All organisms require continuous input of energy to carry on life process. These energy comes from cellular activities. All the cellular activities can be grouped into two categories : anabolism (biosynthetic activities of the cell) and catabolism (breaking- up process of the cell). The anabolic activities are endergonic (utilizes energy in cellular activities), while the catabolic activities are usually exergonic (energy releasing process by oxidation of food material). The sum of total catabolic and anabolic reactions occurring at any time in a cell is called metabolism.
Respiration is a vital process, includes the intake of oxygen. Chemically it is catabolic and brings about the oxidation and decomposition of organic compounds like carbohydrate, fat, protein in the cells of plants and animals with the release of energy. Oxidation of organic compounds by respiration, resulting in the release of chemical energies water and carbon dioxide.

In this reaction, six molecules of oxygen taken up and six molecules each of CO2 and H2O are formed with energy derived from respiration of each molecule of sugar oxidation. The plant cell is able to do chemical work in synthesizing energy- rich materials such as fat and hydrocarbon, osmotic work such as uptake and accumulation of salt and mechanical work such as involved in growth.