Figure 1. Food enters the body as a mixture of lipids, carbohydrates and proteins. These compounds are digested, assimilated and stored as triacylglycerol and glycogen, mainly in adipose tissue, liver and muscle. Over 90% of total energy reserves are in the form of triacylglycerol for long-term, maintenance metabolism. The glycogen reserves are only used when ATP must be produced very quickly or when enough oxygen cannot be supplied to the cells (e.g. during very intensive exercise). When the organism is not eating, the energy contained in triacylglycerol and glycogen is progressively passed on to ATP by the formation of energy-rich phosphate bonds between ADP and P_i. In turn, the energy needed for cellular work can be harnessed from ATP by the breaking of these phosphate bonds, thereby converting it back to ADP + P_i. The utilization of metabolic reserves allows for the maintenance of the ADP/ATP cycle that provides all the energy necessary for biochemical work within living cells. Examples of such work are muscle contraction during exercise, pumping of ions across membranes to maintain concentration gradients and the synthesis of macromolecules to make new tissues during growth.

Figure 2. All cells are capable of producing ATP from carbohydrate reserves in the absence of oxygen. The metabolic pathway used for this transfer of energy from carbohydrates to ATP is called glycolysis and it is located in the cytoplasm. NAD⁺ is an essential cofactor of glycolysis because it must be reduced to NADH in the process of ATP synthesis. To avoid running out of NAD⁺, cells regenerate it by oxidizing NADH back to NAD⁺ in the last reaction of the pathway that converts pyruvate to lactate. The complete pathway of anaerobic glycolysis has two major advantages: (1) it can produce ATP more quickly than any other pathway, and (2) it can do so without oxygen. However, anaerobic glycolysis is particularly inefficient because it only produces 1/18 of the ATP obtainable from the same amount of carbohydrates when complete oxidation to  CO₂ and  H₂O can take place. When ATP does not have to be produced quickly and when  O₂ is provided in sufficient amounts, glycolysis is routinely used without its last reaction to feed pyruvate into the Krebs cycle.

Figure 3. ATP can only be produced efficiently through the complete oxidation of metabolic fuels in the presence of oxygen. The enzymes necessary for the oxidation of lipids and carbohydrates to  CO₂ and  H₂O are located in specialized organelles called mitochondria. Fatty acids derived from triacylglycerol are broken down to acetyl-CoA by a mitochondrial pathway called -oxidation. Glycolysis catabolizes glycogen to pyruvate before converting it to acetyl-CoA. Proteins are broken down to individual amino acids that enter the oxidation pathways as pyruvate or acetyl-CoA or directly as intermediates of the Krebs cycle. -Oxidation, glycolysis and the Krebs cycle produce the energy-rich intermediates NADH and FADH₂. The synthesis of ATP from ADP and P_i is then coupled to the oxidation of these intermediates by oxygen to NAD⁺ and FAD. The complex pathway allowing the transfer of energy from NADH and FADH₂ to ATP is called the respiratory chain and it is located in the inner membrane of mitochondria.