Note: This page was updated with new science.

Lesson Objectives

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

WORKBOOK ASSIGNMENT
Chapter 4.4 workbook pages

Get the workbook here: https://guesthollow.com/store/free-high-school-biology-workbook/

Vocabulary

• alcoholic fermentation — type of anaerobic respiration that converts pyruvic acid (pyruvate) to ethanol and carbon dioxide, and forms NAD⁺.
• fermentation — type of anaerobic respiration that converts pyruvic acid (pyruvate) to one or more other compounds and forms NAD⁺.
• lactic acid fermentation — type of anaerobic respiration that converts pyruvic acid (pyruvate) to lactate (commonly called lactic acid) and forms NAD⁺.

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

Here is a video that shows apple cider fermentation bubbles:

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 fermentation are described below.

 

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.

The following is OUTDATED science! We’ve left it here, so you can see what the textbook used to say:

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. Good thing this isn’t a textbook or maybe your arms would be feeling sore right now. Yay technology!

Here’s what we now know:
Modern research shows a more accurate picture:

• In the body, “lactic acid” instantly becomes lactate + H⁺.
• Lactate is not just waste — it can be used as fuel by the heart, brain, or other muscle cells.
• Fatigue comes from a mix of acidity (extra H⁺ ions), low energy, and ion imbalances, not lactate itself.
• Muscle soreness (DOMS) that happens hours later is due to microscopic muscle damage and inflammation, not lactic acid accumulation.

So next time you feel the burn — that’s not lactic acid piling up. Your body is doing its best to keep producing ATP despite low oxygen.

The above example shows that science is NOT always correct. Sometimes our undertstanding of things change based on new research and information! A big thank you goes to Rachel for bringing our attention to this section about lactate and muscles in the comments!

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.

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?

P.S. If you take our Chemistry in the Kitchen course in the future (which doesn’t even have a textbook or require math, YAY!), you will get to bake some homemade bread and eat it, and that counts as SCHOOL WORK! 

Aerobic vs. Anaerobic Respiration: A Comparison

Aerobic respiration is useful today because the atmosphere is 21% oxygen. However, God also created some anaerobic organisms to be able to live in places where there isn’t very much oxygen.

Advantages of Aerobic Respiration

A major advantage of aerobic respiration is the amount of energy it releases. Without oxygen, organisms can just split glucose into two molecules of pyruvate. This releases only enough energy to make two ATP molecules. With oxygen, organisms can break down glucose all the way to carbon dioxide. This releases enough energy to produce up to 38 ATP molecules. Thus, aerobic respiration releases much more energy than anaerobic respiration.

Advantages of Anaerobic Respiration

One advantage of anaerobic respiration is obvious. It lets organisms live in places where there is little or no oxygen. Such places include deep water, soil, and the digestive tracts of animals such as humans (see Figure below).

A quick video about E. Coli:

Another advantage of anaerobic respiration is its speed. It produces ATP very quickly. For example, it lets your muscles get the energy they need for short bursts of intense activity (see Figure below). Aerobic respiration, on the other hand, produces ATP more slowly.

The muscles of these hurdlers need to use anaerobic respiration for energy. It gives them the energy they need for the short-term, intense activity of this sport.

Watch this video about fermentation:

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 lactate (commonly called lactic acid) and forms NAD+. The NAD+ allows glycolysis to continue so it can make more ATP.
• (Scientists used to think that lactic acid buildup made muscles sore. However, research now shows that lactate is actually a useful fuel and not the cause of muscle soreness.)
• 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.

Lesson Review Questions

Recall

1. What is fermentation?

2. Name two types of fermentation.

3. What is the main advantage of aerobic respiration? Of anaerobic respiration?

4. What process produces fuel for motor vehicles from living plant products? What is the waste product of this process?

Apply Concepts

5. Tanya is on the high school track team and runs the 100-meter sprint. Marissa is on the cross-country team and runs 5-kilometer races. Explain which type of respiration the muscle cells in each runner’s legs use.

Think Critically

6. Compare and contrast lactic acid fermentation and alcoholic fermentation. Include examples of organisms that use each type of fermentation.

7. Explain why bread dough rises when it is set aside in a warm place.

Points to Consider

Two important functions of cells are making food and using it for energy. Photosynthesis and cellular respiration are the processes that carry out these functions. Other important functions of cells are growing and dividing.

• Do you know how cells grow? What do you think controls the growth of cells?
• How do you think cells divide? Do all cells divide in the same way?

Previous: Powering the Cell: Cellular Respiration

Next: The Cell Cycle, Mitosis, and Meiosis

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Changes/edits were made to the original ck12 biology text by Guest Hollow. Changes are not endorsed by ck12 in any way.