Which of the following electron carriers is primarily involved in the synthesis of cholesterol, fatty acids, and steroids?

NADPH is a key electron carrier involved in anabolic reactions, particularly in the biosynthesis of molecules such as cholesterol, fatty acids, and steroids. It provides the reducing equivalents necessary for these synthesis pathways. An example of this is in the fatty acid synthesis process, where NADPH donates electrons and protons that reduce acetyl-CoA derivatives to form fatty acids.

In steroid and cholesterol synthesis, NADPH is also crucial as it helps in the reduction steps of hydroxyl groups and other functional groups, which are integral to the formation of these lipids. The pentose phosphate pathway, which generates NADPH, is thus particularly important in tissues that engage in high levels of lipid biosynthesis, like the liver and adipose tissue.

Other electron carriers like NADH and FADH2 are primarily involved in catabolic pathways and energy production through oxidative phosphorylation rather than in biosynthetic processes. ATP serves as a primary energy currency in the cell but is not directly involved in the transfer of reducing equivalents needed for biosynthetic reactions. Hence, the significant role of NADPH in providing reducing power specifically aligns with the synthesis of cholesterol, fatty acids, and steroids.