Unit C

Module 5~ Lesson 1

 Reduction and Oxidation Reactions

When electrons transfer in a chemical equation, this is called a redox reaction. Here is a chemical equation to illustrate the process.     

Xe- + Y → X + Ye-

Oxidation: Water loses electrons and hydrogen ions to become oxygen. It is oxidized.

Reduction: Carbon dioxide  gains  electrons and hydrogen ions to become glucose. It is reduced. 

The overall reaction of photosynthesis is an oxidation-reduction (redox) reaction. In the first step, the oxygen in water is oxidized by the light energy. This means that the water molecule has lost electrons (LEO). You will learn more about this process when you study light-dependent reactions.

O + light energy → 1/2 O  + 2 H+ 2 e-

The H+ and electron from water are transferred to NADP +. NADPH now carries the electrons to eventually reduce the carbon in CO2 in a series of reactions. The result is a carbohydrate molecule. You will learn about this step during your study of light-independent reactions. A simplified version of the reaction is below.

2H+ + 2e- + CO → (CH  O)   {carbohydrate molecule}

In order for a glucose molecule to be created, more than one water molecule and one carbon dioxide molecule are involved. This situation is indicated by the balanced chemical equation

6 CO (g) + 6 H O(l) + energy → 12 (s) + 6 O (g)

Conversely, in cellular respiration, glucose is oxidized and oxygen is reduced.


Read  pages 167- 168 in your textbook.


There is a simple mnemonic device to remember the definition of the terms oxidation and reduction. All you have to do is remember that LEO the lion goes GER. Spelled out, it looks like this:

ose lectrons → xidation
ain lectrons  → eduction

Clement Bardot. Lion d'Afrique. CC BY-SA. 



NADPH has an electron available to donate for another compound to be reduced. Thus it has the power to reduce another compound.