Alcohol Fermentation: Definition, Equation & Process

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  • 0:00 What is Alcohol Fermentation?
  • 1:00 Alcohol Fermentation Process
  • 4:40 Examples
  • 6:05 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Humans have benefited from the process of alcohol fermentation for thousands of years. Here, we will explore how this process works, its definition, and its chemical equation.

What Is Alcohol Fermentation?

Bread, beer, and Bordeaux: most of us love some or all of these! But they would not exist if not for yeast, a eukaryotic microorganism that can metabolize sugars anaerobically through a pathway called alcohol fermentation. Humans have been using yeasts to make these products for thousands of years, but only learned of their existence in the last two hundred years. How exactly do these tiny creatures make these delicious food and drink items?

Alcohol fermentation, also known as ethanol fermentation, is the anaerobic pathway carried out by yeasts in which simple sugars are converted to ethanol and carbon dioxide. Yeasts typically function under aerobic conditions, or in the presence of oxygen, but are also capable of functioning under anaerobic conditions, or in the absence of oxygen. When no oxygen is readily available, alcohol fermentation occurs in the cytosol of yeast cells. Let's explore the process of alcohol fermentation then see what it means for yeasts and for humans.

The Process of Alcohol Fermentation

The basic equation for alcohol fermentation shows that yeast starts with glucose, a type of sugar, and finishes with carbon dioxide and ethanol. However, to better understand the process, we need to take a look at some of the steps that take us from glucose to the final products.

The process of alcohol fermentation can be divided into two parts. In the first part, the yeast breaks down glucose to form 2 pyruvate molecules. This part is known as glycolysis. In the second part, the 2 pyruvate molecules are converted into 2 carbon dioxide molecules and 2 molecules of ethanol, otherwise known as alcohol. This second part is called fermentation.

The main purpose of alcohol fermentation is to produce ATP, the energy currency for cells, under anaerobic conditions. So from the yeast's perspective, the carbon dioxide and ethanol are waste products. That's the basic overview of alcohol fermentation. Now, let's examine each part of this process in greater detail.

alcohol fermentation pathway yeasts

In the first part of this process, each glucose molecule is broken down into 2 pyruvate molecules. Pyruvate, or pyruvic acid, is an amino acid and will help form ethanol. In the process of breaking glucose down to form pyruvate, several molecules known as electron acceptors are involved.

Electron acceptors are molecules whose job is to give and take the electrons released when a chemical reaction takes place. During this first part, an electron acceptor molecule called NAD+ is reduced to form NADH, gathering up the electrons released by breaking one glucose down to 2 pyruvate molecules. This exchange of electrons that occurs while glucose is being broken down is essentially what helps build ATP.

The conversion of glucose to pyruvate creates a net total of 2 ATP. While this isn't as much ATP as aerobic respiration can produce, it's enough to keep the yeast alive until oxygen is available. This first part may look familiar because it's essentially glycolysis, or the first stage of aerobic respiration.

If oxygen were present, then the pyruvate molecules would enter a mitochondrion to undergo the remainder of aerobic respiration. However, in alcohol fermentation, the pyruvate instead stays in the cytosol, the gooey interior space of the cell. This is where the second part of our reaction, the conversion of pyruvate to ethanol, will take place.

Before pyruvate can be converted to ethanol, it is first converted into an intermediary molecule called acetaldehyde. This releases carbon dioxide. Next, acetaldehyde is converted into ethanol. Key enzymes aid in the conversion of pyruvate to carbon dioxide and ethanol, including the zymases.

Note that this conversion by itself doesn't create any more ATP for the yeast. So, why does the yeast bother to convert pyruvate to ethanol? Well, this conversion takes electrons away from NADH to form NAD+. Remember that NAD+ is the electron carrier that helps build ATP in the first part of our overall process.

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