Respiration is a complex process that happens in both plants and animals. The process involves the breakdown of respiratory substrates like sucrose, starch, glucose or other sugars, fats, organic acids, proteins etc through oxidation and the release of energy. The oxidation involves the release of carbon dioxide along with the release of energy. Respiratory quotient is the ratio of the volume of carbon dioxide released during oxidation to the volume of oxygen taken in during respiration. This quotient is determined for the respiration by plants.

The amount of oxygen required as well as the volume of carbon dioxide released depends on the respiratory substrate that is being broken down. When carbohydrate is being oxidized, the volume of oxygen absorbed is equal to the volume of carbon dioxide released. Carbohydrates are mainly in the form of hexose sugar like glucose.
In the case of fatty acids being used as substrates, more of oxygen is required to break them down. The respiratory quotient thus is always less than one. Proteins are also less in oxygen composition and hence need more oxygen for oxidation. The respiratory quotient is hence less than one. Organic acids on the other hand, are rich in oxygen and hence require less of oxygen to be broken down. Therefore the respiratory quotient of organic acids is always greater than unity.

The significance of respiratory quotient comes from the fact that it helps in providing information regarding the respiratory substrate. The chemical nature of the substrate can be determined by knowing the respiratory quotient. Substrates which have less of oxygen like fatty acids consume more of oxygen for respiration. The oxygen produced during photosynthesis is thus used up for the plant respiration. In some cases, about 50-60% of the oxygen produced during photosynthesis is used up during the respiration. Anaerobic respiration involves no intake of oxygen and the theoretical respiratory quotient is infinity.