Lactic Acid & Alcoholic Fermentation: Comparison, Contrast & Examples

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  • 0:05 Fermentation
  • 1:33 Review of Aerobic…
  • 3:15 Lactic Acid Fermentation
  • 4:33 Alcoholic Fermentation
  • 5:25 Lesson Summary
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Lesson Transcript
Instructor
Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

When in an anaerobic environment, some cells can use glycolysis and fermentation to keep producing ATP. Lactic acid fermentation happens in our muscle cells when we are exercising feverishly, while alcoholic fermentation is used in yeast cells and is what leads to beer, bread, and wine.

Fermentation

Your body's cells normally undergo aerobic cellular respiration, or a process that uses oxygen to convert food into energy. However, there are times when cells undergo anaerobic cellular respiration, or a process that does not use oxygen to convert food into energy. Your muscles' cells, when deprived of air, will use anaerobic cellular respiration. Yeast, which is an organism used to make beer and wine, will follow a similar pathway when deprived of oxygen, too.

Process of aerobic cellular respiration
Aerobic Cellular Respiration

Review of Aerobic Cellular Respiration

Before we delve into how this process works, let's first review cellular respiration as a whole. Remember that in aerobic cellular respiration, there are three stages: glycolysis, the citric acid cycle, and the electron transport chain. In glycolysis, a net of two molecules of ATP, or chemical energy, are produced. The citric acid cycle produces another two molecules, while the electron transport chain produces a whopping 28 molecules of ATP. Oxygen is used in aerobic cellular respiration as the final electron acceptor in the electron transport chain, which is part of why it's able to create so much ATP.

But what happens when oxygen doesn't exist? Like when you're running so hard that you can't catch your breath during the game? The electron transport chain can then no longer be used because there's a shortage of oxygen. Instead, in anaerobic cellular respiration, the only step of this process that occurs is glycolysis. This is unfortunate because ATP is like the chemical currency of the cell, and we lose the ability to 'pay' for all the cellular processes when we run out of ATP. Glycolysis only provides two molecules of ATP. By skipping the final two steps of this process, we lose out on a lot of cash.

However, sometimes some energy is better than no energy at all. Therefore, in anaerobic conditions, some organisms perform fermentation, which is a process that anaerobically generates ATP by performing glycolysis and one extra step. There are two types of fermentation we'll discuss in this lesson - lactic acid fermentation and alcoholic fermentation.

Lactic Acid Fermentation

Glycolysis occurs in the cytosol. Along with 2 ATP, it also produces 2 NADH + H+ electron carriers and two 3-carbon pyruvate molecules. Normally the pyruvate molecules would continue on to the citric acid cycle, while the electron carriers would continue on to the electron transport chain. Here, these electron carriers would drop off their electrons and become oxidized back to NAD+. Then NAD+ molecules would be able to return to glycolysis to perform cellular respiration all over again. But when there is no electron transport chain to go to, what is a poor electron carrier to do with its electrons?

The process of lactic acid fermentation
Lactic Acid Fermentation Process

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Additional Activities

Introduction:

Cells have to be metabolically flexible and have back-up strategies for meeting their energy needs. Although cellular respiration is the preferred metabolic pathway of many cells for meeting their energy needs, when oxygen becomes limited in supply, some cells can switch to an anaerobic pathway for producing ATP. Like the aerobic pathway of cellular respiration, anaerobic pathways begin with glycolysis, which releases some of the energy of glucose to produce some ATP and NADH. For more rounds of glycolysis to occur and make more ATP, the NADH must be reoxidized to NAD+. NAD+ is a vital intermediate in the energy-harvesting phase of glycolysis. Each NAD+ molecule accepts two of the electrons, and their energy, which was originally in a molecule of glucose.

As seen in the video, cells can use either lactic acid or alcoholic fermentation to reform NAD+ from NADH. Pyruvate from glycolysis, or a two carbon derivative of pyruvate called acetaldehyde, serve as the oxidizing agent. To review the two fermentation pathways, complete the crossword puzzle below.

Activity:

Read the 15 puzzle item clues to fill in the blanks of the crossword puzzle. A word bank has been included below to help you.

Word Bank: Choose from among the words below to complete the puzzle. If a term has two words, include a blank space between words in the puzzle.

acetaldehyde

aerobic

anaerobic

ATP

carbon dioxide

ethanol

formaldehyde

glucose

lactic acid

NAD

NADH

oxidation

oxygen

pyruvate

reduction

ten

two

Across:

1. 3-carbon endproduct of glycolysis

5. a gas produced by yeast undergoing fermentation

8. the product of fermentation in human muscle cells

11. loss of electrons

13. gain of electrons

15. a 2-carbon endproduct of fermentation by yeast cells

Down:

2. with oxygen

3. a reduced product of glycolysis

4. abbreviation for nicotinamide dinucleotide

6. lacking oxygen

7. complete oxidation of glucose results in more than 30 of these molecules

9. final acceptor of the electron transport chain

10. the starting carbohydrate for glycolysis

12. a 2-carbon intermediate of alcoholic fermentation

14. number of ATP molecules produced by glycolysis

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