It is the quantitative study of the energy relationships and energy conversions in biological systems. All organisms need free energy to keep themselves alive and functioning. The source of energy is just one; solar energy. Only plants use that energy directly. What the organisms use is the chemical energy in the form of foods. The very first conversion of solar energy into a chemical energy is the sugar molecule. On one side the conversion of solar energy into chemical energy with the help of photosynthesis happens, and on the other hand this photosynthesis makes it possible with the passage of time on earth to accumulate free oxygen in the earth's atmosphere making possible the evolution of respiration. Respiration is important for bioenergetics as it stores the energy to form a molecule ATP; adenosine triphosphate. This molecule is a link between catabolism and anabolisms. The process of photosynthesis is helpful in understanding the principles of energy conversion i.e. Bioenergetics. Photosynthetic organisms and plants capture solar energy and synthesize organic compounds. It is a way of energy input. Energy stored in these organic compounds that are mainly sugars can be used later as a source of energy. Photosynthesis after respiration provides glycolysis, a major substrate, and later this glycolysis with further respiration provides energy in very controlled processes. So respiration and photosynthesis are the main processes dealing with bioenergetics.
