Most of the phototrophic procaryotes are autotrophs, which mean that they are able to fix CO₂ as a sole source of carbon for growth. Just as the oxidation of organic material yields energy, electrons and CO₂, in order to build up CO₂ to the level of cell material (CH₂O), energy (ATP) and electrons (reducing power) are required. The overall reaction for the fixation of CO₂ in the Calvin cycle is CO₂ + 3ATP + 2NADPH₂ ----------> CH₂O + 2ADP + 2Pi + 2NADP. The light reactions operate to produce ATP to provide energy for the dark reactions of CO₂ fixation. The dark reactions also need reductant (electrons). Usually the provision of electrons is in some way connected to the light reactions.

Fig. 18. Comparison of electron transport pathways in oxygenic and anoxygenic photosynthesis

The differences between plant and bacterial photosynthesis are summarized in Table 3 below. Bacterial photosynthesis is an anoxygenic process. The external electron donor for bacterial photosynthesis is never H₂O, and therefore, purple and green bacteria never produce O₂ during photosynthesis. Furthermore, bacterial photosynthesis is usually inhibited by O₂ and takes place in microaerophilic and anaerobic environments. Bacterial chlorophylls use light at longer wave lengths not utilized in plant photosynthesis, and therefore they do not have to compete with oxygenic phototrophs for light. Bacteria use only cyclic photophosphorylation (Photosystem I) for ATP synthesis and lack a second photosystem.

Table 3. Differences between plant and bacterial photosynthesis