Cellular respiration refers to the process by which certain chemical reactions tale place within body cells in presence of oxygen producing energy for functioning of the cell. The energy released by these chemicals and used by the cells is in the form of heat energy. Calorie is a unit of measurement used for measuring quantity of heat energy. One calorie is equal to heat required to raise the temperature of one gram of water by one degree centigrade. This unit of heat is too small to be used conveniently for measuring heat in connection with requirements of energy by human bodies. Therefore, for this a unit called food calories is used for this purpose, which is equal to 1000 ordinary calories. However, food calories are also often called just calories.