The activities of life require a continuous input of energy, which almost all organisms obtain by oxidizing complex organic molecules such as carbohydrates, fats, and proteins. Unless constantly renewed, the existing levels of these molecules would be very quickly exhausted. The supply is continuously replenished by photosynthesis in the eukaryotic algae and higher plants and in the photosynthetic bacteria and blue-green algae. Photosynthesis can be defined as a process in which sunlight is used as an energy source for converting electrons to a high-energy form. These electrons are then used to reduce or fix an electron acceptor, usually an inorganic substance, transforming it to a more complex organic molecule. There are thus two overall steps in photosynthesis: (1) transfer of light energy to electrons and (2) use of the electrons for reductions in cellular synthesis. In the blue-green algae, eukaryotic algae, and higher plants, H2O is used as the source of the electrons in step of this process. The high-energy electrons are used to reduce another inorganic molecule, CO2, in step 2, fixing it into organic compounds. Oxygen, derived from the H2O used in step is released as an important by product of the reaction. The bacteria capable of photosynthesis, the green and purple photosynthetic bacteria, use more highly reduced substances than H2O as an electron source and release molecules such as sulfur or hydrogen rather than oxygen as by-products.
Both steps of photosynthesis in eukaryotes take place inside the organelle specialized for this function, the chloroplast. Within chloroplasts, the enzymes and pigments carrying out step 1 (the absorption of light energy and the excitation of electrons to high energy levels) are concentrated in membranes; step 2 (CO2 fixation) is carried out by enzymes suspended in solution in the chloroplast interior. In prokaryotes, the reactions of photosynthesis are distributed between the plasma membrane and its derivatives, and the background substance of the cytoplasm.
