NADPH differs structurally from NADH in the presence of a phosphoryl group on one of its ribose units. Functionally, NADPH serves a different purpose than NADH. NADH donates electrons (hydride) to the respiratory chain for chemiosmotic generation of ATP. NADPH serves as a hydride donor in biosynthesis, as a source of reducing power. For example, the generation of NADPH is very high in adipose tissue, where large amounts are consumed in the reductive synthesis of fatty acids from acetyl CoA.
To go into the MCAT with this general understanding the differing roles of NADH and NADPH for energy transduction and biosynthesis respectively, you have taken one small step further than the prerequisite basis for the exam. It is really helpful to get a sense of where the line is, and I believe this is just one step across the line, but very close, a bit further into biochemistry than the test will expect you to have gone, with the MCAT not requiring biochemistry itself but a very, very excellent Bio 101 treatment. There is a theory that you should only study the bare MCAT material itself, but that is wrong. Where you want to to get with the material is at least one step over the line, one step more advanced, because it is only from that place that you can get your mind around the MCAT material.