiration.

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Why other options are incorrect:

• • Platelet activating factor: This is a separate phospholipid mediator involved in inflammation and allergic responses.
• • Second messenger for hormones: Common second messengers include cAMP, cGMP, and IP3, not DPPC.
• • Anti-ketogenic compound: While carbohydrates are often considered anti-ketogenic, DPPC plays a structural role in the lungs rather than a metabolic role in ketone body regulation.

Explanation: The Reichert-Meissl number is the number of milliliters of 0.1 N KOH required to neutralize the soluble volatile fatty acids distilled from 5 grams of fat. It is used to measure the amount of volatile fatty acids in butter or other fats.

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Why other options are incorrect:

• • 0.5 KOH: This concentration is typically used in the determination of the Saponification Value, not the Reichert-Meissl number.
• • 0.1 N NaOH: While sodium hydroxide is a base, the standard procedure specifically defines the use of Potassium Hydroxide (KOH) for this test.
• • 0.5 NaOH: This is not the standard titrant for this particular analytical chemistry test.

Explanation: In the typical structure of glycerophospholipids, the fatty acid attached to Carbon 1 (sn-1 position) is usually saturated, while the fatty acid attached to Carbon 2 (sn-2 position) is commonly a polyunsaturated fatty acid.

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Why other options are incorrect:

• • Carbon 1: This position is typically occupied by a saturated fatty acid (e.g., palmitic or stearic acid).
• • Both (Carbon 1) and (Carbon 2): While both carbons hold fatty acids, the specific preference for polyunsaturated chains is overwhelmingly at the sn-2 position.
• • None of these: This is incorrect as the sn-2 position is the specific site for polyunsaturated fatty acid attachment in these molecules.

Explanation: Lysolecithin (lysophosphatidylcholine) is formed by the action of the enzyme Phospholipase A2, which specifically hydrolyzes and removes the fatty acid at the sn-2 position of lecithin (phosphatidylcholine).

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Why other options are incorrect:

• • Fatty acid from position 1: Removal of the fatty acid at the sn-1 position is catalyzed by Phospholipase A1, not resulting in the common lysolecithin structure.
• • Phosphorylcholine: Removal of this group is catalyzed by Phospholipase C, which produces diacylglycerol.
• • Choline: Removal of the choline head group alone is not the standard pathway for producing lysolecithin.

Explanation: The biosynthesis of sphingosine begins in the endoplasmic reticulum with the condensation of palmitoyl-CoA and the amino acid serine, a reaction catalyzed by serine palmitoyltransferase (requiring pyridoxal phosphate).

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Why other options are incorrect:

• • Palmitoyl CoA and Choline: Choline is a component of phosphatidylcholine (lecithin), not a precursor for the sphingosine backbone.
• • Palmitoyl CoA and ethanolamine: While ethanolamine is part of phosphatidylethanolamine, it is not involved in the initial condensation to form sphingosine.
• • Acetyl CoA and choline: Acetyl-CoA is the primary building block for fatty acid synthesis, but it is not used in the initial step of sphingosine formation.

Explanation: Sphingosine synthesis requires pyridoxal phosphate (PLP), NADPH, and FAD at different steps of the pathway. NAD⁺ is not required as a coenzyme for sphingosine synthesis. Therefore, the correct answer is NAD.

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Why other options are incorrect:

• • Pyridoxal phosphate: Required by serine palmitoyltransferase in the initial step of sphingolipid synthesis.
• • NADPH: Provides reducing equivalents during the pathway.
• • FAD: Participates in the oxidative step leading to sphingosine formation.

Explanation: Cerebrosides (or galactocerebrosides) are sphingolipids consisting of a sphingosine backbone, a fatty acid, and a single sugar unit (usually galactose). They do not contain sulphate. Compounds that contain sulphate are known as Sulfatides.

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Why other options are incorrect:

• • Galactose: This is the characteristic sugar component found in cerebrosides.
• • Sphingosine: The fundamental amino alcohol backbone of all sphingolipids, including cerebrosides.
• • Fatty acid: Joined to the sphingosine via an amide bond, forming a ceramide base.

Explanation: Niemann-Pick disease (specifically types A and B) is a lysosomal storage disorder caused by a deficiency of the enzyme sphingomyelinase. This deficiency leads to the accumulation of sphingomyelin in the lysosomes of cells, particularly in the liver, spleen, and brain.

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Why other options are incorrect:

• • Ceramidase: Deficiency leads to Farber disease.
• • Arylsulphatase A: Deficiency leads to Metachromatic leukodystrophy.
• • Hexosaminidase A: Deficiency leads to Tay-Sachs disease.

Explanation: After chylomicrons have delivered their triglyceride content to peripheral tissues (via lipoprotein lipase), the remaining particles, known as chylomicron remnants, are taken up by the liver via receptor-mediated endocytosis and catabolized.

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Why other options are incorrect:

• • Intestine: The intestine is the site of chylomicron synthesis, not their degradation.
• • Adipose tissue: While adipose tissue is a primary site for chylomicron triglyceride hydrolysis, it does not catabolize the remnants themselves.
• • Liver and intestine: The intestine does not play a role in the catabolism (breakdown) of chylomicron remnants; this process is exclusive to the liver.

Explanation: VLDL (Very Low-Density Lipoprotein) is acted upon by lipoprotein lipase to release triglycerides, forming VLDL remnants or IDL (Intermediate-Density Lipoprotein). These IDL particles are further hydrolyzed by hepatic lipase to form LDL (Low-Density Lipoprotein).

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Why other options are incorrect:

• • VLDL: VLDL is the precursor to the remnant, not the product.
• • HDL: HDL is synthesized primarily in the liver and intestine and follows a different metabolic pathway (reverse cholesterol transport).
• • Chylomicrons: Chylomicrons are exogenous lipoproteins formed in the intestine from dietary fats, unrelated to the conversion of VLDL remnants.

Explanation: Chylomicron remnants are cleared from the circulation by the liver. This uptake is mediated primarily by the LDL receptor-related protein (LRP) and the LDL receptor, which recognize Apo E present on the surface of the remnants.

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Why other options are incorrect:

• • Apo A: Primarily associated with HDL particles.
• • Apo B-48: Found on chylomicrons and their remnants, but it does not serve as the specific ligand for the hepatic uptake receptor.
• • Apo C: Acts as a cofactor for lipoprotein lipase, not for receptor-mediated clearance.

Explanation: Nascent HDL (discoidal HDL) secreted by the intestine and liver primarily contains Apo A-I. These nascent particles acquire Apo C and Apo E from other lipoproteins (like VLDL) once they are in the circulation and begin to mature.

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Why other options are incorrect:

• • Apo A: Apo A-I is the primary structural protein of HDL and is present in nascent HDL.
• • Apo C / Apo E: While these are missing in the nascent form, ‘d’ is the most comprehensive answer as both are acquired later in the metabolic cycle.

Explanation: High-Density Lipoprotein (HDL) particles are synthesized and secreted primarily by both the liver and the small intestine. They are released into the blood in a nascent, discoidal form containing mostly Apo A-I and phospholipids.

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Why other options are incorrect:

• • Adipose tissue: While adipose tissue plays a crucial role in lipid storage and mobilization, it is not a primary site for the synthesis of HDL particles.
• • Liver / Intestine (Individual): These are correct locations but are incomplete, as HDL is synthesized in both organs simultaneously.

Explanation: Nascent HDL synthesized by the intestine acquires Apo C and Apo E from circulating HDL of hepatic origin through the exchange of apolipoproteins. This allows the HDL particle to participate in the metabolism and transport of lipids.

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Why other options are incorrect:

• • Chylomicrons: They can exchange apolipoproteins with HDL, but are not the principal source described in this pathway.
• • VLDL: VLDL exchanges Apo C and Apo E with HDL, but HDL of hepatic origin is the expected source in this question.
• • LDL: LDL primarily contains Apo B-100 and is not the principal source of Apo C and Apo E for nascent HDL.

Explanation: Activated LCAT, stimulated mainly by Apo A-I, esterifies free cholesterol on HDL. This drives the maturation of HDL and promotes the conversion of HDL3 into HDL2, a larger and more cholesterol-rich HDL particle.

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Why other options are incorrect:

• • VLDL remnants into LDL: This conversion occurs through the metabolism of VLDL remnants (IDL) and does not require LCAT.
• • Nascent HDL into HDL: LCAT participates in HDL maturation, but the specific conversion asked here is HDL3 into HDL2.
• • HDL2 into HDL3: This remodeling is associated mainly with hepatic lipase and other HDL-remodeling processes, not LCAT.

Explanation: Fatty liver (steatosis) occurs when there is an imbalance between the synthesis/uptake of fatty acids and their oxidation or export as VLDL. All the listed factors contribute to this:

• Methionine deficiency: Limits methyl donors required for choline synthesis, which is needed for VLDL export.
• Puromycin: Inhibits protein synthesis (including the apoprotein components of VLDL), preventing the export of fat from the liver.
• Chronic alcoholism: Increases NADH/NAD+ ratio and inhibits VLDL secretion, leading to triglyceride accumulation.

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Why other options are correct:

Since all three mechanisms effectively block the liver’s ability to export triglycerides (usually via VLDL), “All of these” is the correct answer.

Explanation: The enzyme alcohol dehydrogenase (ADH), located primarily in the liver, oxidizes ethanol to acetaldehyde. In this process, NAD+ is reduced to NADH.

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Why other options are incorrect:

• • Acetyl CoA: Acetaldehyde is further oxidized to acetate by aldehyde dehydrogenase, which is then converted to Acetyl-CoA.
• • Acetate: This is the product of the second step (catalyzed by aldehyde dehydrogenase), not the primary step catalyzed by alcohol dehydrogenase.
• • CO2 and H2O: These are the final products of the oxidation of Acetyl-CoA via the Citric Acid Cycle, not the direct product of alcohol dehydrogenase.

Explanation: Triglycerides (triacylglycerols) are the primary storage form of lipids in the human body. They are stored in specialized cells called adipocytes in adipose tissue, serving as a concentrated source of energy.

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Why other options are incorrect:

• • Phospholipids: These are primarily structural components of cell membranes, not energy storage molecules.
• • Glycolipids: These function mainly as components of cell membranes, particularly in the nervous system, and as cell-surface markers.
• • Cholesterol esters: While cholesterol is stored in esterified form, it acts more as a precursor for steroids and a component of cell membranes rather than an energy storage reservoir.

Explanation: Adipose tissue is the primary site for the storage of triglycerides in the body. Adipocytes are specialized cells designed to store large amounts of lipids, which can be mobilized during periods of energy demand.

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Why other options are incorrect:

• • Liver: While the liver can store lipids (especially in pathological states like fatty liver), it is primarily responsible for lipid metabolism and export, not long-term storage.
• • Brain: The brain is rich in lipids (as structural components like myelin), but it does not serve as an energy storage depot.
• • Muscles: While muscles contain some intramuscular triglycerides as a local energy source, they are not the primary reservoir for body fat.

Explanation: The enzyme glycerol kinase catalyzes the transfer of a phosphate group from ATP to glycerol, producing glycerol-3-phosphate. This reaction is a critical step in the metabolism of glycerol for both lipid synthesis and gluconeogenesis.

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Why other options are incorrect:

• • Thiokinase: An enzyme involved in the activation of fatty acids to acyl-CoA.
• • Triokinase: Involved in the metabolism of fructose and glycerol (specifically acting on dihydroxyacetone).

Explanation: Adipose tissue lacks significant glycerol kinase activity; therefore, it cannot utilize free glycerol for triglyceride synthesis. Instead, it relies on glucose, which is converted to dihydroxyacetone phosphate (DHAP) via glycolysis. DHAP is then reduced to glycerol-3-phosphate by the enzyme glycerol-3-phosphate dehydrogenase.

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Why other options are incorrect:

• • Free glycerol: Adipocytes lack the glycerol kinase enzyme necessary to phosphorylate glycerol.
• • Hydrolysis of triglycerides/phospholipids: These processes release lipids but do not provide the glycerol backbone for new triglyceride synthesis.

Explanation: Adipose tissue lacks the enzyme glycerol kinase, so it cannot metabolize glycerol. The glycerol released during lipolysis diffuses into the blood and is transported to the liver, which contains high levels of glycerol kinase, allowing it to convert glycerol into glycerol-3-phosphate for use in gluconeogenesis or triglyceride synthesis.

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Why other options are incorrect:

• • Reutilised in adipose tissue: Adipocytes lack the glycerol kinase enzyme required to phosphorylate and reuse the released glycerol.
• • Taken up by extrahepatic tissues: While some tissues use glycerol, the liver is the primary site of clearance.
• • Excreted from the body: Glycerol is a valuable metabolic substrate and is not normally excreted.

Explanation: Adipose tissue lacks the enzyme glycerol kinase, which is required to phosphorylate free glycerol to glycerol-3-phosphate. As a result, adipocytes cannot utilize free glycerol for the synthesis of triglycerides and must rely on glucose-derived dihydroxyacetone phosphate (DHAP) for the glycerol backbone.

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Why other options are incorrect:

• • Liver, Kidney, Intestine: These tissues possess high levels of glycerol kinase activity, allowing them to effectively phosphorylate and utilize free glycerol for lipid synthesis.

Explanation: Adipocytes (adipose tissue cells) possess very low or non-existent levels of the enzyme glycerol kinase. Because of this, they are unable to phosphorylate glycerol to glycerol-3-phosphate, forcing them to rely on glucose metabolism (glycolysis) to generate the glycerol-3-phosphate needed for triglyceride resynthesis.

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Why other options are incorrect:

• • Hormone-sensitive lipase: This is highly active in adipose tissue; it is the key enzyme responsible for mobilizing stored fats.
• • cAMP-dependent protein kinase: This is present and is responsible for phosphorylating and activating hormone-sensitive lipase.
• • Glycerol-3-phosphate dehydrogenase: This enzyme is present in adipose tissue to convert DHAP into glycerol-3-phosphate.

Explanation: Bile is produced by the liver and stored in the gallbladder. While it is essential for the digestion and absorption of fats through the emulsification of dietary lipids, it does not contain any digestive enzymes. It mainly consists of bile salts, bile pigments (bilirubin), cholesterol, and phospholipids.

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Why other options are incorrect:

• • Saliva: Contains salivary amylase and lingual lipase.
• • Gastric juice: Contains pepsin and gastric lipase.
• • Pancreatic juice: Contains a wide array of powerful digestive enzymes, including trypsin, chymotrypsin, pancreatic amylase, and pancreatic lipase.

Explanation: Lingual lipase, which is secreted by the Ebner’s glands on the tongue, is specifically specialized to act on triglycerides containing short and medium-chain fatty acids.

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Why other options are incorrect:

• • Medium/Long chain fatty acids: While they are lipids, lingual lipase has high specificity for short-chain triglycerides, which are common in milk fat.
• • All of these: Incorrect, because lingual lipase is not the primary enzyme for long-chain fatty acid digestion; that role is primarily handled by pancreatic lipase in the small intestine.

Explanation: Salivary (lingual) lipase is a highly specific enzyme that preferentially hydrolyzes the ester bond at the sn-3 position of the triglyceride molecule, releasing a fatty acid and a 1,2-diglyceride.

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Why other options are incorrect:

• • Position 1 & 2: Salivary lipase does not typically act on these positions; these are primarily targeted later in digestion by pancreatic enzymes or during specific metabolic processes.
• • All of these: Incorrect, as the enzyme exhibits positional specificity (regiospecificity) rather than acting on all ester bonds equally.

Explanation: Salivary (lingual) lipase acts by partially hydrolyzing triglycerides. Because it primarily acts on the sn-3 ester bond, the products of this reaction are 1,2-diglycerides and free fatty acids.

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Why other options are incorrect:

• • Monoglycerides: These are usually the result of more extensive digestion, often occurring later in the small intestine via pancreatic lipase.
• • Glycerol and fatty acids: This represents the complete breakdown of a triglyceride, which does not occur during the limited action of salivary lipase.
• • All of these: Incorrect, as salivary lipase only initiates the first step of hydrolysis.

Explanation: The optimal activity of pancreatic lipase requires a combination of factors: Co-lipase (which anchors the enzyme to the lipid droplet), Bile salts (which emulsify fats and remove inhibitory products from the interface), and Phospholipids (which help in the formation of an accessible substrate interface).

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Why other options are correct:

• • Co-lipase: Essential for preventing the inhibition of lipase by bile salts.
• • Bile salts: Necessary for emulsifying dietary fats into smaller droplets, increasing the surface area for lipase action.
• • Phospholipids: Assist in the emulsification process and maintain the integrity of the substrate surface.

Explanation: Pancreatic lipase is specific for the primary ester bonds (positions 1 and 3) of triacylglycerols. By hydrolyzing these two positions, it produces two molecules of free fatty acids and one molecule of 2-monoacylglycerol.

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Why other options are incorrect:

• • 1-Monoacylglycerol & 3-Monoacylglycerol: These are not the primary products; they can only form if spontaneous acyl migration occurs from the sn-2 position to the sn-1 or sn-3 position after digestion.
• • 2,3-Diacylglycerol: This is not a standard intermediate in the primary hydrolytic pathway of pancreatic lipase.

Explanation: While fatty acids are activated in the cytosol, the process of beta-oxidation (the primary pathway for fatty acid oxidation) takes place within the mitochondrial matrix.

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Why other options are incorrect:

• • Cytosol: This is where fatty acid synthesis occurs, not oxidation.
• • Inner mitochondrial membrane: While some components (like the electron transport chain) are located here, the enzymes for beta-oxidation are located in the soluble matrix.
• • Microsomes: These are involved in the desaturation and elongation of fatty acids or omega-oxidation, but not the primary beta-oxidation pathway.

Explanation: The activation of fatty acids (forming Acyl-CoA) in the cytosol requires ATP, Coenzyme A, and the enzyme Thiokinase (also known as Acyl-CoA synthetase).

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Why other options are incorrect:

• • Carnitine: This is not involved in the activation step. It is required for the transport of long-chain fatty acyl-CoA into the mitochondrial matrix via the carnitine shuttle system.
• • ATP, Coenzyme A, & Thiokinase: These three are essential components of the thiokinase-catalyzed reaction that converts a free fatty acid into a fatty acyl-CoA molecule.

Explanation: Carnitine acts as a carrier molecule for long-chain fatty acids. It is essential for the “Carnitine Shuttle,” which transports these fatty acids across the inner mitochondrial membrane so they can undergo beta-oxidation.

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Why other options are incorrect:

• • Triglycerides: These are far too large to be transported across the mitochondrial membrane as a whole; they must be broken down first.
• • Short chain fatty acids: These are generally able to cross the mitochondrial membrane via simple diffusion, independently of the carnitine shuttle system.

Explanation: Carnitine acylcarnitine translocase (CACT) is a protein located within the inner mitochondrial membrane. It acts as an antiporter, exchanging acylcarnitine for free carnitine across the membrane, which is a critical step in the fatty acid transport process.

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Why other options are incorrect:

• • Mitochondrial matrix: This is where beta-oxidation enzymes are found, but the translocase itself is embedded in the membrane structure.
• • Outer/Inner surface of membrane: While the translocase facilitates movement between these spaces, it is an integral membrane protein that spans the entirety of the inner mitochondrial membrane to function.

Explanation: Complete oxidation of stearic acid (C18:0) produces 146 ATP equivalents using the classical ATP yield calculation. Stearic acid undergoes 8 cycles of β-oxidation, producing 9 acetyl-CoA, 8 NADH, and 8 FADH₂. After subtracting 2 ATP equivalents for fatty acid activation, the net yield is 146 ATP.

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Why other options are incorrect:

• • 129: This is not the classical net ATP yield for complete oxidation of stearic acid.
• • 131: This is the classical net ATP yield for palmitic acid (C16:0), not stearic acid.
• • 148: This does not account for the 2 ATP-equivalent cost of fatty acid activation.

Explanation: The oxidation of odd-chain fatty acids results in the production of Acetyl-CoA units and one molecule of Propionyl-CoA. Propionyl-CoA is carboxylated by Propionyl-CoA carboxylase to form D-Methylmalonyl-CoA, which is then converted to L-Methylmalonyl-CoA and finally to Succinyl-CoA to enter the TCA cycle.

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Why other options are incorrect:

• • Acetyl CoA: While generated during the process, it is not the immediate conversion product of Propionyl-CoA.
• • Acetoacetyl CoA: This is an intermediate in ketone body synthesis and fatty acid oxidation, not the primary product of Propionyl-CoA metabolism.
• • Butyryl CoA: This is a 4-carbon acyl-CoA intermediate involved in fatty acid synthesis and some beta-oxidation steps, but not the Propionyl-CoA pathway.

Explanation: α-Oxidation primarily occurs in the brain (nervous tissue) and other tissues to metabolize branched-chain fatty acids like phytanic acid, which cannot undergo β-oxidation due to the presence of a methyl group at the β-carbon.

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Why other options are incorrect:

• • Liver: While the liver is the primary site for many metabolic processes (like β-oxidation), α-oxidation is specialized for specific substrates and is most significant in neural tissues.
• • Muscles & Adipose Tissue: These tissues primarily rely on β-oxidation for energy production from long-chain fatty acids and do not play a major role in the α-oxidation pathway.

Explanation: Refsum’s disease is a rare neurological disorder caused by a deficiency of the peroxisomal enzyme phytanoyl-CoA hydroxylase. This enzyme is required for the alpha-oxidation of phytanic acid. Since alpha-oxidation is the defective pathway, the other options (Beta, Gamma, and Omega) are the correct answers to the “except” criteria.

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Why other options are incorrect:

• • Alpha-oxidation: This is the primary pathway affected in Refsum’s disease, which is why it is not the answer to the “except” question.
• • Beta, Gamma, and Omega-oxidation: These are separate metabolic pathways that are not the primary cause of phytanic acid accumulation in Refsum’s disease.

Explanation: Omega-oxidation of fatty acids produces dicarboxylic acids. Fatty acids with an even number of carbon atoms can form dicarboxylic acids such as adipic acid (C6) and suberic acid (C8), depending on the chain length. Therefore, both adipic acid and suberic acid are end products of omega-oxidation.

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Why other options are incorrect:

• • Adipic acid: It is a dicarboxylic acid formed from omega-oxidation of certain even-chain fatty acids, but it is not the only possible product.
• • Suberic acid: It is also a dicarboxylic acid formed from omega-oxidation of longer even-chain fatty acids, but it is not the only possible product.
• • None of these: Incorrect because adipic and suberic acids can be produced by omega-oxidation.

Explanation: Adipose tissue (fat depots) is strategically distributed throughout the body to store energy in the form of triglycerides. These depots are found in subcutaneous areas, intermuscular connective tissue, and visceral areas like the mesentery (which anchors the intestines) and the omentum (which covers the abdominal organs).

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Why these are all correct:

• Fat storage is not restricted to one location; the body utilizes various connective tissue spaces to house adipocytes, which serve as an essential energy reserve for metabolic processes.

Explanation: Salivary lipase, commonly referred to as lingual lipase, is secreted by the Ebner’s glands (also known as von Ebner’s glands). These glands are located in the circumvallate papillae on the dorsal surface of the tongue.

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Why other options are incorrect:

• • Parotid glands: These are the largest salivary glands and primarily secrete saliva rich in salivary amylase (ptyalin).
• • Sub-maxillary glands: These glands contribute to the production of mucin and serous saliva, but are not the primary source of lingual lipase.

Explanation: Colipase is a small protein secreted by the pancreas as procolipase and activated by trypsin. It helps pancreatic lipase digest dietary triglycerides in the presence of bile salts by anchoring lipase to the lipid-water interface.

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Why other options are incorrect:

• • Bile salt: Bile salts emulsify fats but are not colipase.
• • Vitamin: Colipase is not a vitamin; it is a pancreatic protein.
• • Phospholipid: Colipase is a protein, not a phospholipid.

Explanation: Plasma often becomes milky or “lipemic” after a fat-rich meal. This occurs due to the presence of high concentrations of chylomicrons, which are large lipoproteins that transport dietary triglycerides from the intestines to the rest of the body.

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Why other options are incorrect:

• • HDL/LDL: These lipoproteins are small and do not significantly scatter light to make plasma appear milky at physiological levels.
• • Fasting: During fasting, plasma is typically clear because chylomicrons have been cleared from the bloodstream by the enzyme lipoprotein lipase.

Explanation: The inner mitochondrial membrane is readily permeable to short-chain fatty acids. They can enter the mitochondrial matrix without the carnitine shuttle. Long-chain fatty acids require the carnitine transport system for entry into mitochondria.

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Why other options are incorrect:

• • Medium chain fatty acids: They can enter mitochondria relatively easily, but the standard MCQ answer for direct mitochondrial membrane permeability is short-chain fatty acids.
• • Long chain fatty acids: They cannot directly cross the inner mitochondrial membrane and require the carnitine shuttle.
• • All of these: Long-chain fatty acids are not directly permeable through the inner mitochondrial membrane.

Explanation: During each spiral of β-oxidation, two carbon atoms are removed in the form of Acetyl-CoA from the carboxyl end of the fatty acid chain.

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Why other options are incorrect:

• • One carbon atom: The process removes a 2-carbon unit, not 1.
• • Methyl end: Oxidation occurs sequentially from the carboxyl (acid) end of the fatty acid, not the methyl (omega) end.

Explanation: The complete oxidation of one molecule of palmitic acid (a 16-carbon saturated fatty acid) yields 106 ATP. This is calculated by 7 cycles of beta-oxidation (generating 7 FADH2 and 7 NADH) and 8 acetyl-CoA molecules entering the Citric Acid Cycle, minus the 2 ATP equivalents consumed during the initial activation of the fatty acid.

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Why other options are incorrect:

• • 131/146/148 ATP: These figures are often cited in older textbooks or do not account for the updated P/O ratios (2.5 for NADH and 1.5 for FADH2) and the energy cost of activating the fatty acid.

Explanation: A 17-carbon fatty acid is an odd-chain fatty acid. Its complete oxidation produces one propionyl-CoA in addition to acetyl-CoA. Therefore, its net ATP yield is lower than that of an 18-carbon fatty acid but higher than that of a 16-carbon fatty acid, placing its energy yield between the two.

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Why other options are incorrect:

• • Equal to 16-carbon fatty acid: The 17-carbon fatty acid produces a different number of reducing equivalents and a propionyl-CoA-derived succinyl-CoA.
• • Equal to 18-carbon fatty acid: An 18-carbon fatty acid undergoes an even number of β-oxidation cycles and produces more net energy.
• • Less than 16-carbon fatty acid: The 17-carbon fatty acid contains one additional carbon and therefore yields more energy overall than a 16-carbon fatty acid.

Explanation: Extramitochondrial (cytosolic) fatty acid synthesis is not limited to the liver and adipose tissue. It is a vital process that also occurs in other specialized tissues including mammary glands (for milk fat production), lungs (for the synthesis of pulmonary surfactant), and the brain (for the synthesis of myelin and other lipids).

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Why all these are correct:

• These tissues possess the necessary enzyme complex (Fatty Acid Synthase) in the cytosol to convert Acetyl-CoA into long-chain fatty acids to meet their specific physiological requirements.

Explanation: The fatty acid synthase (FAS) complex contains two functional —SH groups per monomeric functional unit: one on the phosphopantetheine arm of ACP and one on the active-site cysteine of the β-ketoacyl synthase (KS) domain. These sulfhydryl groups are essential for carrying and transferring the growing fatty acyl chain.

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Why other options are incorrect:

• • One —SH group: The FAS functional unit has two essential sulfhydryl groups.
• • Three —SH groups: More than the two functional —SH groups involved in the fatty acid synthesis cycle.
• • Four —SH groups: Exceeds the two essential —SH groups present in one functional unit.