At first, glucose is broken down into three carbon molecule called pyruvate. This process happens in the cytoplasm of cells of all organisms. On the basis of presence or absence of oxygen, it is two types- Aerobic respiration and Anaerobic respiration.

Aerobic Respiration: This type of respiration happens in the presence of oxygen. In this respiration, glucose is complete oxidized into carbon dioxide and water. Energy is also released at the end of this process.

Anaerobic Respiration: This process takes place in the absence of oxygen. In this case, pyruvic acid either converted into ethyl alcohol or lactic acid. Ethyl alcohol is usually formed in case of anaerobic respiration in microbes; like yeast or bacteria. Lactic acid is formed in some microbes as well as in the muscle cells.