Cell metabolism is the process that converts dietary fuels from carbohydrates, proteins, and fats into ATP, which provides for the energy needs of the cell. ATP is formed through three major pathways: (1) the glycolytic pathway, (2) the citric acid cycle, and (3) the electron transport chain. In fuel metabolism, which is an oxidation–reduction reaction, the fuel donates electrons and is oxidized, and the coenzymes NAD+ and FAD accept electrons and are reduced.
Glycolysis, which occurs in the cytoplasm of the cell, involves the split-ting of the six-carbon glucose molecule into two three-carbon molecules of pyruvic acid. Because the reaction that splits glucose requires two molecules of ATP, there is a net gain of only two molecules of ATP from each molecule of glucose that is metabolized. The process is anaerobic and does not require oxygen (O2 ) or produce carbon dioxide (CO2 ). When O is present, pyruvic acid moves into the mitochondria, where it enters the aerobic citric acid cycle. Under anaerobic conditions, pyruvate is converted to lac tic acid, allowing glycolysis to continue as a means of supplying cells with ATP when O2 is lacking.
Citric Acid Cycle
Under aerobic conditions, both of the pyruvic acid molecules formed by the glycolytic pathway enter the mitochondria, where each combines with acetyl-coenzyme to form acetyl-coenzyme A (acetyl-CoA). The formation of acetyl-CoA begins the reactions that occur in the citric acid cycle. Some reactions release CO2 and some transfer electrons from the hydrogen atom to NADH or FADH. In addition to pyruvic acid from the glycolysis of glucose, fatty acid and amino acid breakdown products can also enter the citric acid cycle. Fatty acids, which are the major source of fuel in the body, are oxidized by a process called beta oxidation to acetyl-CoA for entry into the citric acid cycle.
The Electron Transport Chain
At the completion of the citric acid cycle, each glucose molecule has yielded four new molecules of ATP (two from glycolysis and two from the citric acid cycle). In fact, the principal function of these earlier stages is to make the electrons (e−) from glucose and other food substrates available for oxidation. Oxidation of the electrons carried by NADH and FADH is accomplished through a series of enzymatically catalyzed reactions in the mitochondrial electron transport chain. During these reactions, protons (H+) combine with O2 to form water (H2O), and large amounts of energy are released and used to add a high-energy phosphate bond to ADP, converting it to ATP. There is a net yield of 36 molecules of ATP from 1 molecule of glucose (2 from glycolysis, 2 from the citric acid cycle, and 32 from the electron transport chain). In general, the net amount of ATP formed from each gram of protein that is metabolized is less than for glucose, whereas that obtained from fat is greater (e.g., each 16-carbon fatty acid molecule yields about 129 molecules of ATP).