'Photosynthesis [is] the process by which green plants and certain other organisms transform light energy into chemical energy. Photosynthesis in green plants harness[es] the energy of sunlight to convert carbon dioxide, water, and minerals into organic compounds and gaseous oxygen. (...)
All photosynthetic organisms - with the exeption of a minor group of bacteria, the halobacteria - contain the light-absorbing pigment chlorophyll, which plays a key role in the transfer of energy from light to chemical compounds.

Photosynthesis is the fundamental process that maintains life on Earth.

Living cells convert food into energy and structural components.
Almost all organisms derive this food, directly or indirectly, from the organic compounds formed within plants during photosynthesis. The stored energy in these compounds is essential for growth, repair, reproduction, movement, and other vital functions. Without photosynthesis, not only would replenishment of the fundamental food supply halt but the Earth would eventually become devoid of oxygen.

Just as the organic molecules in the bodies of living organisms contain energy converted by photosynthesis from the energy of the Sun, so do the molecules of fossil fuels. The energy provided by coal, oil, and gas comes from photosynthesis carried on by plants of earlier times and preserved down through the ages, to be released by combustion in modern industrial process. Most of the energy released both by the burning of fossil fuels and by the metabolism of living cells is given off as heat and must be replaced by the continued input of radiant energy from the Sun.

The principal organic products of plant photosynthesis are carbohydrates. Formation of the simple carbohydrate glucose is shown by the equation

The molecules of glucose produced are usually linked with other molecules to form more complex carbohydrates. Other products of photosynthesis are formed by incorporating mineral elements into the process. The energy required to break the chemical bonds in the reactants and to create new bonds in the products is provided by light.
The excess energy not used up in the chemical reactions is stored as chemical energy in the organic products formed. (...)

The manner in which plants convert light energy into chemical energy began to be understood somewhat in the 19th century. Investigation of the mechanisms of photosynthesis is still going on, aided bythe use of radioactive isotopes and fluorescence techniques and by the study of photosynthetic bacteria and algae.'


(Encyclopedia Britannica 1988)