Fatty acid synthesis begins with a preparatory step in which is transferred from to the 1. However, it cannot pass through the membrane, so it is transported as J.which is cleaved to and In the cytosol, acetyl COA is converted to ,a three-carbon compound. Fatty acid synthesis begins with the transfer of the acetyl group from acetyl CoA to fatty acid synthase. -carbon groups, supplied by are added to the growing acyl chain in a series of steps involving condensation, reduction and dehydration reactions. Elongation of the fatty acid chain stops at carbon atoms, after cycles, as the free fatty acid is released.