These are vital to living systems and they occur simultaneously. The reaction may involve solitary electrons or the electron may travel with a proton. It is often hydrogen. It is just the flow of electrons from one molecule to another to provide energy. Oxidation is the removal of hydrogen atoms.
Reduction is the gain of electrons.
The input of energy from the sun causes the electron to move to a higher level. The carbon dioxide is reduced to form glucose. The hydrogen atoms are transferred from water to carbon dioxide for photosynthesis. Metabolism Energy capturing processes or reactions are vital to all living process beginning with sunlight. The guiding principle for all cell metabolism involves these two processes.