Cellular respiration is an aerobic process that depends on oxygen to break apart sugar molecules for the purpose of producing ATP. Fermentation also breaks apart sugar molecules for the purpose of producing ATP. However, it does so without the presence of oxygen, meaning it is an anaerobic process. Plus, it generates only two molecules of ATP for each molecule of glucose, making it a less efficient means of energy production.

There are different types of fermentation, and they can be identified by three things during the process. One is the kind of molecules that form as a result of glycolysis. Another is the different enzymes complex proteins that speed up chemical reactions during the process. The third is the kind of waste the process creates.

Two common kinds of fermentation are lactic acid fermentation and alcohol fermentation. Both are anaerobic processes, but lactic acid fermentation occurs in animal tissue such as human muscle, while alcohol fermentation occurs in plant and yeast cells. Both are used in food production. You may already be familiar with some fermented products, including yogurt, cheese, wine, beer, soy sauce, salami, and pepperoni. Fermentation also causes breads to rise and to produce their assorted shapes and textures.


Lactic Acid Fermentation

When you exercise, your muscles work harder than normal, and you breathe more heavily to take in the additional oxygen your muscle cells need for cellular respiration. However, after prolonged exercise, you may “run out of breath;” you may not be able to take in as much oxygen. In the absence of oxygen, the process of lactic acid fermentation replaces cellular respiration in your body’s muscle cells.

As in cellular respiration, glycolysis occurs in lactic acid fermentation, too. The process generates two molecules of pyruvate, which then change to lactic acid. Lactic acid then becomes lactate, a waste product. The buildup of lactate in your muscles causes a burning sensation and muscle fatigue. When you slow down, your muscle cells begin to recover. More oxygen enters the cells, allowing cellular respiration to resume. The muscle cells release lactate into the bloodstream, where it travels to the liver. The liver changes the lactate back into glucose that is stored in the body or enters the bloodstream for cells to pick up.

Here is a simplified description of the process of lactic acid fermentation:

One molecule of sugar (glucose) → 2 molecules of pyruvate (which are later changed to lactate) + 2 molecules of ATP

Alcohol Fermentation

Unlike human muscle cells that produce lactate during fermentation, yeast cells change glucose into ethyl alcohol, or ethanol. The process produces carbon dioxide as a waste product.

When yeast cells are added to bread dough, they spread out. During fermentation the yeast cells release carbon dioxide as waste. Bubbles of carbon dioxide form in the dough and cause the bread to rise. When the bread is baked, the yeast cells die, and the burst bubbles leave the holes you see.

Here is a simplified description of the process of alcohol fermentation:

1 molecule of glucose → 2 molecules of ethanol + 2 molecules of carbon dioxide + 2 molecules of ATP

Conduct Your Own Investigation
Blow Up a Balloon You need only a few materials to conduct this investigation. Begin by getting safety glasses and an apron. Then collect a small clear, plastic soda bottle, a balloon, water, sugar, and a packet of ‘ baker’s yeast.
Fill the bottle with warm water. The water must be warm, not hot, because yeast are living organisms and hot water will kill them. Next, pour the baker’s yeast into the warm water. Swirl the bottle gently.
Add a teaspoon of sugar to the water. Swirl the bottle again. Stretch the balloon and then attach it to the bottle’s opening. Put the bottle ma warm place and let it sit for twenty minutes. Observe what happens.

Ethanol: An Alternative Energy Source

Fossil fuels are a non-renewable resource. It took millions of years for them to form. So, once they are gone, they cannot be replaced. Fossil fuel production and use also harm the environment.

Scientists have been seeking alternatives to fossil fuels . One such alternative is ethanol. Ethanol can be produced through the fermentation of plant crops, such as corn. Recall that when plants perform photosynthesis, they produce food in the form of glucose. Energy, captured from the Sun, is stored in the bonds of the glucose molecules, which are stored in the plant.

When you eat a plant such as corn, enzymes in your saliva break down the carbohydrates in corn into simpler sugars. Your body then breaks down the sugars during cellular respiration.

To make fuel, the corn is cooked and enzymes are added. The result is a simple sugar to which yeast cells are added. During alcohol fermentation, the yeast cells feed on the sugar and produce ethanol. One acre of corn can produce more than 300 gallons of ethanol. The ethanol holds much of the energy that was in the sugar, making it a good source of fuel.

For every 85 parts ethanol, 15 parts gasoline are added. The result is a fuel that decreases the demand for fossil fuel.