Fundamentals of Microbiology

3.1.1 Energy yielding reactions

Oxidation – Reduction

Oxidation is the removal of electrons (e^-) from an atom or molecule, a reaction that often produces energy. Figure shows an example of an oxidation in which molecule A loses an electron to molecule B. Molecule A is oxidized (meaning that it has lost one or more electrons), whereas molecule B has undergone reduction (meaning that it has gained one or more electrons). Oxidation and reduction reactions are always coupled. When one substance is oxidized, another is simultaneously reduced. The pairing of these reactions is called oxidation – reduction.

Substrate-Level phosphorylation

In substrate level phosphorylation, ATP is generated when a high-energy p is directly transferred from a phosphorylated compound (a substrate) to ADP. Generally, the p has acquired its energy during an earlier reaction in which the substrate itself was oxidized. The following example shows only the carbon skeleton and the p of a typical substrate:

C – C – C ~ p + ADP → C – C – C + ATP

Oxidative phosphorylation

When oxidative phosphorylation occurs, electrons transferred from organic compounds to electron carriers (usually to NAD⁺) are passed through a series of different electron carriers to molecules of oxygen (O₂) or other inorganic molecules. This process occurs in the plasma membrane of prokaryotes and in the inner mitochondrial membrane of eucaryotes. The series of electron carriers used in oxidative phosphorylation is called an electron transport chain (Figure). 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.

Photophosphorylation

Photophosphorylation, occurs only in photosynthetic cells, which contain light trapping pigments such as chlorophylls. In photosynthesis, organic molecules, are synthesized with the energy of light from the energy poor building blocks carbon dioxide and water. photophosphorylation starts this process by converting light energy to the chemical energy of ATP and NADPH, which, in turn, are used to synthesize organic molecules. As in oxidative phosphorylation, an electron transport chain is involved.