Lipid Chemistry and Metabolism Questions and Answers

Answer: C

The correct match is P-4, Q-1, R-2, S-3.

• 17-β estradiol is a steroid hormone made from cholesterol (P-4).
• Thromboxane A2 is an eicosanoid made from arachidonic acid by cyclooxygenase (Q-1).
• Epinephrine is a catecholamine made from tyrosine via DOPA and norepinephrine (R-2).
• Retinoic acid is made from vitamin A, which comes from β-carotene (S-3).

The other options pair estradiol with arachidonic acid or epinephrine with cholesterol or β-carotene, none of which is a precursor of those hormones.

Answer: D

The products are (8 Acetyl CoA) and (7 Acetyl CoA + 1 Propionyl CoA), respectively.

Each round of β-oxidation removes two carbons as acetyl CoA. A 16-carbon chain goes through 7 rounds, the last of which splits a four-carbon unit into two acetyl CoA, so it yields 8 acetyl CoA.

An odd-chain, 17-carbon acid also goes through 7 rounds, releasing 7 acetyl CoA, but the final unit left over has three carbons: propionyl CoA, which is then carboxylated to methylmalonyl CoA and converted to succinyl CoA.

β-oxidation releases no CO₂, so the options with CO₂ are wrong, and propionyl CoA arises only once, at the end of an odd-chain acid.

Answer: A

Testosterone is converted to estradiol by aromatase.

Aromatase (CYP19A1) removes the C-19 methyl group and aromatises the A ring of androgens, turning testosterone into estradiol and androstenedione into estrone.

The others act on steroids without aromatising them. 3β-hydroxysteroid dehydrogenase converts Δ5-3β-hydroxysteroids to Δ4-3-ketosteroids, for example pregnenolone to progesterone. 5α-reductase reduces testosterone to dihydrotestosterone. 17β-hydroxysteroid dehydrogenase interconverts 17-keto and 17β-hydroxy steroids, such as androstenedione and testosterone or estrone and estradiol.

Answer: A

Both [a] and [r] are true and [r] is the correct reason for [a].

Complete oxidation of palmitate (16 carbons) yields about 106 ATP, roughly 6.6 ATP per carbon, while glucose (6 carbons) yields about 30 to 32 ATP, roughly 5 per carbon. So per carbon, palmitic acid gives more ATP.

The reason is the oxidation state of the carbons. The carbons of a fatty acid are mostly \(-CH_{2}-\) groups, highly reduced, so oxidising each to \(CO_{2}\) releases more electrons (as NADH and \(FADH_{2}\)) for the electron transport chain. The carbons of glucose already carry hydroxyl groups and are partly oxidised. The reason therefore explains the assertion directly.

Answer: D

The Kennedy pathway is associated with the endoplasmic reticulum.

The Kennedy pathway is the CDP-choline and CDP-ethanolamine route by which cells make phosphatidylcholine and phosphatidylethanolamine. Choline or ethanolamine is phosphorylated, activated to its CDP derivative, and finally transferred onto diacylglycerol by a phosphotransferase bound to the ER membrane, where most membrane phospholipid is assembled.

Peroxisomes carry out the first steps of ether-lipid (plasmalogen) synthesis, mitochondria make cardiolipin and convert phosphatidylserine to phosphatidylethanolamine by decarboxylation, and the nucleus does not host this pathway.

Answer: A

The most abundant phospholipid is phosphatidyl ethanolamine.

The inner (plasma) membrane of E. coli is roughly three quarters phosphatidylethanolamine (PE), with phosphatidylglycerol making up most of the rest and cardiolipin a few per cent. This simple, zwitterion-rich composition is typical of Gram-negative bacteria.

Phosphatidylcholine, the dominant phospholipid of eukaryotic membranes, is essentially absent from E. coli. Phosphatidylserine occurs only as a short-lived intermediate, since it is decarboxylated to PE as soon as it is made. Phosphatidylinositol is a eukaryotic lipid that E. coli does not make.

Answer: D

The specific gravity of lipid is 0.8.

Fats and oils are lighter than water, with specific gravities a little below 1, typically about 0.8 to 0.9. That is why oil floats on water and why fat separates as a layer on top of milk.

1.0 is the specific gravity of water itself, and 1.5 would make lipids sink. 0.5 is far too low for fats and oils, so of the options, 0.8 is the value the key accepts.

Answer: C

The correct matching is a-4, b-1, c-2, d-3.

The systematic name counts the carbon atoms in the chain:

• Acetic acid, 2 carbons: ethanoic acid.
• Butyric acid, 4 carbons: n-butanoic acid.
• Palmitic acid, 16 carbons: n-hexadecanoic acid.
• Lignoceric acid, 24 carbons: n-tetracosanoic acid.

Option D swaps palmitic and lignoceric acids, giving the 16-carbon acid a 24-carbon name. The first two codes start by pairing acetic acid with the wrong chain length, and acetic acid is the easiest anchor to fix: it is the acid of vinegar, with two carbons.

Answer: C

The one that is not a steroid is insulin.

Insulin is a peptide hormone made of two polypeptide chains, A and B, joined by disulphide bonds, secreted by the beta cells of the pancreas.

Steroids share the four-ring steroid nucleus. Cholesterol is the parent steroid, bile acids are made from cholesterol in the liver, and vitamin D is a secosteroid formed from a cholesterol derivative in the skin. So all three of the other options are steroids.

Answer: B

The enzyme not in the fatty acid synthase complex is β-hydroxy acyl CoA dehydrogenase.

That enzyme belongs to β-oxidation, the breakdown of fatty acids in mitochondria, where it oxidises the β-hydroxy group using NAD⁺. The synthase runs the reverse sequence with different enzymes: condensation, then reduction of the β-keto group (β-ketoacyl reductase), dehydration (β-hydroxyacyl dehydratase) and a second reduction (enoyl reductase).

Enoyl reductase, β-keto acyl reductase and acetyl transacylase are all activities of the synthase complex, so they are the distractors.