Overview

Photosynthesis is the process by which autotrophic biological organisms make use of solar energy for the synthesis of their biomolecules. It has large-scale planetary consequences, such as the storage of fuels and the accumulation of oxygen in the atmosphere. On the molecular scale, all the initial steps of conversion of light energy into chemical energy are carried out in photosynthetic organisms by molecular energy converting proteins called reaction centres (RCs). Several classes of biological organisms perform photosynthesis; some photosynthetic bacteria are prokaryotic (lacking cell nuclei), while algae and higher plants are eukaryotic (possessing cell nuclei). Eukaryotic photosynthesisers fall into two groups, oxygenic (liberating oxygen during photosynthesis) and anoxygenic (liberating no oxygen). Oxygenic organisms (higher plants, algae and cyanobacteria) possess two types of reaction centre called Photosystem I and Photosystem II and often abbreviated as PSI and PSII, respectively. These work in series, allowing oxygenic organisms to use water as a source of electrons to reduce CO ₂. Photosynthetic bacteria (purple non-sulphur bacteria, green sulphur bacteria etc.) are anoxygenic. These possess only one type of RC, either of the PSI or of the PSII type, and they use sulphur compounds such as H ₂S or oxidisable organic molecules as their source of electrons. In other ways, for instance in size and ecological requirements, photosynthetic organisms...