Now, what about fatty acid synthesis? What if we actually want to synthesize fatty acids? Why would we want to synthesize fatty acids? Well, if we have plenty of ATP molecules we don't want to form anymore, our cells will essentially take these acetyl units and we'll synthesize fatty acids from them. Or if we want to build some type of hormone or modify a protein or build up our cell membrane, we can also build up these fatty acids and use them in those processes. Now, as we'll see in just a moment, the synthesis is actually the opposite of the breakdown.