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.