Respiration is often compared to the process of combustion, because in both the instances the substrate material gets burnt (oxidized) up. But the process of respiration differs from mere combustion in many respects. In combus­tion the burning up or oxidation of the substrate is sudden while in respi­ration it is stepwise.

In combustion, the burning up or oxidation of large amount of energy is wasted as heat, while in respiration it is channelised and stored up for further usage. Hence respiration may be termed as an orderly combustion found in living organisms.