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How do organisms generate energy when oxygen is not available? Let's explore how humans, bacteria, yeast, and other organisms undergo fermentation to generate energy from food in the absence of oxygen. Before you get started, don’t forget to print out your OnTRACK Biology Journal.

TEKS Standards and Student Expectations

B(4) The student knows that cells are the basic structures of all living things with specialized parts that perform specific functions and that viruses are different from cells. The student is expected to:

B(4)(B) investigate and explain cellular processes, including homeostasis, energy conversions, transport of molecules, and synthesis of new molecules

Learning Objectives

Identify and describe the processes organisms use to release energy from food when oxygen is not available.

Describe the process human muscle cells use to release energy during strenuous exercise.

Explain the benefits and the challenges of fermentation.

Compare and contrast fermentation and cellular respiration.

Compare and contrast lactic acid fermentation and alcoholic fermentation.

Essential Questions

How do organisms generate energy when oxygen is not available?

How is fermentation similar to cellular respiration and how is it different?

How is fermentation in yeast similar to fermentation in human muscle cells and how is it different?

How do humans use fermenting bacteria and yeast to generate useful products?


  • ATP
  • Fermentation
  • Glycolysis
  • Glucose
  • Pyruvate (Pyruvic Acid)
  • Lactic Acid Fermentation
  • Lactic Acid (Lactate)
  • Alcoholic Fermentation
  • Ethyl Alcohol (Ethanol)


Fermentation: An Introduction

Pause for a moment and take a deep breath in. As you do, air fills your lungs. Your lungs and bloodstream work to supply your cells with plenty of oxygen to generate the energy the cells need to function. Remember, cells use oxygen to generate usable energy, or ATP, from the food we eat. This is usually done through the process of cellular respiration. In cellular respiration, oxygen accepts electrons at the end of the electron transport chain where the majority of ATP is formed. Without oxygen, the electron transport chain stops generating ATP.

When you perform strenuous exercise like sprinting in a race, your muscles require energy production faster than your lungs and bloodstream can deliver oxygen. Your muscles are forced to work without enough oxygen. 

In these situations, your working muscles generate ATP anaerobically (i.e., without oxygen) using a process called fermentation. Fermentation is beneficial in that it can generate ATP quickly for working muscle cells when oxygen is scarce.



Fermentation is glycolysis followed by a process that makes it possible to continue to produce ATP without oxygen. Glycolysis is the first series of reactions that occur during cellular respiration. Glycolysis does not require oxygen to produce ATP. 

During glycolysis, glucose is broken down into two molecules of pyruvate (pyruvic acid). Two ATP molecules are required, and four ATP molecules are produced, resulting in a net gain of two ATP. Electrons are also transferred to two NAD+ molecules, forming two NADH molecules.

The following diagram summarizes glycolysis.

When cells generate large amounts of ATP through the process of glycolysis, they quickly use up the cell's available NAD+ molecules. Once all available NAD+ molecules are converted to NADH, glycolysis stops producing ATP. Without oxygen, the citric acid cycle (Krebs cycle) and electron transport chain will not run, so there is nowhere for NADH molecules to deposit their electrons.

Directions: Watch What Happens When You Run Out Of Oxygen! to see an animation showing NADH build up when a cell runs out of oxygen.

What Happens When You Run Out Of Oxygen!
Picardposer. What Happens When You Run Out Of Oxygen! Retrieved from https://www.youtube.com/watch?v=StXlo1W3Gvg&feature=youtu.be

As you can see, without oxygen, the electron carriers in the electron transport chain cannot accept electrons from NADH. No NADH gets converted to NAD+. Without NAD+, cells cannot keep going through glycolysis, and ATP production stops.

To solve this problem, cells convert NADH back into the election carrier, NAD+, through fermentation. This allows glycolysis to continue to produce ATP.

As with glycolysis, fermentation takes place in the cytoplasm of the cell. There are two different forms of fermentation—lactic acid fermentation and alcoholic fermentation. Let's first take a look at lactic acid fermentation.

Lactic Acid Fermentation

Most organisms carry out fermentation through a chemical reaction that converts the pyruvate from glycolysis into lactic acid or lactate. Lactic acid fermentation also converts NADH into NAD+ so that glycolysis can continue. 

The following diagram shows a summary of lactic acid fermentation.

Humans undergo lactic acid fermentation when the body needs a lot of energy in a hurry. When you are sprinting full speed, your cells will only have enough ATP stored in them to last a few seconds. Once the stored ATP is used, your muscles will start producing ATP through lactic acid fermentation. Fermentation makes it possible for cells to continue generating ATP through glycolysis. 

Lactic acid is a byproduct of fermentation. Lactic acid will build up in fermenting cells and eventually limit the amount of fermentation that can occur. The only way to get rid of lactic acid is through a chemical pathway that requires oxygen. As a result, after a quick sprint, a runner will need to supply oxygen to cells with plenty of heavy breathing. An intense effort that lasts just a few seconds may require several minutes of heavy breathing to deliver enough oxygen to cells to clear the lactic acid build up.

Many bacteria are also lactic acid fermenters. For example, bacteria used in the production of cheese, yogurt, buttermilk, sour cream, and pickles are lactic acid fermenters. Yogurt and cheese both start with a source of sugar (i.e., lactose from milk). Then certain bacteria are added (e.g., Lactobacillus). The bacteria carry out lactic acid fermentation in the absence of oxygen. The bacteria convert the lactose sugar to glucose, which enters glycolysis and is followed by lactic acid fermentation. Many other pathogenic microorganisms are killed when the acidity rises due to lactic acid build up. Lactic acid also imparts a sharp, sour flavor typically associated with yogurt and sour cream.


Renee Comet. A Giant brand yogurt container of plain yogurt. Retrieved from https://commons.wikimedia.org/wiki/File:Yogurt_(1).jpg

Alcoholic Fermentation

Yeast (a microscopic fungus) are also capable of both cellular respiration and fermentation. When yeast cells are kept in an anaerobic environment (i.e., without oxygen), they switch to alcoholic fermentation to generate usable energy from food. Like lactic acid fermentation, alcoholic fermentation generates NAD+ so that glycolysis can continue to produce ATP. However, alcoholic fermentation in yeast produces ethyl alcohol instead of lactic acid as a waste product. Alcoholic fermentation also releases carbon dioxide.

The diagram below shows a summary of alcoholic fermentation.


Alcoholic fermentation is the process that causes bread dough to rise. When yeast cells in the dough run out of oxygen, the dough begins to ferment, giving off tiny bubbles of carbon dioxide. These bubbles are the air spaces you see in a slice of bread. The small amount of ethyl alcohol that is produced in the dough evaporates when the bread is baked.

Directions: Watch Bread Time Lapse to see the results of fermenting yeast cells producing carbon dioxide.

Bread Time Lapse
Steven McCann. Bread Time Lapse. Retrieved from https://www.youtube.com/watch?v=vrKA4TYngFk&feature=youtu.be

Fermentation Review

Let's review the processes of fermentation. Here are some key points:

  • Fermentation happens in anaerobic conditions (i.e.,without oxygen).
  • Fermentation begins with glycolysis which breaks down glucose into two pyruvate molecules and produces two ATP (net) and two NADH.
  • Fermentation allows glucose to be continuously broken down to make ATP due to the recycling of NADH to NAD+. (Without fermentation, the electron carrier would be full of electrons, the entire process would back up, and no ATP would be produced.)
  • Lactic acid (i.e., lactate) fermentation occurs in some strains of bacteria and in skeletal muscle and produces lactic acid (i.e., lactate).
  • Alcoholic fermentation occurs in yeast and produces ethanol and carbon dioxide.
  • Fermentation only produces two ATP per glucose molecule through glycolysis, which is much less ATP than cellular respiration.


Journal Activity