Redox Reactions and Electron Carriers
Redox reactions involve the transfer of charge from one molecule to another.
An anabolic pathway, one of the two types of metabolic reactions, consumes energy in the form of ATP. A catabolic pathway, the other type of metabolic reaction, produces energy in the form of ATP. But just how is this energy used by cells? The answer lies in the electron (the subatomic, negatively charged particle found in all atoms). In a chemical reaction, when an electron is transferred from one reactant to another, the reaction is known as an oxidation-reduction reaction, or redox reaction. The loss of electrons from a molecule is called oxidation, while the gain of electrons is called reduction (due to the molecule's overall decrease in charge because of the addition of negatively charged particles). The electrons in molecules have potential energy, which is the stored energy of something based on its position. Energy must be added to pull an electron from one molecule and add it to another molecule. Recall that the second law of thermodynamics drives a system toward a state of equilibrium. Energy is released when electrons freely move from one molecule to another molecule. Some energy is always lost as heat during chemical reactions. When energy is given off quickly, the loss of heat will be greater (consider the difference between a fiery explosion and the fire in a fireplace). To capture the greatest amount of energy from chemical reactions in metabolic pathways, cells rely on electron carriers. These are molecules that shuttle charged particles between steps in a metabolic pathway. One of these electron carriers is nicotinamide adenine dinucleotide (denoted NAD+ in its oxidized form). When reduced, it is written NADH, to show that it has gained a hydrogen ion. Similarly, in the Calvin cycle, the phosphorylated form of NAD, called NADP, acts as the electron carrier. These molecules, along with similar electron carriers, capture electrons given off during catabolic reactions and donate them again later in the pathway, or in subsequent pathways. In this way, the loss of heat energy during chemical reactions is minimized.