Cellular respiration is the process that allows organisms such as humans to use the energy in the form of ATP. It begins with glycolysis which is where glucose is broken down into two pyruvic acids. In this reaction 4ATP is made and NAD+ is made into NADH. However, it takes 2ATP to begin, so only 2 out of the 4ATP made is gained. Next, in the Krebs Cycle, the products of glycolysis are taken and made into another 2ATP. This happens by first combining the pyruvates with oxygen, making them oxidized. One of the carbons bonds with an oxygen molecule and leaves as carbon dioxide. Acetyl coA is what’s left. Next, another NAD+ comes and takes a hydrogen to become NADH. The two pyruvates in this series of reactions created 2 molecules of NADH and