Definition

 Important plant life processes such as proteins, fats, and carbohydrates, require a certain expenditure of energy. This energy is mainly derived from the breakdown of complex macromolecules. During photosynthesis, light energy is converted into chemical energy and stored in the bonds of complex organic molecules. The major portion of stored energy in plants is found in the form of carbohydrates like glucose and starch.

The energy stored in carbohydrate molecules during photosynthesis is released during cellular oxidation of carbohydrates into CO2 and H2O. This is called Respiration. During this process, O2 is consumed and CO2 released out.

In respiration, oxidation of various organic food substances like carbohydrates, fats, proteins, etc., may take place. Among these, glucose is the commonest. Its oxidation proceeds a shown below in the simplest equation :

                                   C6 H12O6 + 6O2 → 6CO2 + 6H2O + Energy (686 K.Cal.)

Breakdown of glucose involves many steps releasing energy in the form of ATP molecules and forming a number of carbon compounds (intermediates) in a very well-organized sequence.