Fatty acids are synthesized in the cytosol, but acetyl CoA is produce from pyruvate in the mitochondria. Therefore the acetyl CoA must be transferred from the mitochondria into the cytosol to allows fatty acid synthesis to occur. Furthermore, the inner mitochondrial membrane is not readily permeable to this molecule. This problem is overcome by the condensation of acetyl CoA with oxaloacetate to form citrate. This is then transported into the cytosol where it is cleaved to regenerate acetyl CoA and oxaloacetate by ATP-citrate lyase in an energy-requiring procedure.