Photosynthesis is a plant process (in chloroplasts) whereby energy from light is harvested to provide carbohydrates for energy production (Figure 17.1). It is the major path through which carbon reenters the biosphere (from CO2). Photosynthesis is also the major source of oxygen in the earth's atmosphere.

Photosynthetic organisms probably first appeared about 3.5 billion years ago. The Earth's atmosphere before that was probably devoid of oxygen (though rich in carbon dioxide).

The reaction for photosynthesis can be written in a general way as shown in here. The term, [CH2O], represents a generalized carbohydrate. Because the reverse reaction is an oxidation, the reaction as written must be a reduction. In this case, H2O is shown as the source of electrons (ultimate reducing agent). This is the case in plants, most algae, and cyanobacteria. Other photosynthetic bacteria use other reductants, so a more general reaction for photosynthesis is that given here, where H2A is the general reductant and A is the oxidized product. Some example reactions of this type are shown in Table 17.1.

The source of the oxygen released as a result of photosynthesis in plants, algae, and cyanobacteria is H2O. To reflect this, the reaction for photosynthesis can also more clearly be written as shown here. Light energy has no direct effect on this reaction and H2O has no known way to directly reduce CO2. Thus, photosynthesis is a much more intricate process than these equations suggest.

Figure 17.3 depicts schematically the way in which light energy transfers electrons from H2O to CO2 and how oxygen is released.

1. Photochemical oxidation of H2O - Energy from light causes electrons from H2O to be transferred (with protons) to NADP⁺, forming NADPH, and releasing O2. This series of steps is sometimes called the light reactions.

2. Part of the energy of from the sunlight is captured in a process called photophosphorylation in which ADP is phosphorylated to form ATP. The remaining reactions of photosynthesis are not dependent upon light and are part of what are called the dark reactions of photosynthesis. They do NOT, however, occur only in the dark. They are, in fact, stimulated by light.

In the dark reactions of photosynthesis, NADPH and ATP produced by the light reactions are used in the reductive synthesis of carbohydrate from CO2 and water.