Negative entropy is vital in sustaining life. Life is based on carrying out metabolic reactions, enabling the conversion of chemical energy into usable forms. This could be through various metabolic pathways: photosynthesis, respiration, the Krebs cycle, … Negative entropy is necessary to enable unfavourable reactions to occur. For example, ATP synthesis is catalysed by the ATP synthase enzyme. This enzyme, found in the mitochondrial inner membrane, allows the use of a proton (H+) gradient to phosphorylate adenine diphosphate (ADP) molecules. This is done by binding the H+ ions to the Fo rotor, producing rotational energy and enabling conformational change in the F1 head (where the catalysis occurs). These conformational changes follow the binding change model, cycling through open, loose, and tight conformations:
- Open: during this conformation, the F1 head allows the release of ATP
- Loose: this conformation allows ADP and Pi to bind to the catalytic site
- Tight: this is the catalytic conformation, enabling the phosphorylation of ADP to occur, producing an ATP
There are three catalytic sites within the F1 head, so every rotation of the Fo rotor produces three ATPs. The number of H+ ions required to rotate the Fo rotor once depends on the species, ranging from 8 (in animals) to 15 (in plants). This is due to the variability in energy supply experienced by plants, such as the sun going in, or not being able to synthesise further sugars during the night. Animals do not have this issue, due to the more stable energy supply they access.