One of the distinctive features of living organisms is their capacity to maintain internal order. Cells perform this function through a network of millions of interconnected chemical reactions known as metabolism. Most of these reactions are not spontaneous since they are not energetically favorable. They are only possible because of the chemical energy contained in adenosin triphosphate molecules, also known as ATP. This energy is provided by the decomposition of molecules from food and the oxygen from the air.
Before they can be used, glucose and oxygen must reach the cells. Food molecules are digested into nutrients and absorbed through the small intestine. Blood carries them from here to every part of the organism. Oxygen is also transported to the cells through blood from air inhaled through the trachea, bronchi, bronchioles, and pulmonary alveoli.
Cellular respiration occurs in three main stages. Glycolysis takes place in the hyaloplasm and consists of splitting a glucose molecule into two pyruvates and creating two ATP molecules and two reduced compounds. These pyruvates then enter the mitochondrion's matrix, where they are oxidized through the Krebs cycle, creating six carbon dioxide molecules, two ATP molecules, and ten reduced compounds. Finally, the respiratory chain occurs in the internal mitochondrial membrane, where electrons are passed on to oxygen and combine with it to form water. This final stage generates 32 ATP molecules, resulting in an overall energy recovery of 36 ATP molecules for every oxidized glucose molecule.