Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
The process of photosynthesis consists of two phases: the light-dependent and light-independent reactions

Photosynthesis in green plants proceeds in two stages: the first encompasses light reactions, which occur only when the plant is illuminated, whereas the second involves dark reactions, which can take place both in the dark and in the light. During the light reactions, light energy is absorbed by chlorophyll and other pigments of photosynthetic Cells and is stored in chemical form as two high-energy products, ATP and NADPH; oxygen is evolved simultaneously. In the dark reactions, the ATP and NADPH generated during the light reactions are utilized to reduce carbon dioxide to glucose and other organic products (Fig. 23-5). The production of oxygen, which occurs exclusively in the light, and the reduction of carbon dioxide, which does not require light, thus represent two completely distinct processes.

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Fig. 23-5. In the light reactions, the high-energy compounds NADPH and ATP are synthesized at the expense of solar energy. These compounds are utilized in the dark reactions to reduce CO₂, leading to The formation of glucose.

However, an important point should be noted here, which we will discuss in more detail later. Although the reactions resulting in the reduction of CO₂ to glucose can proceed in the dark, they are regulated by light.