Question

why can't oxaloacetate or acetyl coA cross the membrane on their own?

Answer 1

Acetyl CoA and Oxaloacetate both cannot cross the mitochondrial membrane(s). However, when the two join together and become citrate (citrate synthase from the TCA cycle catalyzes this), citrate can cross the mitochondrial membrane. Once the citrate is out in the cytosol, it can be cleaved back into oxaloacetate and acetyl CoA by citrate lyase. The oxaloacetate can be converted to malate, which is converted into pyruvate by an oxidative decarboxylation reaction catalyzed by malic enzyme. The pyruvate can cross the mitochondria membrane to get back into the mitochondria to be converted into oxaloacetate, which can join another acetyl CoA to allow the process to repeat and send as many acetyl coa molecules into the cytosol as necessary.