[Taotao]

What is the chemical reaction equation for when pyruvate is broken down into ethanol in alcoholic fermentation? Specifically for when it happens in yeast.

"If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms. Pyruvate from glycolysis is converted by fermentation to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation, or to acetaldehyde and then to ethanol in alcoholic fermentation."

I read this from the Wikipedia and I just want to know the balanced reaction equation with the chemical formulas for each product and reactant, I also read that there is carbon dioxide produced so if you can include that as well it will be greatly appreciated.

[Babcock_Hall]

We cannot help you unless you first show your attempt.  Fortunately, this is a straightforward problem, especially when you assign oxidation numbers to the carbon atoms.

[Taotao]

I'm in grade 11, this is just for a chemistry report I have to do on the chemical reactions that occur when cooking. I haven't learned oxidation yet, just need the equation to put in my report.

[Babcock_Hall]

Try writing the structures of pyruvate, acetaldehyde, carbon dioxide, and ethanol, and arranging them in the order of the process, then post what you have.

[Taotao]

Something like this?

[Babcock_Hall]

What you wrote is basically correct, but it is important to be aware that each hydrogen nucleus also has an electron.  I chose to write this as two hydrogen atoms (the small dot signifies one electron in my drawing), although what actually happens chemically is equivalent, but a little bit different.  I also chose to write the reaction as if pyruvic acid were the reactant.  That makes it easier to check that the equation is balanced, but is not physiologically realistic.  We can talk about either of these two points further if you like.

[Taotao]

In the context of baking and in the environment of yeast, where would the 2 H atoms come from? Are they already there as part of the environment or were the produced from a reaction that happened earlier?

[Babcock_Hall]

In the conversion of glucose to pyruvate, two electrons (two hydrogen atoms if you like) were taken away.  There is an important molecule in cells abbreviated NAD.  At one point in the conversion of glucose to pyruvate, NAD becomes NADH (it gained two electrons and one proton).  But if NADH does not give back the electrons at some point, all of the NAD in the cell would be in the form of NADH, and the process would stop.  So the NADH gives acetaldehyde the two electrons and one proton to produce ethanol (a second proton comes from solution).  All of these reactions are catalyzed by enzymes.

In some bacteria, an enzyme reduces pyruvic acid to lactic acid, and this process also requires electrons in the form of NADH.

[Taotao]

Would you say these steps accurately/correctly describe what happens in yeast during alcoholic fermentation.


The overall reaction can be described by this equation: C6H12O6 → 2 C2H5OH + 2 CO2. This type of alcoholic fermentation is catalyzed by an enzyme found in the yeast known as Zymase.


1.   Sucrose from the dough is broken down by water and the enzyme invertase, into two glucose molecules. (C12H22O11 + H2O → 2 C6H12O6) 

2.   Then, the process of glycolysis separates each glucose molecule into two pyruvate molecules and other products.
 (C6H12O6 + 2 ADP + 2 Pi + 2 NAD+ → 2 CH3COCOO− + 2 ATP + 2 NADH + 2 H2O + 2 H+)

3.   Each pyruvate molecule accepts a proton and becomes pyruvic acid, which then is converted into acetaldehyde and carbon dioxide.
(CH3COCOO− + H+ → CH3COCOOH → CH3CHO + CO2)

4.   In the process of glycolysis shown above, 2 NAD+ molecules were converted into 2 NADH molecules, they accepted a proton and two electrons. For the process to last, the NADH has to become NAD+ so that it can go through the reaction again and keep separating glucose. "..."

Of these next 2 sentences, which one better describes the final step when acetaldehyde becomes ethanol? It would go right after the last sentence, where the "..." is.

This happens when NADH releases that proton and two electrons, becoming the NAD+ needed to repeat the reaction in step 2, to join acetaldehyde, which in the meantime also accepted a proton (H+) that was produced in step 2,  forming ethanol. (CH3CHO + H + e- + H+ → C2H5OH)

This happens when NADH reacts with acetaldehyde, forming NAD+  and a compound that accepts a proton (H+) from step 2 to become ethanol. (CH3CHO + NADH + H+ → NAD+ + C2H5OH)

[Babcock_Hall]

0. The process of breaking down glucose into ethanol and CO₂ requires at least a dozen enzymes.  I have forgotten what the term "zymase," denoted, but I suspect it is of historical significance only.

1. Sucrose produces one glucose and one fructose upon hydrolysis.  Fructose can also be fermented, but the details are slightly different.

2.  I did not notice any errors (other than formatting).

3.  Pyruvate is not protonated at physiological pH (the pK_a is between 2 and 3).  It is easier to write it as pyruvic acid, but that is convenient fiction.  The enzyme is pyruvate decarboxylase.

4.  This looks correct.

5.  Why not just show the reaction catalyzed by alcohol dehydrogenase?

[Taotao]

1. Is there a proper or standard way to differentiate glucose and fructose in an equation since their formulas are the same? Or would I just use: C6H12O6 (glucose) + C6H12O6 (fructose)

3. Sorry but I didn't understand that sentence, how would I show the fact that the pyruvate, product in step 2, is now pyruvic acid in step 3?

5. (CH3CHO + NADH + ADH + H+ → NAD+ + C2H5OH) Like this? Or does adding the ADH change the equation

[Babcock_Hall]

5.  Enzymes are catalysts.  Does that help?

3.  Pyruvate does not get protonated at physiological pH.  In other words pyruvate is the major form of this chemical species; pyruvic acid is present only at extremely low concentration.  My earlier drawing contained a simplification, but perhaps it would have been better not to have drawn it the way I did initially.  Here is a newer version that is more chemically realistic.  PDC is the enzyme pyruvate decarboxylase, and ADH is the enzyme alcohol dehydrogenase.  Glycolysis produces two protons, which are consumed in the production of ethanol and carbon dioxide from pyruvate.  NADH carries two electrons and a single proton temporarily in the process of converting glucose into ethanol.