Energy production:

There are two general aspects of energy production; the concept of oxidation-reduction and the mechanism of ATP generation.

Oxidation – reduction reactions:

Oxidation – is removal of electron from an atom or molecule, a reaction that often produces energy.

Reduction – is addition of one or more electrons to an atom or molecule.

Oxidation and reductions reactions are always coupled. The pairing of these reactions is called oxidation-reduction or redox reactions. Most biological oxidation reactions involve the loss of hydrogen atoms, they are also dehydrogenation. Hydrogen atom contains both electrons and protons and in cellular oxidations, electrons and protons are removed at the same time. An organic molecule is oxidized by the loss of two hydrogen atoms, and a molecule of NAD⁺ is reduced by accepting two electrons and one proton. One proton is left over and is released into the surrounding medium. The reduced coenzyme contains more energy than NAD⁺. This energy can be used to generate ATP in later reactions.

Cells use oxidation- reduction (biological) reactions in catabolism to extract energy from nutrient molecules. Ex. Cell oxidizes a molecule of glucose to CO₂ and H₂O. The energy in the glucose molecule is removed in stepwise manner and ultimately is trapped by ATP, which can then serve as an energy source for energy requiring reactions. Thus glucose is a valuable nutrient for organisms.

The generation of ATP:

Much of the energy released during oxidation – reduction reactions is trapped within the cell by the formation of ATP. A phosphate group is added to ADP with the input of energy to form ATP. Addition of a phosphate to a chemical compound is called phosphorylation. Organisms use three mechanisms of phosphorylation to generate ATP from ADP.

Substrate – level phosphorylation: ATP is generated when a high energy phosphate is directly transferred from a phosphorylated compound (a substrate) to ADP. Generally the phosphate has acquired its energy during an earlier reaction in which the substrate itself was oxidized.

C-C-C~ P + ADP C-C-C + ATP

Oxidative phosphorylation: Electrons are transferred from organic compounds to one group of electron carriers (usually to NAD⁺ and FAD). Then, the electrons are passed through a series of different electron carriers to molecules of O₂ or other oxidized inorganic and organic molecules. This process occurs in the plasma membrane of prokaryotes and in the inner mitochondrial membrane of eukaryotes. The sequence of electron carriers used in oxidative phosphorylation is called an electron transport chain. The transfer of electrons from one electron carrier to the next releases energy, some of which is used to generate ATP from ADP through a process called chemiosmosis.