Photosynthesis is a photochemical process that requires light to convert carbon dioxide and water to carbohydrates, starch and sugars. Most plants respond to increasing light intensity with increasing photosynthetic rates. Some plants require the shade to grow well and must avoid exposure to sunlight. These shade loving plants respond to photosynthesis with more sensitivity to low levels of solar irradiance than do sun loving plants but the sun plant has a greater photosynthetic rate in full sunlight than does the shade plant.
Aquatic plants respond to light in the same way as that of land plants.
Animals respond to light in a number of different ways. Some organisms have a biological clock, which is a physiological mechanism for determining time. Some of these biological clocks seem to be regulated by external factors. Some plants and animals possess responses having about a 24 hour periodicity. These are called circadian raythorns. These timing mechanisms regulate endocrine changes, gonadal development, color changes in birds in spring and autumn mating behavior in moths, insect feeding, flowering and many other responses. Light intensity and day length clearly are important in these cases. If environmental changes occur it may have a direct impact on the biological clock of many plants and animals.