The amphibolic pathway involves both catabolism and anabolism where catabolism stands for the breakdown of complex substances to produce energy and anabolism stands for the synthesis of complex substance from simple substances. Plants also use substances such as fats and proteins as respiratory substrates. Though pure fats or proteins cannot be directly used as substrates, they can enter as intermediary substrates of glycolysis and the Krebs’ Cycle.
Fats are first broken down into glycerol and fatty acids. Glycerol is then converted into PGAL, an intermediary substrate of glycolysis. In the same way, fatty acids are broken down to form acetyl coA, an intermediary substrate of the Krebs’ Cycle. Similarly, proteins are broken down by protease enzymes into amino acids. These amino acids are converted into different intermediary substrates of the Krebs’ Cycle such as pyruvate. Thus, in addition to glucose, plants use fats and proteins as respiratory substrates to produce energy. These processes describe the role of the respiratory pathway in catabolism and anabolism. When the plant requires proteins or carbohydrates, it can synthesise them by withdrawing some of the substrates in the respiratory pathway. The PEP formed during glycolysis is withdrawn and used for the synthesis of proteins or carbohydrates. Likewise, if the plant requires fats, it withdraws acetyl coA and citrate from the Krebs’ Cycle to synthesise them. Thus, the respiratory pathway is involved in both catabolism and anabolism and is called an ‘amphibolic pathway’. Aerobic respiration involves the consumption of oxygen and expulsion of carbon dioxide.
The Respiratory Quotient is calculated based on the volume of carbon dioxide expelled to the volume of oxygen consumed. As plants use more than one substrate for respiration, RQ depends on the respiratory substrate used. When carbohydrates are used as the respiratory substrate, the RQ value is 1 which means the volume of carbon dioxide expelled equals the volume of oxygen consumed. But when fats or proteins are used as the respiratory substrate, the RQ value is less than 1that is, 0.7 for fats and 0.9 for protein which means the volume of carbon dioxide expelled is less than the volume of oxygen consumed. This is why glucose is used as the primary substrate for respiration by plants.