Redox reactions are fundamental to the way in which all cells obtain, convert, and use energy. Photosynthesis converts the energy of sunlight into chemical form. Photosynthesis is a series of light-driven redox reactions, one of which involves a single molecule of the green pigment chlorophyll. In plant cells, photosynthesis occurs in chloroplasts, and uses water as a source of hydrogen atoms. This natural, chemical oxidation of water produces and replenishes the oxygen in the earth's atmosphere.
Respiration is the useful release of the energy that has been stored by photosynthesis .
In animal and plant cells, respiration takes place in mitochondria, and is a series of redox reactions reactions that finally dump hydrogen atoms onto the oxygen molecules originally produced in photosynthesis. We depend totally on respiration and on photosynthesis - for the food we eat and the air we breathe. Directed mutation is used to alter specific structural features such as sites of phosphorylation. We have recently resolved up to seventeen chloroplast proteins whose phosphorylation is controlled by redox potential, and find parallel control mechanisms in cyanobacteria, purple photosynthetic bacteria, and mitochondria. Chloroplasts and mitochondria are descendants of bacteria, from which they retain genes for a small but constant sub-set of their own components.