Britannica Sources used in this answer

Why are the light-independent reactions sometimes called dark reactions?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

The light-independent reactions are commonly referred to as "dark reactions" because they do not require light energy directly to proceed. While these reactions can occur in the presence of light, they are distinguished from the light-dependent reactions because they rely on the chemical energy (ATP) and reducing power (NADPH) generated during the initial light-harvesting stage of photosynthesis.123

It is important to note that the term "dark reactions" does not imply that they only occur at night. In fact, because these reactions depend on products from the light reactions, they typically function during the day when those energy sources are available. Furthermore, some enzymes involved in the Calvin-Benson cycle are actually inactivated in the dark to prevent them from counteracting other metabolic processes like glycolysis. Therefore, while they are light-independent in their mechanism, they are physiologically integrated into the broader, light-driven process of carbon assimilation.134