The term “Redox”, is an abbreviated term for reduction and oxidation reactions, where reduction is the gaining of electrons and oxidation is the losing of electrons within a reaction. Biological reactions such as cellular respiration and photosynthesis are the best examples of Redox reactions. For example, in cellular respiration, redox reactions occur when glucose is oxidized (losing electrons) to become carbon dioxide, and oxygen is reduced (gaining electrons) to become water.

In a Redox reaction, there are four aspects to observe; the oxidizing agent (gains e− during reaction and is therefore reduced during reaction), the reducing agent (loses e− during reaction and is therefore oxidized during reaction), oxidized form (form of molecule lacking (it’s all relative) an e−), and reduced form (form of the molecule having an additional (again, relative) e−).

There are three ways to represent a redox reaction; these are shown below with a representative biological redox reaction:

(1) Overall reaction:

acetaldehyde + NADH + H+ → ethanol + NAD+

(2) Electron-transfer diagram:

 Acetaldehyde          e-                NAD+

             ↓                     ←                     ↓

      ethanol                               NADH + H+

(3) Half-reactions

 Acetaldehyde + 2 H+ + 2e− → Ethanol

    +

                                         NADH → NAD+ + H+ + 2e– 

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 Acetaldehyde + NADH + H+ → Ethanol + NAD

In the reaction shown above:

NADH is oxidized to NAD+

acetaldehyde is reduced to ethanol

acetaldehyde is the oxidizing agent

NADH is the reducing agent

NADH and ethanol are the reduced forms

NAD+ and acetaldehyde are the oxidized forms

References:

https://ocw.mit.edu/courses/biology/7-014-introductory-biology-spring-2005/readings/l17_redox_handou.pdf

Picture: https://goo.gl/images/71eLjf