Lipid Metabolism Practice Questions

Q: The fatty acid present in breast milk which is important for growth and CNS development is:

Docosahexaenoic acid. ***Docosahexaenoic acid*** - **Docosahexaenoic acid (DHA)** is a crucial **omega-3 fatty acid** abundantly found in breast milk. - It plays a vital role in the **development of the brain** and **retina**, particularly during infancy. *Palmitic acid* - **Palmitic acid** is a **saturated fatty acid** and a major component of fats in both plants and animals, including breast milk. - While it serves as an **energy source**, it is not primarily recognized for its specific role in CNS development like DHA. *Linolenic acid* - **Alpha-linolenic acid (ALA)** is an **omega-3 essential fatty acid** that is a precursor to DHA and EPA, meaning the body can convert it into these longer-chain fatty acids. - While important as a precursor, **ALA itself is not the direct fatty acid** primarily responsible for brain and CNS development in the same way DHA is. *Linoleic acid* - **Linoleic acid** is an **omega-6 essential fatty acid**, meaning the body cannot produce it and it must be obtained from the diet. - It is important for overall growth and development, but it is not specifically highlighted for its direct role in **CNS development** in the same way DHA is for breast milk.

Q: Dietary cholesterol is transported from the intestine to the liver by:

Apo-B. ***Apo-B*** - **Apolipoprotein B-48** (Apo-B48) is the key structural protein of **chylomicrons**, which are large lipoprotein particles formed in intestinal enterocytes. - Chylomicrons are responsible for transporting **dietary triglycerides** and **cholesterol** from the intestine via the lymphatic system into the bloodstream and ultimately to the liver. - Apo-B48 is essential for chylomicron assembly and secretion from the intestine. *Apo-A* - **Apolipoprotein A-I** (ApoA-I) is the primary apolipoprotein of **high-density lipoprotein (HDL)**. - HDL is mainly involved in **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. *Apo-C* - **Apolipoprotein C-II** (ApoC-II) is an activator of **lipoprotein lipase (LPL)**, which metabolizes triglycerides in chylomicrons and VLDL. - **Apolipoprotein C-III** (ApoC-III) inhibits LPL activity and hepatic uptake of triglyceride-rich lipoproteins. *Apo-E* - **Apolipoprotein E** (ApoE) is crucial for the receptor-mediated uptake of **chylomicron remnants** and **VLDL remnants** by the liver. - While involved in remnant clearance, it is not the primary apolipoprotein for the initial transport of dietary cholesterol from the intestine within intact chylomicrons.

Q: What is the primary glycerol-containing absorption product of dietary triglycerides in the gut?

Mono acyl glycerol. ***Monoacylglycerol (2-MAG)*** - **Pancreatic lipase** hydrolyzes dietary triglycerides at positions 1 and 3, producing **2-monoacylglycerol** and **free fatty acids** as the two major products. - As the **primary glycerol-containing product**, 2-monoacylglycerol constitutes approximately **25% of digested lipid mass** and is essential for efficient **triglyceride resynthesis** in enterocytes. - Both 2-MAG and fatty acids are packaged into **mixed micelles** and co-absorbed, but 2-MAG is the predominant form containing the **intact glycerol backbone**. *Fatty acids* - **Free fatty acids** are co-absorbed with 2-monoacylglycerol in approximately a **2:1 molar ratio** (2 fatty acids per 1 monoacylglycerol). - While quantitatively significant, fatty acids do **not contain the glycerol backbone** structure. - Both products are required for efficient absorption and subsequent **triglyceride resynthesis** within enterocytes. *Diacylglycerol* - **Diacylglycerol** is a minor intermediate product during triglyceride hydrolysis. - It is rapidly hydrolyzed further by **pancreatic lipase** to yield **monoacylglycerol** and free fatty acids. - Not a significant absorption product in the intestinal lumen. *Triglycerides* - **Intact triglycerides** are too large and hydrophobic to be absorbed directly across the intestinal brush border. - They must first undergo **enzymatic hydrolysis** by pancreatic lipase into smaller absorbable units (2-MAG and fatty acids).

Q: Acetyl CoA carboxylase is stimulated by all except which of the following?

Acyl CoA. ***Acyl CoA*** - **Acyl CoA** (specifically long-chain fatty acyl CoAs) is an **inhibitor** of acetyl CoA carboxylase (ACC), signifying an abundance of fatty acids and a need to reduce further synthesis. - This feedback inhibition helps regulate **fatty acid synthesis**, ensuring that the pathway is downregulated when sufficient fatty acids are present. *Citrate* - **Citrate** is a potent **allosteric activator** of acetyl CoA carboxylase, indicating a high energy state and excess mitochondrial acetyl CoA, which can be channeled into fatty acid synthesis. - Its presence promotes the polymerization of ACC monomers into active polymers, enhancing enzyme activity. *ATP* - **ATP** is required as a substrate for the carboxylation reaction catalyzed by ACC, providing the energy for the formation of **malonyl CoA**. - High levels of ATP indirectly signal a state of energy abundance, which favors anabolic processes like fatty acid synthesis. *Insulin* - **Insulin** is a hormonal activator of acetyl CoA carboxylase, promoting its dephosphorylation via **protein phosphatase 2A**. - This dephosphorylation leads to increased enzyme activity, stimulating **fatty acid synthesis** in response to high blood glucose after a meal.

Q: Insulin promotes lipogenesis by all except?

Inhibits PDH. ***Inhibits PDH*** - Insulin **activates pyruvate dehydrogenase (PDH)**, not inhibits it, converting pyruvate to acetyl-CoA for fatty acid synthesis. - Since insulin **does not inhibit PDH**, this is the correct answer to this EXCEPT question. - Inhibiting PDH would decrease lipogenesis, which is opposite to insulin's effect. *Increasing acetyl CoA carboxylase activity* - **Acetyl-CoA carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis. - Insulin **activates ACC** (by dephosphorylation), which is a key mechanism for promoting lipogenesis. *Increases the transport of glucose into the cell* - Insulin enhances glucose uptake into adipose tissue and muscle cells by promoting **GLUT4 transporter** translocation to the cell membrane. - This increased glucose provides substrate (via glycolysis to acetyl-CoA) for fatty acid synthesis. *Decreases intracellular cAMP level* - Insulin **lowers intracellular cAMP levels** by activating phosphodiesterase, which degrades cAMP. - Lower cAMP reduces **hormone-sensitive lipase (HSL)** activity, leading to decreased lipolysis and thereby favoring lipogenesis.

Q: Apolipoprotein A-I is found in which of the following lipoproteins?

HDL. ***HDL*** - **Apolipoprotein A-I (ApoA-I)** is the primary protein component of **high-density lipoprotein (HDL)**. - Its main roles are the activation of **lecithin-cholesterol acyltransferase (LCAT)** and to facilitate the transport of **cholesterol** from peripheral tissues back to the liver (**reverse cholesterol transport**). *VLDL* - **Very-low-density lipoprotein (VLDL)** primarily contains **apolipoprotein B-100 (ApoB-100)**, **ApoC-II**, and **ApoE**. - Its main function is the transport of **triglycerides** synthesized in the liver to peripheral tissues. *LDL* - **Low-density lipoprotein (LDL)** contains a single molecule of **apolipoprotein B-100 (ApoB-100)**. - LDL is responsible for delivering **cholesterol** from the liver to peripheral cells. *Chylomicrons* - **Chylomicrons** primarily contain **apolipoprotein B-48 (ApoB-48)**, **ApoC-II**, and **ApoE**. - They are responsible for transporting **dietary triglycerides** and **cholesterol** from the intestines to peripheral tissues and the liver.

Q: All of the following are products of the cyclooxygenase pathway from arachidonic acid, except for:

LTD4. ***LTD4*** - Leukotriene D4 (**LTD4**) is a product of the **lipoxygenase pathway**, not the cyclooxygenase pathway, from arachidonic acid. - It plays a significant role in **bronchoconstriction** and inflammation, especially in asthma. *PGE2* - **Prostaglandin E2 (PGE2)** is a major product of the **cyclooxygenase (COX) pathway**. - It is involved in mediating **fever**, pain, and inflammation. *PGD2* - **Prostaglandin D2 (PGD2)** is also produced via the **cyclooxygenase (COX) pathway**. - It is primarily associated with allergic reactions, **bronchoconstriction**, and sleep regulation. *PGF2α* - **Prostaglandin F2 alpha (PGF2α)** is a product derived from arachidonic acid through the **cyclooxygenase (COX) pathway**. - It plays a role in **uterine contractions**, fertility, and vascular tone.

Q: Enzymes involved in polyunsaturated fatty acid synthesis are

Desaturase. ***Correct: Desaturase*** - **Desaturases** introduce double bonds into fatty acid chains, a key step in synthesizing **polyunsaturated fatty acids (PUFAs)**. - This process is essential for converting saturated and monounsaturated fatty acids into increasingly unsaturated forms. *Incorrect: Pyruvate carboxylase* - **Pyruvate carboxylase** is involved in **gluconeogenesis** and **anaplerosis** by converting pyruvate to oxaloacetate. - It does not play a direct role in the synthesis of **fatty acids**, especially polyunsaturated ones. *Incorrect: Acetyltransferase* - **Acetyltransferases** catalyze the transfer of an acetyl group from **acetyl-CoA** to another molecule. - While acetyl-CoA is a precursor for fatty acid synthesis, acetyltransferases themselves are not the enzymes responsible for introducing **double bonds** into fatty acid chains to make them polyunsaturated. *Incorrect: Acyltransferase* - **Acyltransferases** transfer an acyl group (a fatty acid chain) from one molecule to another, often involved in **lipid synthesis** and modification. - They are crucial for forming esters like triglycerides and phospholipids but do not introduce **double bonds** to create polyunsaturated fatty acids.

Q: Which cooking oil is known to have the highest percentage of polyunsaturated fatty acids?

Safflower oil. ***Safflower oil*** - Safflower oil contains one of the **highest concentrations of polyunsaturated fatty acids**, particularly **linoleic acid**, among commonly used cooking oils. - This composition makes it a popular choice for health-conscious consumers looking to increase their intake of beneficial fats. *Coconut oil* - Coconut oil is predominantly composed of **saturated fatty acids**, notably **lauric acid**, which makes it solid at room temperature. - Its high saturated fat content distinguishes it significantly from oils rich in polyunsaturated fats. *Butter* - Butter is a dairy product that is very high in **saturated animal fats** and **cholesterol**. - Its fatty acid profile is dominated by short- and medium-chain saturated fatty acids, making it distinct from oils rich in polyunsaturated fats. *Vanaspati* - Vanaspati is a **partially hydrogenated vegetable oil** that is high in **trans fatty acids** and saturated fats due to its processing. - While it originates from vegetable oils, hydrogenation alters its fatty acid structure, reducing polyunsaturated content and increasing less healthy fats.

Q: A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?

VLDL. ***VLDL*** - A low-fat diet with increased **carbohydrates** can lead to increased hepatic synthesis of triglycerides, which are then packaged into **VLDL** particles for transport from the liver. This is because excess carbohydrates can be converted to fatty acids and then to triglycerides in the liver. - The liver's increased triglyceride production, driven by abundant **glucose** from carbohydrates, directly corresponds to a rise in **VLDL** secretion to export these lipids. *Chylomicron* - **Chylomicrons** primarily transport **dietary fats** (exogenous triglycerides) absorbed from the intestine. - Switching to a low-fat diet would typically lead to a *decrease* in chylomicron production, as less dietary fat is available for absorption. *IDL* - **IDL** (Intermediate-Density Lipoprotein) is a remnant of **VLDL** metabolism, formed after VLDL loses some triglycerides. - While VLDL may increase, leading to *more* IDL formation, IDL itself is not the primary component that *increases* directly due to high carbohydrate intake; rather, the precursor **VLDL** is directly affected. *HDL* - **HDL** (High-Density Lipoprotein) is involved in **reverse cholesterol transport**, picking up excess cholesterol from peripheral tissues and returning it to the liver. - High carbohydrate intake, especially refined carbohydrates, can sometimes lead to a *decrease* in HDL levels, not an increase.

Question 1

The fatty acid present in breast milk which is important for growth and CNS development is:

Accepted Answer

Docosahexaenoic acid

Answer

Palmitic acid

Answer

Linolenic acid

Answer

Linoleic acid

Question 2

Dietary cholesterol is transported from the intestine to the liver by:

Accepted Answer

Apo-B

Answer

Apo-E

Answer

Apo-A

Answer

Apo-C

Question 3

What is the primary glycerol-containing absorption product of dietary triglycerides in the gut?

Accepted Answer

Mono acyl glycerol

Answer

Fatty acids

Answer

Diacyl glycerol

Answer

Triglycerides

Question 4

Acetyl CoA carboxylase is stimulated by all except which of the following?

Accepted Answer

Acyl CoA

Answer

ATP

Answer

Insulin

Answer

Citrate

Question 5

Insulin promotes lipogenesis by all except?

Accepted Answer

Inhibits PDH

Answer

Increases the transport of glucose in to the cell

Answer

Increasing acetyl co A carboxylase activity

Answer

Decreases intracellular cAMP level

Question 6

Apolipoprotein A-I is found in which of the following lipoproteins?

Accepted Answer

HDL

Answer

VLDL

Answer

LDL

Answer

Chylomicrons

Question 7

All of the following are products of the cyclooxygenase pathway from arachidonic acid, except for:

Accepted Answer

LTD4

Answer

PGE2

Answer

PGD2

Answer

PGF2α

Question 8

Enzymes involved in polyunsaturated fatty acid synthesis are

Accepted Answer

Desaturase

Answer

Acetyltransferase

Answer

Pyruvate carboxylase

Answer

Acyltransferase

Question 9

Which cooking oil is known to have the highest percentage of polyunsaturated fatty acids?

Accepted Answer

Safflower oil

Answer

Coconut oil

Answer

Butter

Answer

Vanaspati

Question 10

A person switches from a high-fat diet to a low-fat diet with a compensatory increase in carbohydrates to maintain the same caloric intake. Which lipoprotein is likely to increase?

Accepted Answer

VLDL

Answer

Chylomicron

Answer

IDL

Answer

HDL

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?