Re: What is the chemical equation for alcohol fermentation?

Hi Abby,

The alcohol ethanol is one of the waste products produced when cells extract energy from glucose. The process of extracting energy from glucose is called glycolysis, and the end product of glycolysis is a three-carbon molecule known as pyruvate. With the generation of pyruvate, the cell has the choice of converting it into three different molecules; lactic acid, ethanol, or acetyl-CoA. Lactic acid and ethanol are basically waste products that are produced in the absence of oxygen in processes that we call fermentation (or anaerobic respiration), but acetyl-CoA is used to generate even more energy in the mitochondrion in a process known as oxidative phosphorylation (or aerobic respiration).

I wrote an answer to a previous question asking about the chemical equation for the souring of milk (1146763939.Bc), which you should read before continuing with this answer, because the souring of milk is caused by the formation of lactic acid from pyruvate.

So, here, I am going to talk about the formation of ethanol from pyruvate, and you can review the process of the formation of pyruvate from glucose in the previous answer.

Ethanol is a two-carbon molecule, so one of the three carbons of pyruvate has to be removed. This is called a decarboxylation. The enzyme pyruvate decarboxylase does this to the pyruvate, using a free proton (H+), and forms acetaldehyde and carbon dioxide.

Acetaldehyde is then reduced by the enzyme alcohol dehydrogenase, using a proton (H+) and one of the NADH generated in glycolysis, to form ethanol and regenerate the NAD+ needed for more glycolysis.

Note that for both the formation of lactic acid and ethanol from pyruvate, the NAD+ used in glycolysis is recycled through the formation of these molecules.

So, the overall equation for alcoholic fermentation is:

C6H12O6 + 2Pi + 2ADP + 2H+ --> 2 C2H6O + 2 CO2 + 2 ATP + 2 H2O.

However, as with the answer I wrote about lactic acid fermentation, you can see that this is not a single-step process. The action of 12 different enzymes are required to generate ethanol from glucose!

As I wrote in the other answer, you can find much more detail about glycolysis and anaerobic respiration in a good college- level biochemistry textbook, such as Biochemistry, by L. Stryer. For example, if you count up all of the hydrogens and oxygens in the equation above, they won't balance out. This is because I haven't written out the chemical formulas for the ATP, ADP or phosphate (Pi) molecules (or discussed the oxidation of Glyceraldehyde-3- phosphate to 1,3-Bisphosphoglycerate).

You can also search our archives for the many answers that we already have about anaerobic respiration, glycolysis, lactate, lactose and lactase, and metabolism in general.

Keep asking questions!