Photosynthesis is the process where light energy is converted to chemical energy by photosynthetic organisms, such as plants, algae and photosynthetic bacteria, and covers a wide range of biochemical reactions, from the capture of light energy by photosynthetic pigments ("light phase") to the fixation of carbon to organic compounds during the so-called "dark-phase" of photosynthesis. Thus, photosynthesis is considered the primary mechanism of energy input into living organisms. The assimilation of CO₂ and the simultaneous release of oxygen through photosynthesis is responsible for maintaining the concentrations of these gases in the atmosphere, and is thus a factor influencing the climate on earth. Photosynthesis is probably the single most important biological process on earth, maintaining the aerobic environment necessary for life, as well as fulfilling our requirements for energy. Scientific research into photosynthesis and the related field of photobiology allows us to elucidate the mechanisms and evolution of the photosynthetic process, giving us the potential to increase crop yields and adaptability, and providing an insight into new ways to harvest and utilize solar energy.

The photosynthetic organisms are the major organic recipient of solar irradiation, and the regulation of photosynthetic electron transport is one of their primary means of energy management. One of the main goals of electron transport regulation in photosynthetic organisms is the maintenance of a poised system of electron mediators. Over-reduction of the photosynthetic electron transport mechanism leads to the photooxidative damage of these components by active oxygen species (PAPERS I-II), processes also believed to be involved in the aging mechanisms of other organisms, including humans. Amazingly, grazing animals become light sensitive due to the accumulation of chlorophyll metabolites that are photosensitizers. This knowledge, together with our ever growing understanding of energy transfer in photosynthetic and other biological membranes, has lead to the development of photochemical therapies in cancer research. Clearly, photochemical changes have important effects on most organisms. Once the mechanisms of these changes are understood, it will be possible to modify the process and improve the efficiency of beneficial reactions, such as active oxygen scavenging, or inhibit adverse reactions, such as the destabilizing effects of UV-light on DNA.

Photobiologists and plant biochemists are engaged in the exiting task of elucidating the mechanisms and physiological importance of electron transport in photosynthesis. Recent developments in protein chemistry, molecular biology and biophysics have opened new possibilities for the study of electron transport. These developments include an increase in our understanding of gene structure, expression and manipulation, developments in the field of enzyme isolation and purification, as well as in the detection and assay of specific enzymes (PAPER III-V). Amino acid sequences of most photosynthetic polypeptides are available, as are high-resolution 2- and 3-dimensional structures obtained from electron microscopy and x-ray diffraction. Model systems are being devised to explain the chemistry and physics of photosynthetic systems, the results of which are leading to the development of efficient solar cells and artificial photosynthesis prototypes. Spectroscopic techniques such as optical spectroscopy and NMR are useful in measuring levels, fluxes and stoichiometries of electron transport components (PAPERS IV-V), while EPR techniques are allowing us to probe the structures of these components. Of the many spectroscopic techniques available to photochemists, the most exciting and potent are those based on lasers. Photoacoustic techniques measure thermal emissions associated with cyclic electron transport in vivo, while cyclic electron flow has been further quantified in recent years through the use of absorbance-difference measurements of electron flow through P₇₀₀ (PAPERS I-II, IV), through the use of PSII fluorescence-quenching analyses, and through measurements of transthylakoidal electrochemical potential.