When Does Photosynthesis Occur?

Keita Kuroki/CC-BY 2.0

Photosynthesis, the process by which plants convert the sun’s energy to glucose, takes place during daylight hours when the plant’s leaves are exposed to sunlight. This process only occurs in green plants and in some protist species. Carbon dioxide and water must also be present in order for photosynthesis to occur.

Photosynthesis is a series of chemical reactions that take place in the chloroplasts, which are specialized structures found in plant cells. Through this series of reactions, carbon dioxide and water are converted to glucose and oxygen. The energy from the sunlight is used to drive the reaction forward. Since it is an endothermic reaction, it requires the input of this energy to occur. Photosynthesis is classified as an oxidation-reduction reaction because it involves the loss of electrons by one chemical species, the water, and the gain of electrons by another chemical species, the carbon dioxide.

The specialized process of photosynthesis takes place mainly in the mesophyll, or middle layer, of the plant’s leaves. The carbon dioxide necessary for the process enters the leaves through the stoma, which are small holes in the leaf’s outer layer. The water needed for photosynthesis is transported from the roots to the leaves through the plant’s vascular tissue. Very little photosynthesis takes place in the stems of the plant.