ABSTRACT
Sunlight is the ultimate source of energy for all living organisms, which derive their energy, directly or indirectly, from the radiant energy of the sun arising from the thermonuclear fusion reactions. Plants evolved as photosynthetic organisms, absorbing the sun’s radiant energy and using it to drive electrons from water to carbon dioxide, thus forming energy-rich carbon com pounds such as sucrose, starches, and cellulose. The molecular oxygen released by the photo synthetic organisms into the early atmosphere has paved the way for the existence of non-photo synthetic organisms, which in turn depend on plants as the source of energy. The latter obtain their energy needs by oxidizing the energy-rich carbon compounds of photosynthesis, by passing elec trons to atmospheric oxygen to form water, carbon dioxide and other end products. Plants are thus the forerunners of energy source for most heterotrophs.
