Fatty acids are made two carbon atoms at a time. Many of the metabolic processes taking place in cells, including the breakdown of carbohydrates for energy, result in the production of a two-carbon molecular fragment called an acetyl group (CH3-CO-). This is a very tiny molecular fragment that could easily get lost in the soup of similar tiny molecules which pack the cytoplasm of all metabolically active cells, so it is joined to a much larger molecule called CoEnzyme A (CoA).
This hybrid molecule, acetyl-CoA, is a central player in the synthesis of all fatty acids.
Acetyl-CoA is first made in the mitochondria either by the removal of hydrogen from a molecule pyruvate or by the oxidation of other fatty acids. This is a delicate balancing act. When the cell needs lots of ATP energy, all the pyruvate and oxidized fatty acids are broken down further in the tricarboxylic acid (TCA) cycle so as to make more and more ATP.
However, if the need for energy supplies decreases, the cells switch off these breakdown reactions and switch over to those metabolic pathways that join acetyl units together to form fatty acids, lipids and fat. These lipids are then stored and used as long-term fuel supplies as and when they are needed.
However, all of these two-carbon acetyl units are in the wrong place. Before they can be used in fatty acid synthesis, they have to be moved into the cytoplasm of the cell, where the fatty acids will be made.
Acetyl-CoA is moved through the mitochondrial membrane, and enters the cytoplasm of the cell, as the molecule citrate. In the cytoplasm, these citrate molecules are once again converted back to acetyl-CoA. This reaction requires that the cell use up some energy by breaking down an ATP molecule.