The citric acid series otherwise referred to as the tricarboxylic acid series performs aerobic metabolism in the mitochondria part of both plant and animal cells. When needed to produce energy, several processes such as oxidation are undertaken by organisms that thrive in oxygen on substances such as proteins and carbohydrates. The purpose of the citric acid cycle is to break down 2-carbon units of acetyl into carbon dioxide while at the same instance transforming the energy from the acetyl group into GTP, which will be equivalent to ATP (Murray et al., 2011, p. 5). During this process, CoA from pyruvate that was produced at the time of glycolysis is completely oxidized into carbon dioxide, thus leading to the reduction of NAD and FAD to NADH and FADH2 in that order. The main purpose that the citric acid cycle serves is to supply hydrogen atoms to the mitochondria. However, the cycle serves other different purposes. One of them is amphibolic, which means that the cycle serves both anabolism and catabolism. Acetyl groups are derived when food is first broken down before it can enter the cycle. Both plants and animals have this process happening in the mitochondria of their cells. It is usually a part of the cell respiration process.
Reference
Murray, R. et al. (2011). Harpers Illustrated Biochemistry. New York: McGraw-Hill.