Why Correct: William Cumming Rose's 1935 research conclusively established 20 standard amino acids as the building blocks of proteins in all living organisms, encoded by the universal genetic code.
Distractor Analysis: 10 represents an incomplete subset of the standard amino acids. 30 includes some rare modified amino acids like selenocysteine and pyrrolysine that occur in specific contexts but aren't part of the universal set. 40 approximates the total count of known naturally occurring amino acids including non-proteinogenic ones like ornithine, citrulline, and beta-alanine.
Takeaway: Selenocysteine, incorporated via UGA codon recoding, functions as the 21st proteinogenic amino acid in specific enzymes like glutathione peroxidase and thioredoxin reductase.

Why Correct: Enzymes function as biological catalysts by providing an alternative reaction pathway with a lower activation energy barrier, allowing more reactant molecules to overcome it per unit time.
Distractor Analysis: Increasing activation energy would slow down the reaction. Lowering temperature generally decreases reaction rates by reducing molecular kinetic energy. Increasing temperature can speed up reactions but is not the mechanism of enzyme catalysis and can denature enzymes.
Takeaway: Enzymes do not alter the equilibrium constant or the free energy change (ΔG) of a reaction—they only accelerate the rate at which equilibrium is reached.

Why Correct: Riboflavin (vitamin B2) forms FAD (flavin adenine dinucleotide), which serves as the prosthetic group for acyl-CoA dehydrogenase in fatty acid beta-oxidation.
Distractor Analysis: Nicotinic acid (niacin) forms NAD+/NADP+ coenzymes. Pyridoxine (vitamin B6) forms pyridoxal phosphate for transamination. Pantothenic acid forms coenzyme A, which carries acyl groups but is not the prosthetic group of acyl-CoA dehydrogenase.
Takeaway: FAD-dependent dehydrogenases include succinate dehydrogenase (Krebs cycle) and glycerol-3-phosphate dehydrogenase (electron transport shuttle).

Why Correct: Metabolism encompasses all biochemical reactions in living organisms, including both anabolic (building) and catabolic (breaking down) processes that maintain life.
Distractor Analysis: Anabolism specifically refers to synthesis reactions that build complex molecules from simpler ones, catabolism describes breakdown reactions that release energy from complex molecules, and 'None of the above' is incorrect since metabolism accurately defines the complete set of biochemical processes.
Takeaway: Basal metabolic rate measures energy expenditure at complete rest, while metabolic pathways like glycolysis and Krebs cycle are specific sequences of biochemical reactions.

Why Correct: Cholesterol is a steroid alcohol with a characteristic four-ring hydrocarbon structure and a hydroxyl group, classified as a sterol.
Distractor Analysis: Saturated fats contain only single bonds between carbon atoms, like butter and lard. Unsaturated fats have one or more double bonds, like olive oil and fish oil. Diglycerides consist of two fatty acids attached to a glycerol backbone.
Takeaway: All steroids share the same four-ring structure, including hormones like testosterone and cortisol.