# 0.21 Bis2a 06.1 red/ox in metabolism v1.2

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In this module we will review Red/Ox reactions and relate them to metabolic process. We will introduce the electron tower, a tool to be used to understand red/ox half reactions and the energy associated with different reactions.

## Red/ox reactions in biology

Earth’s atmosphere contains about 20% molecular oxygen, O 2 , a chemically reactive gas that plays an essential role in the metabolism of aerobic organisms and in many environmental processes that shape the world. The term oxidation was originally used to describe chemical reactions involving O 2 , but its meaning has evolved to refer to a broad and important reaction class known as oxidation-reduction (red/ox) reactions . Our current working definition of oxidation is based on the ability of a compound to lose electrons, and removes all references to the involvement of molecular oxygen. A few examples of such reactions will be used to develop a clear picture of this classification of essential biochemical reactions.

The chemical reactions underlying metabolism involve the transfer of electrons from one compound to another by processes catalyzed by enzymes. The electrons in these reactions commonly come from hydrogen atoms, which consist of an electron and a proton. A molecule gives up a hydrogen atom, in the form of a hydrogen ion or proton, (H + ) and an electron, breaking the molecule into smaller parts. During the loss of an electron(s), or oxidation from one compound, the elctron(s) are then passed to another molecule in a process called reduction , or the gaining of an electron. These two reactions always happen together in an oxidation-reduction reaction (also called a red/ox reaction)—when electrons are passed between two molecules, the donor molecule is oxidized and the recipient molecule is reduced. These reactions are exergonic    . Remember, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy), indicating a spontaneous reaction. Oxidation-reduction reactions often happen in a series: A molecule that has just been reduced may be very quickly re-oxidized, passing on an electron to a new acceptor.

Remember the definitions of oxidation and reduction :

$\begin{array}{lll}\hfill \mathbf{\text{oxidation}}& =& \text{loss of electrons}\hfill \\ \hfill \mathbf{\text{reduction}}& =& \text{gain of electrons}\hfill \end{array}$

Also remember, that if a compound is oxidized another compound must be reduced. The two process go together. The electrons have to go somewhere, we can not have free floating electrons in our system, they must be associated with a molecule or atom. This is an essential concept.

## Red/ox in metabolism

One of the primary sources of cellular energy comes from Oxidation-Reductions reactions, termed red/ox . During the movement of electrons from one molecule to a second, energy is released, and that energy can be used to work (translocate protons) or be stored (ATP synthesis) for future work. This is true as long as the electrons are passed from one compound to a second compound with a higher reduction potential , that is a compound that has a higher affinity for those electrons. The concept of reduction potential is explained below.

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