ATP can be formed during CELLULAR RESPIRATION , either in the general cytoplasm during GLYCOLYSIS or in the MITOCHONDRIA via the KREBS CYCLE and the ELECTRON TRANSPORT SYSTEM if oxygen is present. ATP is also formed during photosynthesis in the CHLOROPLASTS of green plants, again using an electron transport system. ATP molecules act therefore as short-term ‘biological batteries’, retaining energy until required for such processes as active transport, synthesis of new materials, nerve transmission, and muscle contraction. An active cell requires more than two million molecules of ATP per second to drive its biochemical machinery.
Step ATP (used -) (produced +) 1 -1 3 -1 5 - NADH to pyruvic acid to lactic acid. not used 0 6 used twice 1 x 2 = + 2 9 used twice 1 x 2 = + 2 NET 2 ATP
Acetyl coenzyme A or acetyl-CoA is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism .  Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for energy production. Coenzyme A (CoASH or CoA) consists of a β-mercaptoethylamine group linked to the vitamin pantothenic acid through an amide linkage.  The acetyl group (indicated in blue in the structural diagram on the right) of acetyl-CoA is linked to the sulfhydryl substituent of the β-mercaptoethylamine group. This thioester linkage is a "high energy" bond, which is particularly reactive. Hydrolysis of the thioester bond is exergonic (− kJ/mol).