Anaerobic Cellular Respiration & Fermentation Worksheet — 13-Year-Olds (Middle School Biology)

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Instructions

Read the information below about how tiny organisms get energy without oxygen. Use what you learn to complete the four activities that follow. Good luck!

All living things need energy to survive. Most get this energy by breaking down glucose (sugar) in a process called cellular respiration. When oxygen is present, it's called aerobic respiration. This is the most efficient way to get energy, producing lots of ATP (the cell's energy molecule).

But what happens when there's no oxygen? This is where anaerobic respiration comes in! The word "an-aerobic" means "without air" or "without oxygen." Many tiny microorganisms, like bacteria and yeast, are experts at this. A common type of anaerobic process is called fermentation. It doesn't produce nearly as much ATP as aerobic respiration, but it allows organisms to get energy in oxygen-poor environments. Humans have learned to use these amazing microbes and their fermentation processes to create some of our favorite foods!

There are two main types of fermentation we will look at:

  • Lactic Acid Fermentation: Certain bacteria (like Lactobacillus) take glucose and turn it into lactic acid and a small amount of ATP. This is the process that turns milk into yogurt and cheese! Your own muscle cells do this too during very intense exercise when you can't get oxygen to them fast enough, which can lead to that sore, burning feeling.
  • Alcoholic Fermentation: Yeast (a type of fungus) is famous for this process. It breaks down glucose into ethyl alcohol, carbon dioxide (CO2) gas, and a small amount of ATP. This is how bread dough rises (the CO2 gas creates bubbles) and how biofuels like ethanol are produced.

Activity 1: Complete the Sentences

Use the words from the word bank to fill in the blanks in the paragraph below. Each word is used only once.

Word Bank: anaerobic, microorganisms, oxygen, ATP, fermentation, glucose

When cells need to create energy from 1. _______________, they perform cellular respiration. If there is no 2. _______________ available, some organisms must use an alternative path. This path is known as 3. _______________ respiration. A process called 4. _______________ is used by many 5. _______________, like yeast and bacteria, to create a small amount of 6. _______________ to survive without air.

Activity 2: Match the Process to the Product

Draw a line (or write the correct letter in the blank) to match the fermentation process or microbe on the left with its correct description or product on the right.

Process / Microbe Description / Product
1. Yeast _____ A. The process that turns milk into yogurt.
2. Lactic Acid Fermentation _____ B. The gas released during alcoholic fermentation that makes bread rise.
3. Alcoholic Fermentation _____ C. The microorganism used to make bread.
4. Carbon Dioxide (CO2) _____ D. The process that produces alcohol and CO2.

Activity 3: Critical Thinking

Answer the following questions in complete sentences.

  1. Why is yeast so important for a baker who makes bread? Explain what is happening inside the dough.
  2. A marathon runner and a 100-meter sprinter both get tired, but the sprinter is more likely to feel a "burn" in their muscles from lactic acid. Why do you think that is? (Hint: Think about oxygen availability during their respective races).
  3. If you left a cup of grape juice in a sealed container with some yeast, what two products would the yeast create from the sugar in the juice?

Activity 4: Compare and Contrast

Place the phrases from the list below into the correct section of the Venn diagram. Some phrases may describe only one type of fermentation, while others describe both.

Phrase List:
  • Produces CO2 gas
  • Produces lactic acid
  • Done by yeast
  • Used to make yogurt
  • Starts with glucose
  • Anaerobic (no oxygen needed)
  • Produces alcohol
  • Can occur in human muscles
  • Creates a small amount of ATP
Lactic Acid Fermentation





Alcoholic Fermentation





Both







Answer Key

Activity 1: Complete the Sentences

  1. glucose
  2. oxygen
  3. anaerobic
  4. fermentation
  5. microorganisms
  6. ATP

Activity 2: Match the Process to the Product

  1. Yeast: C (The microorganism used to make bread.)
  2. Lactic Acid Fermentation: A (The process that turns milk into yogurt.)
  3. Alcoholic Fermentation: D (The process that produces alcohol and CO2.)
  4. Carbon Dioxide (CO2): B (The gas released during alcoholic fermentation that makes bread rise.)

Activity 3: Critical Thinking

  1. Yeast is important because it performs alcoholic fermentation. Inside the dough, the yeast consumes the sugar and releases carbon dioxide gas. These gas bubbles get trapped in the dough, causing it to expand and rise.
  2. The sprinter is running at maximum intensity for a very short time, so their body can't supply oxygen to the muscles fast enough. The muscles switch to lactic acid fermentation for quick energy, which causes lactic acid to build up. A marathon runner runs at a slower, aerobic pace, allowing their body to consistently supply oxygen to the muscles.
  3. The yeast would create ethyl alcohol and carbon dioxide.

Activity 4: Compare and Contrast

  • Lactic Acid Fermentation Only:
    • Produces lactic acid
    • Used to make yogurt
    • Can occur in human muscles
  • Alcoholic Fermentation Only:
    • Produces CO2 gas
    • Done by yeast
    • Produces alcohol
  • Both:
    • Starts with glucose
    • Anaerobic (no oxygen needed)
    • Creates a small amount of ATP
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