Plant Physiology - Musienko, M.M. 2001

Respiration
Respiratory substrates. Respiratory quotient

Dissimilation most commonly takes the form of Respiration, which involves the uptake of oxygen and the release of carbon dioxide. A distinction should be made between external respiration—the exchange of gases between the Organism and its environment— and internal respiration—intracellular biochemical processes accompanied by the release of energy.

CARBOHYDRATES serve as the primary substrate for respiration. Fats and Proteins are utilized mainly in the respiration of seedlings developing from seeds rich in fats or proteins. The breakdown of substrates during respiration is preceded by Hydrolysis: carbohydrates are broken down into Monosaccharides, fats into glycerol, and proteins into Amino Acids.

The overall equation for respiration can be represented as the breakdown of the substrate and The oxidation of hydrogen:

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It is evident from the summary equation given above that the volumes of gases involved in carbohydrate oxidation are equal.

The ratio of released CO₂ to absorbed O₂ during respiration is termed the respiratory quotient; as seen from the equation above, the respiratory quotient for carbohydrates is equal to CO₂/O₂ = 1. If fats serve as the nutritional substrate, such as stearic acid, the overall respiration equation becomes:

The respiratory quotient can deviate significantly from unity if the respiratory substrate is composed of proteins or fats rather than carbohydrates. If the substrate is rich in hydrogen ions, a portion of the atmospheric oxygen is used not only to oxidize carbon, but also to oxidize the excess hydrogen present in the substrate. This is why the respiratory quotient for fats is less than 1. During respiration relying on proteins, the RQ can also be less than unity, approximately 0.7–0.8. Conversely, if the substrate (for instance, C₄H₆O₅ — malic acid) is rich in oxygen, the respiratory quotient exceeds 1:

An increase in the RQ value is always observed when respiration is accompanied by Fermentation, since fermentation processes inherently involve the release of CO₂ without the uptake of atmospheric oxygen. It is readily apparent that the lower the respiratory quotient, the greater the thermal effect of oxidation, and vice versa. Consequently, proteins and fats are characterized by a high thermal equivalent, whereas organic acids have a very low one.