Fermentation Read the lesson title aloud to students.

Learning Objectives Explain how organisms get energy in the absence of oxygen. Identify the pathways the body uses to release energy during exercise. Click to reveal each of the learning objectives. Bring to class two loaves of bread—one that is leavened and one that is unleavened. Pass the loaves around and have each student take a small piece of the bread. Have students compare the two pieces of bread and consider the differences and similarities. Point out that yeast is added to dough to make it rise. Make sure students understand that at the end of the presentation they should be able to explain how organisms generate energy when oxygen is not available, as well as how the body produces ATP during exercise.

Fermentation In the absence of oxygen, fermentation releases energy from food molecules by producing ATP. Explain that during fermentation, cells convert NADH to NAD+ by passing high-energy electrons back to pyruvic acid. This action converts NADH back into the electron carrier NAD+, allowing glycolysis to produce a steady supply of ATP. Ask for a volunteer to point out where ATP is produced in this process. Click to highlight.

Fermentation There are two slightly different forms of the process: Alcoholic fermentation Lactic acid fermentation Tell students that fermentation is an anaerobic process that occurs in the cytoplasm of cells. Point out that sometimes glycolysis and fermentation are together referred to as anaerobic respiration. Ask: What is missing from the illustration of fermentation that was involved in the Krebs cycle and electron transport chain? Answer: Oxygen. Explain that fermentation is referred to as anaerobic respiration because it takes place in the absence of oxygen. Ask for a volunteer to point out the alcoholic fermentation pathway. Click to highlight. Ask for a volunteer to point out the lactic acid fermentation pathway. Tell students that, while the figure shows both alcoholic and lactic acid fermentation, these processes do not normally occur at the same time in an organism. Most organisms carry out either one or the other. Explain that the figure shows pyruvic acid from glycolysis entering either alcoholic or lactic acid fermentation, not both at once.

Alcoholic Fermentation Yeasts and a few other microorganisms use alcoholic fermentation, which produces ethyl alcohol and carbon dioxide. Pyruvic acid + NADH → Alcohol + CO2 + NAD+ Direct students’ attention to the alcoholic fermentation portion of glycolysis. Click to highlight this segment of the process. Ask for a volunteer to fill in the rest of the summary of alcoholic fermentation. Click to reveal the answer. Tell students that alcoholic fermentation is what causes bread dough to rise. It is also used to produce alcoholic beverages. Have students reexamine the bread that you passed out at the beginning of the lesson. Explain that when yeast cells in the dough run out of oxygen, the dough begins to ferment, giving off tiny bubbles of carbon dioxide. These bubbles form the air spaces visible in a slice of leavened bread. The small amount of alcohol produced in the dough evaporates when the bread is baked.

Lactic Acid Fermentation Most organisms carry out fermentation using a chemical reaction that converts pyruvic acid to lactic acid. Pyruvic acid + NADH → Lactic acid + NAD+ Direct students’ attention to the lactic acid fermentation portion of glycolysis. Click to highlight this segment of the process. Ask for a volunteer to fill in the rest of the summary of lactic acid fermentation. Click to reveal the answer. Ask: Which form of fermentation is carried out by your body? Answer: Lactic acid fermentation Tell students that unlike alcoholic fermentation, lactic acid fermentation does not give off carbon dioxide. Explain that like alcoholic fermentation, lactic acid fermentation also regenerates NAD+ so that glycolysis can continue.

Quick Energy For short, quick bursts of energy, the body uses ATP already in muscles as well as ATP made by lactic acid fermentation. Tell students that humans are lactic acid fermenters. During brief periods without oxygen, many of the cells in our bodies are capable of producing ATP by lactic acid fermentation. Explain that the cells best adapted to doing that are muscle cells, which often need very large supplies of ATP for rapid bursts of activity. Ask: What do you think the effects of lactic acid buildup in an athlete’s muscles might be? Answer: Muscle cramping, soreness, and fatigue. Tell students that, until recently, many people thought that lactic acid buildup was responsible for muscle fatigue and soreness. Point out that researchers have since found that lactic acid can actually be used as a fuel by mitochondria to make energy. Ask: At the beginning of a race, what is the principal source of energy for the runners’ muscles? Answer: ATP that is already in the cell, then ATP produced by lactic acid fermentation.

Long-Term Energy For exercise longer than about 90 seconds, cellular respiration is the only way to continue generating a supply of ATP. Explain that cellular respiration releases energy more slowly than fermentation does. Tell students that the body stores energy in muscle and other tissues in the form of the carbohydrate glycogen. These stores of glycogen are usually enough to last for 15 or 20 minutes of activity. After that, your body begins to break down other stored molecules, including fats, for energy. Ask: What form of exercise would be most beneficial for weight control? Answer: Aerobic forms of exercise such as running, dancing, and swimming are beneficial for weight control. Ask: How would a bear look different after hibernation? Answer: The bear will probably be thinner, because it will have used its stored fat for energy during hibernation.

Fermentation Review During fermentation, cells convert NADH to NAD+ by passing high-energy electrons back to pyruvic acid. Direct students to reexamine the two fermentation processes, and then ask the following questions. Ask: What reactants and products do the two types of fermentation have in common? Answer: Both alcoholic fermentation and lactic acid fermentation have the same two reactants, pyruvic acid and NADH. Both also have one product in common, NAD+. Click to first reveal the reactants. Then click again to reveal the common product: NAD+. Ask: Why does a cell need NAD+ to keep glycolysis going? Answer: The cell needs NAD+ to accept electrons when glucose is broken down into pyruvic acid.