The total of all the chemical reactions occurring in living organisms is metabolism and the sequences in which they occur are the metabolic pathways. Cells can metabolize because they are isolated systems, separated from their environment by membranes. Organisms, and the cells of which they are composed, use enzymes to regulate the reactions and utilize energy carriers to move energy among the parts of the system. Metabolic reactions are linked so that exergonic reactions supply energy for the endergonic.
Most of the energy exchanges in plants are chemical reactions that involve the exchange of energy between one set of chemical bonds and another. These, for the most part, are oxidation‐reduction reactions (commonly called redox reactions). In oxidation, electrons are lost from an atom or molecule, which is then said to be oxidized. (The term “oxidation” is used because oxygen is often the electron acceptor.) Reduction always accompanies oxidation and is the acceptance or gain of electrons by another atom, which, thereby, is reduced. As electrons are lost, so is energy, thus oxidized molecules have less energy than reduced molecules that gain energy as they receive electrons.
In organisms, electrons rarely move alone and are usually accompanied by a proton (a hydrogen atom and its single electron). Oxidation, accordingly, entails the removal of a hydrogen atom, and reduction entails the addition of hydrogen atoms.
The collections of biological molecules within the cells are reduced and electron‐rich and so have relatively weak chemical bonds. In the surrounding environment, most of the molecules are oxidized (and electron‐poor) and have much stronger bonds. Living organisms thus continually add energy to their systems, to prevent operation of the Second Law of Thermodynamics, i.e. to avert becoming a disordered collection of oxidized molecules.
Photosynthesis and respiration are both oxidation‐reduction processes. Photosynthesis requires the input of energy, while respiration releases energy; photosynthesis is thus an endergonic process, respiration an exergonic.