Chapter 2, Section 2
Fermentation
Some cells are able to obtain energy from food without using oxygen. For example, some single-celled organisms live where there is no oxygen, such as deep in the ocean or in the mud of lakes or swamps. These organisms obtain their energy through fermentation, an energy-releasing process that does not require oxygen. Fermentation provides energy for cells without using oxygen. The amount of energy released from each sugar molecule during fermentation, however, is much lower than the amount released during respiration.
Alcoholic Fermentation
One type of fermentation occurs when yeast and some other single-celled organisms break down sugars. This process is sometimes called alcoholic fermentation because alcohol is one of the products. The other products are carbon dioxide and a small amount of energy.
The products of alcoholic fermentation are important to bakers and brewers. The carbon dioxide produced by yeast creates air pockets in bread dough, causing it to rise. Carbon dioxide is also the source of bubbles in alcoholic drinks such as beer and sparkling wine.
Lactic Acid Fermentation
Another type of fermentation takes place at times in your body. You’ve probably felt its effects. Think of a time when you ran as fast as you could for as long as you could. Your leg muscles were pushing hard against the ground, and you were breathing quickly.
No matter how hard you breathed, your muscle cells used up the oxygen faster than it could be replaced. Because your cells lacked oxygen, fermentation occurred. The fermentation supplied your cells with energy. One product of this type of fermentation is an acid known as lactic acid. When lactic acid builds up, you feel a painful sensation in your muscles. Your muscles feel weak and sore.
Figure 9Lactic Acid Fermentation When an athlete’s muscles run out of oxygen, lactic acid fermentation supplies the cells with energy.
