ATP is generated from ADP and phosphate ions by a complex set of processes occurring in the cell. These processes depend on the activities of a special group of coenzymes. Three important coenzymes are nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and flavin adenine dinucleotide (FAD).
NAD and NADP are structurally similar to ATP. Both molecules have a nitrogen-containing ring called nicotinic acid, which is the chemically active part of the coenzymes. In FAD, the chemically active portion is the flavin group. The vitamin riboflavin is used in the body to produce this flavin group.
All coenzymes perform essentially the same work. During the chemical reactions of metabolism, coenzymes accept electrons and pass them on to other coenzymes or other molecules. The removal of electrons or protons from a coenzyme is oxidation. The addition of electrons to a molecule is reduction. Therefore, the chemical reactions performed by coenzymes are called oxidation-reduction reactions.
The oxidation-reduction reactions performed by the coenzymes and other molecules are essential to the energy metabolism of the cell. Other molecules participating in this energy reaction are called cytochromes. Together with the coenzymes, cytochromes accept and release electrons in a system called the electron transport system. The passage of energy-rich electrons among cytochromes and coenzymes drains the energy from the electrons to form ATP from ADP and phosphate ions.
The actual formation of ATP molecules requires a complex process called chemiosmosis. Chemiosmosis involves the creation of a steep proton (hydrogen ion) gradient. This gradient occurs between the membrane-bound compartments of the mitochondria of all cells and the chloroplasts of plant cells. A gradient is formed when large numbers of protons (hydrogen ions) are pumped into the membrane-bound compartments of the mitochondria. The protons build up dramatically within the compartment, finally reaching an enormous number. The energy released from the electrons during the electron transport system pumps the protons.
After large numbers of protons have gathered within the compartments of mitochondria and chloroplasts, they suddenly reverse their directions and escape back across the membranes and out of the compartments. The escaping protons release their energy in this motion. This energy is used by enzymes to unite ADP with phosphate ions to form ATP. The energy is trapped in the high-energy bond of ATP by this process, and the ATP molecules are made available to perform cell work. The movement of protons is chemiosmosis because it is a movement of chemicals (in this case, protons) across a semipermeable membrane. Because chemiosmosis occurs in mitochondria and chloroplasts, these organelles play an essential role in the cell’s energy metabolism. Chapter 5 explains how energy is trapped in the chloroplasts in plants, while Chapter 6 explains how energy is released in the mitochondria of plant and animal cells.