Life on earth is dependent on photosynthesis, the process in which solar light is converted into chemical energy and stored as carbohydrates. The carbohydrates are, finally, degraded to carbon dioxide and water in the cell respiration in a reaction requiring molecular oxygen. The liberated energy is utilized to power the life processes. In photosynthesis, the carbon dioxide and water are used to resynthesize carbohydrate, while molecular oxygen is released into the atmosphere as a waste product. Thus, not only is present life on earth largely a result of photosynthesis but so is the air we all breathe.
Photosynthesis and respiration are based on the transfer of electrons between donor and acceptor molecules bound to biological membranes – sheet-like structures composed of lipids and proteins which surround the cells and their inner compartments. The photosynthetic reactions in plants take place in the inner membranes of the chloroplasts, the organelles which contain the chlorophyll. Some bacteria have a simpler form of photosynthesis, to some extent similar to that in plants but without the ability to form oxygen.
In all types of photosynthesis, the light energy absorbed by chlorophyll is transferred to membrane-bound protein-pigment complexes, known as reaction centers. In these complexes the light energy initiates electron-transfer reactions which are coupled to the translocation of hydrogen ions across the membrane. The resulting pH gradient is utilized by another membrane-bound protein, ATPase, to synthesize ATP, a compound used as a fuel in energy-demanding biological processes. In cell respiration, too, electron transport is coupled to proton translocation and ATP synthesis.
Our knowledge about photosynthesis, respiration and other membrane-associated processes is limited due to the lack of information about the molecular organization of the membrane proteins involved, a problem which derives from difficulties in crystallizing these proteins. The crystals are used for the determination of the 3-dimensional structure of the proteins by X-ray diffraction, a technique in which X-rays are analyzed after being scattered by the molecules in a crystal.