The oxidation-reduction progression involving alcohols, aldehydes/ketones, carboxyl groups, and carbon dioxide is important both for biosynthesis and energy metabolism. Here are a few more examples which are good to contemplate for the MCAT. Contemplation is not the same thing as memorizing dozens of biochemical pathways before the exam.

The conversion of pyruvate to lactate involves reduction of a carbonyl group to a hydroxyl group.

The carbonyl group in an acetoacetyl group is reduced, on its way to forming a purely aliphatic chain, in fatty acid synthesis.

The corticosteroids cortisol and cortisone differ from each other by the reduction of a single carbonyl group.