Solar energy in photosynthesis

Solar energy provides the reducing power within green leaves to convert CO₂ and H₂O into sugars. The CO₂ is supplied by the atmosphere and enters the leaf by diffusion. Factors affecting the rate of photosynthesis must either change the CO₂ diffusive resistances or the CO₂ concentration gradient along the diffusion pathways. Therefore, these effects can be described in terms of diffusive control mechanisms.

Light affects CO₂ diffusion by initiating photosynthesis, which removes CO₂ at the chloroplast and establishes a diffusion gradient. Light also triggers stomatal opening, thereby sharply decreasing the diffusive resistance. However, intense radiation can cause desiccation of stomatal guard cells, a mechanism whereby the diffusive resistance increases.

During illumination, leaf cells have both a source (respiration) and sink (photosynthesis) for CO₂. Respiration in some species appears to be greatly stimulated by light. This additional internal CO₂ flux is a possible reason for a lower efficiency of energy utilization than in species whose respiration is not enhanced by light.

Physiological growth responses or movements often occur that position leaves in the light. Plants lacking this capability are often excluded in ecological succession in nature.