Biochemistry sits at the point where chemistry stops being abstract and starts describing living systems. Practice covers carbohydrates, proteins and amino acids, lipids, nucleic acids, enzymes and enzyme kinetics, metabolic pathways, and vitamins and coenzymes. Pathway questions include the regulation step in the explanation, because that is usually what the question is really testing rather than the sequence itself.
Which of the following statements about HDL (High-Density Lipoprotein) is CORRECT?
Answer: B
HDL mediates reverse cholesterol transport, removing excess cholesterol from peripheral tissues and transporting it to the liver for excretion. High HDL levels are protective against cardiovascular disease.
Q.62Easy
Linoleic acid (18:2) and alpha-linolenic acid (18:3) are classified as:
Answer: A
Linoleic acid and alpha-linolenic acid are essential fatty acids that cannot be synthesized by humans and must be obtained from dietary sources.
Q.63Easy
Which of the following lipids is amphipathic in nature and forms the structural basis of cell membranes?
Answer: A
Phospholipids have both hydrophilic (phosphate group) and hydrophobic (fatty acid chains) regions, making them amphipathic. This property allows them to spontaneously form bilayers in aqueous solutions, serving as the structural foundation of biological membranes.
Q.64Easy
Which of the following fatty acids is essential and must be obtained from dietary sources?
Answer: B
Linoleic acid (omega-6 PUFA) is essential because humans lack the enzyme delta-12 desaturase needed to introduce double bonds beyond carbon-9. Only plant-based sources provide linoleic and alpha-linolenic acids. Other fatty acids can be synthesized de novo.
Q.65Easy
Which of the following factors would increase the fluidity of a cell membrane?
Answer: C
Unsaturated fatty acids with cis double bonds introduce kinks in the carbon chain, preventing tight packing and increasing membrane fluidity. Cholesterol and saturated fatty acids decrease fluidity. Temperature and cholesterol content can be modulated to maintain optimal fluidity.
Q.66Easy
Which of the following lipids serves as a precursor for the synthesis of steroid hormones and bile acids?
Answer: C
Cholesterol is the universal precursor for steroid hormone synthesis (cortisol, testosterone, estrogen) and bile acid synthesis. The side-chain cleavage enzyme (P450scc) catalyzes the first committed step of hormone synthesis, while 7-alpha-hydroxylase initiates bile acid synthesis.
Q.67Easy
Which lipoprotein fraction is responsible for the reverse cholesterol transport from peripheral tissues to the liver?
Answer: C
HDL mediates reverse cholesterol transport, removing cholesterol from peripheral tissues and arterial walls via ABCA1 and SR-BI pathways, then transferring it to the liver via CETP-mediated exchange with apoB-containing lipoproteins. Higher HDL is associated with reduced cardiovascular risk.
Q.68Easy
Which of the following is the primary storage form of lipids in adipose tissue?
Answer: A
Triglycerides are the primary storage form of lipids in adipose tissue, accounting for >95% of stored lipids. They are mobilized during energy deficit states.
Q.69Easy
The rate-limiting enzyme in cholesterol synthesis is:
Answer: B
HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate and is the rate-limiting step in cholesterol biosynthesis. It is inhibited by statins.
Q.70Easy
Which lipid component is essential for maintaining the fluidity of cell membranes?
Answer: A
Cholesterol molecules insert between phospholipids in the bilayer, regulating membrane fluidity. At high temperatures, it reduces fluidity; at low temperatures, it increases it.
Q.71Easy
Which of the following statements about saturated fatty acids is correct?
Answer: A
Saturated fatty acids have higher melting points and are typically solid at room temperature due to tight packing. They are more stable and less prone to oxidation than unsaturated fatty acids.
Q.72Easy
Which of the following statements about prostaglandins is most accurate?
Answer: B
Prostaglandins are eicosanoids derived from arachidonic acid (20-carbon polyunsaturated fatty acid). They act as paracrine and autocrine signaling molecules.
