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Anaerobic Respiration

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Anaerobic Respiration

Lesson Objectives

• Define fermentation.
• Describe lactic acid fermentation and alcoholic fermentation.
• Compare the advantages of aerobic and anaerobic respiration.

Vocabulary

$&\mathbf{alcoholic \ fermentation} & &\mathbf{fermentation} & &\mathbf{lactic \ acid \ fermentation}$

Introduction

Today, most living things use oxygen to make ATP from glucose. However, many living things can also make ATP without oxygen. This is true of some plants and fungi and also of many bacteria. These organisms use aerobic respiration when oxygen is present, but when oxygen is in short supply, they use anaerobic respiration instead. Certain bacteria can only use anaerobic respiration. In fact, they may not be able to survive at all in the presence of oxygen.

Fermentation

An important way of making ATP without oxygen is called fermentation. It involves glycolysis but not the other two stages of aerobic respiration. Many bacteria and yeasts carry out fermentation. People use these organisms to make yogurt, bread, wine, and biofuels. Human muscle cells also use fermentation. This occurs when muscle cells cannot get oxygen fast enough to meet their energy needs through aerobic respiration. There are two types of fermentation: lactic acid fermentation and alcoholic fermentation. Both types of are described below. You can also watch animations of both types at this link: http://www.cst.cmich.edu/users/schul1te/animations/fermentation.swf.

Lactic Acid Fermentation

In lactic acid fermentation, pyruvic acid from glycolysis changes to lactic acid. This is shown in Figure below. In the process, NAD+ forms from NADH. NAD+, in turn, lets glycolysis continue. This results in additional molecules of ATP. This type of fermentation is carried out by the bacteria in yogurt. It is also used by your own muscle cells when you work them hard and fast.

Lactic acid fermentation produces lactic acid and NAD+. The NAD+ cycles back to allow glycolysis to continue so more ATP is made. Each circle represents a carbon atom.

Did you ever run a race and notice that your muscles feel tired and sore afterward? This is because your muscle cells used lactic acid fermentation for energy. This causes lactic acid to build up in the muscles. It is the buildup of lactic acid that makes the muscles feel tired and sore.

Alcoholic Fermentation

In alcoholic fermentation, pyruvic acid changes to alcohol and carbon dioxide. This is shown in Figure below.NAD+ also forms from NADH, allowing glycolysis to continue making ATP. This type of fermentation is carried out by yeasts and some bacteria. It is used to make bread, wine, and biofuels.

Alcoholic fermentation produces ethanol and NAD+. The NAD+ allows glycolysis to continue making ATP.

Have your parents ever put corn in the gas tank of their car? They did if they used gas containing ethanol. Ethanol is produced by alcoholic fermentation of the glucose in corn or other plants. This type of fermentation also explains why bread dough rises. Yeasts in bread dough use alcoholic fermentation and produce carbon dioxide gas. The gas forms bubbles in the dough, which cause the dough to expand. The bubbles also leave small holes in the bread after it bakes, making the bread light and fluffy. Do you see the small holes in the slice of bread in Figure below?

The small holes in bread are formed by bubbles of carbon dioxide gas. The gas was produced by alcoholic fermentation carried out by yeast.

Lesson Summary

• Fermentation is a way of making ATP from glucose without oxygen. There are two types of fermentation: lactic acid fermentation and alcoholic fermentation.
• Lactic acid fermentation changes pyruvic acid to lactic acid and forms NAD+. The NAD+ allows glycolysis to continue so it can make more ATP.
• Alcohol fermentation changes pyruvic acid to ethanol and carbon dioxide and forms NAD+. Again, the NAD+ allows glycolysis to keep making ATP.
• Aerobic respiration produces much more ATP than anaerobic respiration. However, anaerobic respiration occurs more quickly.