Biochemistry : Lipid Synthesis Reactants, Intermediates, and Products

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Lipid Synthesis Reactants, Intermediates, And Products

Which statement about fatty acids is false?

Possible Answers:

None of the other answers is false

Fatty acid synthesis takes place in the mitochondria

Beta-oxidation takes place in the mitochondria

Fatty acids are aliphatic

Carnitine transports fatty acids into the mitochondrial matrix

Correct answer:

Fatty acid synthesis takes place in the mitochondria

Explanation:

Beta-oxidation is the process by which fatty acid molecules are broken down in the mitochondria to generate acetyl-CoA, which then enters the Krebs cycle. Fatty acids are not aromatic (they do not have aromatic rings), rather they are organized in straight chains of hydrocarbons and are therefore aliphatic. Carnitine transports long-chain acyl groups from fatty acids into the mitochondria (so that they can undergo beta-oxidation). Fatty acid synthesis, however, takes place in the cytosol.

Example Question #2 : Lipid Synthesis Reactants, Intermediates, And Products

Which of the following are true?

Possible Answers:

None of the other answers are true

Palmitic acid is the precursor of stearic acid

 is synthesized during ketogenesis

Only one malonyl-CoA molecule is required for palmitate synthesis from acetyl-CoA

The pentose pathway produces a coenzyme that is needed for fatty acid degradation

Correct answer:

Palmitic acid is the precursor of stearic acid

Explanation:

During palmitate synthesis, malonyl-CoA molecules keep on being added to the fatty acid chain, seven malonyl CoA molecules total. Ketogenesis does not involve the synthesis of ; rather, NADH is oxidized to  as -hydroxybutyrate is formed from acetoacetate. The cofactor produced by the pentose phosphate pathway is NADPH, whereas the -oxidation-mediated degradation of fatty acids requires  and FAD. Palmitic acid is indeed the precursor of stearic acid, as well as of many other fatty acids.

Example Question #3 : Lipid Synthesis Reactants, Intermediates, And Products

Statins are a class of drugs that act to decrease the levels of LDL in blood. Which of the following could be the mechanism of action of statins?

Possible Answers:

Decreasing the production of acetyl-CoA

Decreasing the production of HMG-CoA

Increasing the production of malonyl-CoA

Decreasing the production of mevalonate

Correct answer:

Decreasing the production of mevalonate

Explanation:

Statins function to decrease the activity of HMG-CoA reductase, an important enzyme in the cholesterol synthesis pathway. This enzyme converts HMG-CoA to mevalonate. This step is the rate-limiting (and irreversible) step in this pathway. Statins inhibit this enzyme; therefore, statins prevent the production of mevalonate and cause an accumulation of HMG-CoA. The HMG-CoA can be converted into acetyl-CoA, which can now be used for many other processes.

Example Question #4 : Lipid Synthesis Reactants, Intermediates, And Products

A researcher is analyzing HMG-CoA. He isolates a region of the cell and observes that the HMG-CoA found in this region participates in the synthesis of ketone bodies. What can you conclude about this location in the cell?

Possible Answers:

It has enzymes for degradation of cell debris

It is bound by an inner and an outer membrane

Ribosomes found here to synthesize cytosolic proteins

It has histones

Correct answer:

It is bound by an inner and an outer membrane

Explanation:

HMG-CoA can be found in two locations: cytosol and mitochondria. In the cytosol, HMG-CoA participates in the production of cholesterol. In mitochondria, it participates in the production of ketone bodies. The question states that ketone bodies are produced; therefore, the researcher must be analyzing the mitochondria. Recall that mitochondria has two membranes: an inner and an outer membrane. Ribosomes in cytosol synthesize cytosolic proteins. The nucleus contains histones, which are proteins that facilitate packaging of DNA molecules. Degradative enzymes are found in organelles such as lysosomes and peroxisomes. These organelles clean the cell by removing unwanted cell debris.

Example Question #5 : Lipid Synthesis Reactants, Intermediates, And Products

What molecule is the source for all of the carbons in cholesterol?

Possible Answers:

Acetoacetic acid

Pyruvate

Acetyl-CoA

Malonyl-CoA

Correct answer:

Acetyl-CoA

Explanation:

Cholesterol is synthesized from acetyl-CoA. A cholesterol molecule contains 27 carbons and an acetyl-CoA molecule contains 2 carbons. Cholesterol is synthesized from a total of 18 acetyl-CoA molecules. These 18 molecules undergo reactions that yield a 30 carbon molecule and 6 carbon dioxide molecules (total of 36 carbons). The 30 carbon molecule loses 3 methyl groups and becomes the 27-carbon cholesterol molecule. Malonyl-CoA, acetoacetic acid, and pyruvate are not involved in this pathway.

Example Question #6 : Lipid Synthesis Reactants, Intermediates, And Products

The molecule repeatedly added to the growing carbon chain of an endogenously synthesized fatty acid contains how many carbons?

Possible Answers:

Correct answer:

Explanation:

The molecule which is repeatedly added to a growing fatty acid is malonyl-CoA. Malonyl-CoA is synthesized from acetyl-CoA (two carbons) and  (one carbon), and, thus, contains three carbons. Of course, it is important to remember that the  of malonyl-CoA leaves during the reaction with the acyl chain being synthesized. 

Example Question #7 : Lipid Synthesis Reactants, Intermediates, And Products

Which of the following is true about fatty tissue?

Possible Answers:

Chylomicrons are secreted by enterocytes

The lipoprotein lipase digests chylomicrons, yielding glycerol and triacyglycerides

Adipocytes stock mainly glycerol

Fatty tissue is composed of lipids, which originate mostly in the liver

Correct answer:

Chylomicrons are secreted by enterocytes

Explanation:

Adipocytes stock mainly fatty acids, not glycerol; glycerol produced during triacylglycerol degradation is shuttled through the blood to the liver. Lipase does not digest chylomicrons, but rather triacylglycerol, producing glycerol and fatty acids. The lipids (fatty acids) in fatty tissue mostly originate in our diet, not in our liver. Mammals specifically require certain polyunsaturated fatty acids which they are unable to synthesize, like linoleate; these are known as the essential fatty acids. Chylomicrons are proteins which carry triacylglycerols, cholesterol, and other lipids, obtained by the diet, away from the intestine. Chylomicrons are created in the endoplasmic reticulum of small intestine cells i.e., enterocytes and exoctyosed into lymphatic capillaries.

Example Question #8 : Lipid Synthesis Reactants, Intermediates, And Products

In what order does fatty acid synthesis occur?

Possible Answers:

Reduction, reduction, dehydration, condensation

Oxidation, hydration, oxidation, cleavage

Condensation, reduction, dehydration, reduction

Condensation, reduction, reduction, cleavage

Reduction, condensation, dehydration, reduction

Correct answer:

Condensation, reduction, dehydration, reduction

Explanation:

The proper order for fatty acid synthesis is condensation, reduction, dehydration, and reduction once again.  This creates an activated acyl group that has been lengthened by two carbons through this anabolic biosynthetic pathway.

Example Question #9 : Lipid Synthesis Reactants, Intermediates, And Products

Triglycerides (triacylglycerols) contain three fatty acid molecules and a glycerol molecule. Glycerol-3-phosphate is necessary for triglyceride synthesis. Which of these are the sources of glycerol-3 phosphate in the body?

I. Phosphorylation of glycerol by glycerol kinase in the liver forms glycerol-3 phosphate

II. Glycerol 3-phosphatase reduces dihydroacetone phosphate to glycerol 3-phosphate in the liver and adipose tissue

III. Phosphorylation of glycerol by glycerol kinase in the adipose tissue

Possible Answers:

I and III

I and II

I only

II only

I, II, and III

Correct answer:

I and II

Explanation:

The sources of glycerol-3 phosphate for triglyceride synthesis are glycerol in the liver, but not the adipose tissue (adipose tissue does not have glycerol kinase) and from the conversion of dihydroxyacetone phosphate (obtained in glycolysis) to glycerol-3 phosphatase in liver and adipose tissue. Triglycerides are one of the most important forms of storage of lipids in the body. 

Example Question #10 : Lipid Synthesis Reactants, Intermediates, And Products

Triglycerides and glycerophospholipids are lipids similar in structure. What are some structural and functional characteristics of triglycerides and glycerophospholipids?

Possible Answers:

Triglycerides are formed by three fatty acids and a glycerol molecule

All of these are correct

Triglycerides are a form of storing lipids in the body

Glycerophospholipids contain choline or inositol groups

Glycerophospholipids are part of the cellular membrane and are sources of second messengers such as diacylglycerol

Correct answer:

All of these are correct

Explanation:

Triglycerides are the major form of storing dietary lipids in the body.Triglycerides are composed of three fatty acids and a glycerol molecule. In glycerophospholipids the third fatty acid of a triglyceride particle is replaced by a phosphate group and a choline or inositol group. Choline groups are ammonium salt groups in neurotransmitters or phospholipids on cell membranes. Inositol groups are found in second messengers. Glycerophospholipids are part of the cellular membrane and are sources of second messengers such as diacylglycerol and inositol-3-phosphate. 

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