Biochemistry - Oxidation and Reduction

Oxidation and Reduction are two often very confused processes in science. Both of these processes involve a molecule that can gain or lose electrons.

One common biological molecule that undergoes these two processes is Nicotinamide adenine dinucleotide, more commonly known as NAD+ / NADH. This molecule is especially important in Cellular Respiration.

Before trying to understand the processes, understanding why they can be confused is important. The word reduction typically makes us think "less." Thus, by looking at the chemical formulas NAD+ and NADH, which one seems to have less? - NAD+! In these reactions, however, we are not looking at the number of molecules, but the charge. 

Think about the parts of an atom. 
 Photo 1:
 

• Neutrons are neutral with no net charge
• A proton has a single positive charge
• An electron has a single negative charge

A neutrally charged molecule then, would have the same number of protons and electrons - the positives and negatives cancel out, like the equations below where each number represents a charge, p is protons, and e is electrons. For the sake of this lesson, I am not including neutrons in the equation, due to them having no charge.

1. -2e + 2p = 0
2. 5p - 5e = 0
3. 10p - 10e = 0

However, especially in biological molecules, ions often exist. Basically, an ion is an atom with a charge. In redox reactions (reduction + oxidation), that charge is typically (but not always) a positive charge on the oxidized molecule.   

Compare the atom in Photo 2 below with the atom in the Photo 1 above. What is the same? What has changed?
Photo 2:



In Photo 1, there are:

• 3 Protons (red) = 3 positive charges
• 3 Neutrons (yellow) = no charge
• 3 Electrons (black) = 3 negative charges

Therefore, the molecule in Photo 1 has no net charge because 3 - 3 = 0.

 In Photo 2, there are:

• 3 Protons (red) = 3 positive charges
• 3 Neutrons (yellow) = no charge
• 2 Electrons (black) = 2 negative charges

Now, there is a difference in math. For photo 2, the equation is actually 3 - 2 which means there is a + 1 charge on that molecule. Therefore, this molecule is an ion, just like the NAD+ molecule!
  So then, what is actually being reduced in the reduction portion of the redox reaction? Answer: the charge.

In our example, NAD+ (a +1 positive molecule) is reduced to NADH (a neutral molecule). How is that, though? - Through the addition of an electron. That is reduction.

Oxidation, then, is the opposite. To be oxidized, a molecule loses electrons, usually becoming positive. 

Using what you learned: In the animation below, you can see that one molecule is being reduced, while the other is being oxidized. Notice, here, unlike the previous example, both molecules start off with neutral charges. After the reaction occurs, the sodium molecule (Na) becomes positive and the fluoride (F) atom becomes negative.  The blue dots represent the electrons currently associated with the sodium molecule and the red dots represent the electrons currently associated with the fluoride molecule. Notice, a blue coded electron moves from the sodium molecule to the fluoride molecule.

So, which molecule is being reduced?
(animation courtesy of wikipedia, NaF.gif)

Below are the equations for these reactions. Redox Reaction equation courtesy of wikipedia, Redox Halves

 


Answer to the using what you have learned question: Think about which molecule has a charge that is being reduced. To reduce the charge, an electron (a negative charge) has to be added. Look at the equation above. In the animation, the fluoride atom gains an electron, which takes it from a neutral charge to a - 1 charge. Therefore, it is the fluoride that is being reduced. Oppositely, the sodium ion loses and electron (or a negative charge), becoming a + 1 charge. Thus, the sodium molecule is oxidized.