The macromolecule glucose is produced by plants and burned in the mitochondria to produce free energy. The free energy is released in the form of adenosine triphosphate, commonly referred to as ATP.
Plants are considered autotrophs due to their ability to create their own energy sources. Plant cells contain chloroplasts, an organelle that undergoes photosynthesis to harvest energy. During photosynthesis, the plant uses light energy obtained from the sun to convert carbon dioxide and water to the chemical energy present in glucose and releases the byproduct oxygen.
Animals, because they are heterotrophs, lack the ability to create their own energy sources and must instead consume organic carbon created from other organisms. Animals consume the glucose produced by plants and burn that using oxygen through the process of aerobic respiration. This process is the reverse of photosynthesis. The free energy obtained is harvested as ATP. Carbon dioxide and water are released as byproducts.
The first step of aerobic metabolism is the 10-step process of glycolysis, which occurs in the cell's cytoplasm. One molecule of glucose is broken down into two molecules of pyruvate and two molecules of NADH. This process requires two ATP molecules and releases four ATP, so the overall yield is two ATP molecules. After glycolysis, the pyruvate enters the mitochondria where it is oxidized into acetyl coenzyme A. Two ATP molecules, eight NADH molecules and two FADH2 molecules are also produced.
In the mitochondria, all of the acetyl coenzyme A, NADH, and FADH2 produced in the first two steps of aerobic metabolism are completely broken down through the citric acid cycle. In this final step 32 molecules of ATP are produced. The overall process of aerobic metabolism yields 36 molecules of ATP, which can then be transported throughout the body and used for cell processes.