There are two main types of respiration: Aerobic respiration - This is the most common type of respiration and requires oxygen. Aerobic respiration takes place in the mitochondria of cells and involves the breakdown of glucose to produce energy, carbon dioxide, and water. Anaerobic respiration - This type of respiration occurs in the absence of oxygen and is less efficient than aerobic respiration. Anaerobic respiration involves the breakdown of glucose without the use of oxygen and produces lactic acid or ethanol as a byproduct. It is commonly seen in microorganisms, some plants, and certain muscle cells during strenuous exercise.