Lipid Metabolism — MCQs
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Number of ATP formed by the oxidation of one molecule of palmitic acid (16 carbon atoms):
All are polyunsaturated fatty acids except:
Which among the following is a primary bile acid?
Essential fatty acids are helpful in controlling which of the following?
End point of fatty acid synthesis is the formation of
What is the major metabolic pathway for saturated fatty acids in the mitochondria?
Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?
Which of the following enzymes is not part of the fatty acid synthase complex?
In beta-oxidation of fatty acids, carnitine is required for:
Which of the following is not an action of HDL?
Lipid Metabolism Indian Medical PG Practice Questions and MCQs
Question 731: Number of ATP formed by the oxidation of one molecule of palmitic acid (16 carbon atoms):
- A. 146
- B. 106 (Correct Answer)
- C. 135
- D. 34
Explanation: ***106*** - Palmitic acid (16 carbon atoms) undergoes **7 cycles of beta-oxidation**. Each cycle produces 1 FADH2, 1 NADH, and 1 acetyl-CoA. - The 7 cycles yield **7 FADH2** (7 x 1.5 ATP = 10.5 ATP), **7 NADH** (7 x 2.5 ATP = 17.5 ATP), and **8 acetyl-CoA** (8 x 10 ATP = 80 ATP in the TCA cycle), totaling 108 ATP. Two ATP equivalents are used for activation, resulting in a net yield of **106 ATP**. *146* - This number is a common misconception, often arising from older calculations that used different ATP yields per NADH and FADH2 (e.g., 3 ATP for NADH and 2 ATP for FADH2). - Modern bioenergetics typically uses **2.5 ATP per NADH** and **1.5 ATP per FADH2** based on more precise proton pump stoichiometry. *135* - This value is not consistent with the current understanding of ATP yields from the complete oxidation of palmitic acid. - It might stem from errors in calculation or using an **incorrect number of beta-oxidation cycles** or ATP equivalents for coenzymes. *34* - This number is significantly **too low** for the complete oxidation of a 16-carbon fatty acid. - It is closer to the ATP yield from the complete oxidation of a **glucose molecule** (around 30-32 ATP), not a fatty acid.
Question 732: All are polyunsaturated fatty acids except:
- A. Linoleic acid
- B. Linolenic acid
- C. Arachidonic acid
- D. Palmitic acid (Correct Answer)
Explanation: ***Palmitic acid*** - **Palmitic acid** is a **saturated fatty acid**, meaning it has no double bonds in its carbon chain. - Its chemical structure is CH₃(CH₂)₁₄COOH, indicating it is **fully saturated** with hydrogen atoms. *Linoleic acid* - **Linoleic acid** is an **omega-6 polyunsaturated fatty acid** with two double bonds. - It is an **essential fatty acid**, meaning the human body cannot synthesize it and it must be obtained through diet. *Linolenic acid* - **Linolenic acid** typically refers to **alpha-linolenic acid (ALA)**, an **omega-3 polyunsaturated fatty acid** with three double bonds. - Like linoleic acid, ALA is an **essential fatty acid** crucial for various physiological functions. *Arachidonic acid* - **Arachidonic acid** is an **omega-6 polyunsaturated fatty acid** with four double bonds. - It is a precursor for the synthesis of **eicosanoids**, such as prostaglandins and leukotrienes, which are important signaling molecules.
Question 733: Which among the following is a primary bile acid?
- A. Deoxycholic acid
- B. Lithocholic acid
- C. Chenodeoxycholic acid (Correct Answer)
- D. None of the options
Explanation: ***Chenodeoxycholic acid*** - **Chenodeoxycholic acid** is one of the two primary bile acids synthesized in the **liver** from **cholesterol**. - The other primary bile acid is **cholic acid**. *Deoxycholic acid* - **Deoxycholic acid** is a **secondary bile acid**, formed from **cholic acid** by bacterial action in the gut. - It is not directly synthesized in the liver. *Lithocholic acid* - **Lithocholic acid** is also a **secondary bile acid**, derived from **chenodeoxycholic acid** through bacterial dehydroxylation in the intestine. - Due to its low solubility, it is considered **toxic** and is efficiently excreted. *None of the options* - This option is incorrect because **chenodeoxycholic acid** is indeed a primary bile acid. - The other common primary bile acid, **cholic acid**, was not listed but is also synthesized directly in the liver.
Question 734: Essential fatty acids are helpful in controlling which of the following?
- A. Atherosclerosis (Correct Answer)
- B. Nephritis
- C. Diabetes Mellitus
- D. Oedema
Explanation: ***Atherosclerosis*** - **Essential fatty acids**, particularly omega-3 fatty acids, help reduce **triglyceride levels** and have **anti-inflammatory** properties, both of which are beneficial in preventing and controlling atherosclerosis. - They also contribute to improving **endothelial function** and reducing **platelet aggregation**, decreasing the risk of plaque formation. *Nephritis* - While some **omega-3 fatty acids** might have general anti-inflammatory effects, there is no strong evidence to suggest they are a primary treatment or control measure for **nephritis** as a specific condition. - **Nephritis** involves inflammation of the kidneys, and its management typically focuses on addressing underlying causes like autoimmune diseases or infections. *Diabetes Mellitus* - **Essential fatty acids** do not directly control **blood glucose levels**, which is the primary challenge in **diabetes mellitus**. - Their role in diabetes management is indirect, mainly through improving **lipid profiles** and reducing cardiovascular risk, but not controlling the core metabolic dysfunction. *Oedema* - **Oedema** (swelling) is primarily related to fluid retention and imbalances in fluid regulation, often due to issues with the heart, kidneys, or lymphatic system. - **Essential fatty acids** do not have a direct mechanism to control **fluid retention** or resolve **oedema**.
Question 735: End point of fatty acid synthesis is the formation of
- A. Palmitic acid (Correct Answer)
- B. Stearic acid
- C. Oleic acid
- D. Linoleic acid
Explanation: ***Palmitic acid*** - **Palmitic acid** (palmitate) is the primary 16-carbon saturated fatty acid that is synthesized **de novo** in fatty acid synthesis. - All other fatty acids are typically derived from palmitic acid through elongation and desaturation reactions. *Stearic acid* - **Stearic acid** is an 18-carbon saturated fatty acid, which is formed by the **elongation** of palmitic acid, not as the initial end-product of de novo synthesis. - This elongation primarily occurs in the **endoplasmic reticulum**. *Oleic acid* - **Oleic acid** is an 18-carbon **monounsaturated fatty acid** formed by the desaturation of stearic acid, making it a derivative rather than the initial end-product of de novo synthesis. - The double bond is introduced by an enzyme called **stearoyl-CoA desaturase**. *Linoleic acid* - **Linoleic acid** is an 18-carbon **polyunsaturated fatty acid** that is an **essential fatty acid**, meaning it cannot be synthesized by the human body and must be obtained from the diet. - It is not produced through de novo fatty acid synthesis in humans.
Question 736: What is the major metabolic pathway for saturated fatty acids in the mitochondria?
- A. α-oxidation
- B. beta-oxidation (Correct Answer)
- C. ω-oxidation
- D. None of the above
Explanation: ***beta-oxidation*** - **Beta-oxidation** is the primary metabolic pathway for **saturated fatty acids** in the **mitochondria**, progressively breaking them down into **acetyl-CoA** units. - Each cycle of beta-oxidation shortens the fatty acid chain by two carbons, producing **FADH2** and **NADH** for ATP synthesis in the electron transport chain. *a-oxidation* - **Alpha-oxidation** is a minor pathway primarily used for degrading **branched-chain fatty acids**, such as **phytanic acid**, in peroxisomes. - It removes one carbon at a time from the carboxyl end of the fatty acid, rather than two carbons like beta-oxidation. *ω-oxidation* - **Omega-oxidation** is a minor pathway of fatty acid metabolism that occurs in the **endoplasmic reticulum** and primarily targets **medium-chain fatty acids**. - It introduces a hydroxyl group at the omega (ω) carbon, the farthest carbon from the carboxyl group, which is then oxidized to a carboxyl group. *None of the above* - This option is incorrect because **beta-oxidation** is indeed the major metabolic pathway for saturated fatty acids in the mitochondria.
Question 737: Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?
- A. Dienoyl CoA isomerase and Enoyl CoA isomerase
- B. Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase
- C. Enoyl CoA isomerase and Enoyl CoA reductase
- D. Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase (Correct Answer)
Explanation: **Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase** - **Enoyl CoA isomerase** is necessary to convert *cis* double bonds to *trans* double bonds at the 3,4 position, which allows the beta-oxidation enzymes to continue processing the fatty acid. - **2,4 Dienoyl CoA reductase** is required to reduce *cis-2, cis-4* or *trans-2, cis-4* dienoyl intermediates into a single *trans-3* enoyl CoA, which can then be isomerized by enoyl CoA isomerase. *Dienoyl CoA isomerase and Enoyl CoA isomerase* - This option is incorrect because **Dienoyl CoA isomerase** is not a commonly recognized single enzyme directly involved in PUFA beta-oxidation in the way described. The key is to reduce a diene, which reductase does. - While **Enoyl CoA isomerase** is crucial, pairing it with another isomerase in this context does not fully address the reduction step needed for certain PUFAs. *Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase* - This option incorrectly names **Dienoyl CoA isomerase** as one of the two main required enzymes. A 2,4 Dienoyl CoA reductase does exist. - While **2,4 Dienoyl CoA reductase** is essential, the other enzyme should be Enoyl CoA isomerase to handle the initial *cis* to *trans* isomerizations. *Enoyl CoA isomerase and Enoyl CoA reductase* - This option is incorrect because **Enoyl CoA reductase** without the "2,4" prefix generally refers to the enzyme involved in fatty acid synthesis, not beta-oxidation of PUFAs. - **Enoyl CoA isomerase** is correctly identified, but the other enzyme specifically for PUFA oxidation is the **2,4 Dienoyl CoA reductase**.
Question 738: Which of the following enzymes is not part of the fatty acid synthase complex?
- A. Enoyl reductase
- B. Ketoacyl synthase
- C. Acetyl-CoA carboxylase (Correct Answer)
- D. Ketoacyl reductase
Explanation: ***Acetyl-CoA carboxylase*** - **Acetyl-CoA carboxylase (ACC)** is a crucial enzyme in fatty acid synthesis, catalyzing the committed and rate-limiting step of converting **acetyl-CoA to malonyl-CoA**. - While essential for providing the substrates for fatty acid synthase, ACC is a **separate, distinct enzyme** and not structurally part of the fatty acid synthase complex itself. *Ketoacyl reductase* - **Ketoacyl reductase** is an integral enzymatic domain of the fatty acid synthase complex. - It catalyzes the **first reduction step** in the fatty acid synthesis cycle, converting a $\beta$-ketoacyl group to a $\beta$-hydroxyacyl group using NADPH. *Enoyl reductase* - **Enoyl reductase** is an intrinsic enzymatic domain of the fatty acid synthase complex. - It catalyzes the **second reduction step**, converting a trans- $\alpha$, $\beta$-enoyl group to a saturated acyl group using NADPH. *Ketoacyl synthase* - **Ketoacyl synthase (or $\beta$-ketoacyl-ACP synthase)** is a core enzymatic domain within the fatty acid synthase complex. - It catalyzes the **condensation reaction** between the growing acyl chain and malonyl-ACP, forming a $\beta$-ketoacyl-ACP.
Question 739: In beta-oxidation of fatty acids, carnitine is required for:
- A. Transport of long chain fatty acid to mitochondrial inner layer (Correct Answer)
- B. Conversion of long chain fatty acids to short chain fatty acids
- C. Transport of long chain fatty acid across the outer mitochondrial membrane
- D. Conversion of short chain fatty acids to medium chain fatty acids
Explanation: ***Transport of long chain fatty acid to mitochondrial inner layer*** - **Carnitine** acts as a shuttle, transporting **long-chain fatty acids** from the cytosol across the **inner mitochondrial membrane** for beta-oxidation. - This process involves the enzyme **carnitine palmitoyltransferase I (CPT-I)** on the outer membrane and **CPT-II** on the inner membrane. - The carnitine shuttle system is essential because the inner mitochondrial membrane is impermeable to long-chain fatty acyl-CoA molecules. *Conversion of long chain fatty acids to short chain fatty acids* - **Carnitine** is not involved in the conversion or shortening of fatty acid chains. - The breakdown of long-chain fatty acids into shorter chains occurs *during* beta-oxidation, not as a function of carnitine transport. *Transport of long chain fatty acid across the outer mitochondrial membrane* - Long-chain fatty acids do not require carnitine to cross the **outer mitochondrial membrane**, which is freely permeable to fatty acyl-CoA. - Carnitine is specifically required for transport across the **inner mitochondrial membrane**, which is impermeable to fatty acyl-CoA. - CPT-I on the outer membrane surface and CPT-II on the inner membrane work together to facilitate this transport. *Conversion of short chain fatty acids to medium chain fatty acids* - **Carnitine** does not facilitate the elongation or interconversion of fatty acid chains. - These processes relate to fatty acid synthesis pathways, not transport for beta-oxidation.
Question 740: Which of the following is not an action of HDL?
- A. Esterification of cholesterol
- B. Transfer of malate into mitochondria (Correct Answer)
- C. Storage of apolipoproteins
- D. Reverse transport of cholesterol to the liver
Explanation: ***Reverse transport of cholesterol to the liver*** - This statement is actually a primary and well-known function of **HDL**, also known as **reverse cholesterol transport**. - HDL collects excess cholesterol from peripheral tissues and transports it back to the liver for excretion or recycling. *Storage of apolipoproteins* - **HDL** functions as a **circulating reservoir** for apolipoproteins such as **apoC-II** and **apoE**, which it can donate to other lipoproteins. - This storage and transfer of apolipoproteins are crucial for the metabolism of other lipoproteins like chylomicrons and VLDL. *Transfer of malate into mitochondria* - The transfer of **malate into mitochondria** is a function of the **malate shuttle system**, a metabolic pathway involved in gluconeogenesis and energy production. - This process is not directly related to the functions of High-Density Lipoprotein (HDL) in lipid metabolism. *Esterification of cholesterol* - **HDL** contains and activates the enzyme **lecithin-cholesterol acyltransferase (LCAT)**, which esterifies cholesterol on its surface. - This process is essential for trapping cholesterol within the HDL particle, facilitating its transport and preventing its release back into peripheral tissues.
Practice by Chapter
Lipid Classification and Chemistry
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Fatty Acid Oxidation
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Ketone Body Metabolism
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Fatty Acid Synthesis
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Metabolism of Triacylglycerols
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Phospholipid Metabolism
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Cholesterol Metabolism and Biosynthesis
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Bile Acids and Bile Salts
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Lipoprotein Metabolism and Transport
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Dyslipidemias and Atherosclerosis
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Prostaglandins and Eicosanoids
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Fatty Liver and Lipotropic Factors
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