For the MCAT, I don't think memorizing minute mechanisms that we learn in Orgo I and II is too important. Definitely know electrophilic and nucleophilic sites of Carbonyl centers, know the reactivity of carboxylic acid and its derivates in the context of biologically relevant molecules such as peptides, triacylglycerides, etc. and definitely learn sugars the in the context of hemiacetal formation and stereospecificity (anomeric carbon, D vs. L). For Biochem, pathways and how they relate rather than the mechanisms are more important IMO (gluconeogenesis, PPP, Krebs)