Every organism uses glucose as a source of energy. This process is called respiration. During this process, glucose is broken down into pyruvate. Which later is converted into other molecules.
Breakdown of pyruvate can happen through three different processes. In all these process energy is released which is utilized by the cell to meet its energy requirement.
These three processes are:
a. Anaerobic respiration: Some organisms, for example, yeast break pyruvate into ethanol and carbon dioxide with the release of energy. In this process, no oxygen is consumed. So this process is called anaerobic respiration.
b. Aerobic respiration: Humans and other animals and some microorganism use oxygen in the breakdown of pyruvate. They break pyruvate into carbon dioxide and water releasing energy. Since this takes place in the presence of oxygen, the process is called aerobic respiration.
c. Lack of oxygen: Apart from aerobic and anaerobic respiration there is a third type of respiration which happens in our muscle cell during heavy work. During heavy and continuous work proper amount of oxygen does not reach the muscle cells, so they resort to another pathway and break down pyruvate into lactic acid with the release of energy.