Lesson focus: When oxygen supply is insufficient, cells can release some energy anaerobically, but the pathways and products differ among organisms.
Compare aerobic respiration with anaerobicWithout oxygen. pathways in animals and yeast and interpret fermentationWhen yeast or bacteria respire without oxygen, such as yeast making bread rise. evidence.
Build on earlier learning
Aerobic respirationAerobic respiration established the high-yield oxygen-dependent pathway. This lesson examines alternatives when oxygen availability limits that route.
Practical
Yeast fermentation
- Mix yeast with sugar solution in a tube fitted with a delivery tube.
- Count the bubbles of gas per minute at different temperatures.
- Bubble the gas through limewater to show it is carbon dioxide.
Record: A table and graph of temperature against bubbles per minute.
You will be successful when you can…
- State animal and yeast anaerobic equations.
- Compare energy released with aerobic respiration.
- Explain lactate accumulation and recovery.
- Plan a controlled yeast investigation.
Words that carry the science
- anaerobic
- Occurring without the use of oxygen.
- lactate
- The product formed from glucose during anaerobic respiration in human muscle cells.
- fermentation
- Anaerobic metabolic pathways used by microorganisms, including alcoholic fermentation in yeast.
- ethanol
- An alcohol produced with carbon dioxide during yeast fermentation.
- oxygen debt
- An older shorthand for elevated recovery oxygen use after exercise; excess post-exercise oxygen consumption is more precise.
- yield
- The amount of useful product obtained, or energy released, from a process.
Compare pathways by products and energy released
01Animal cells form lactate
In human muscle cells the simplified equation is glucose → lactate. The pathway releases less energy per glucose molecule than aerobic respiration because glucose is only partly broken down.
lactateThe product of anaerobic respiration in muscles when they get too little oxygen; it is later broken down using oxygen (oxygen debt). accumulation is associated with falling muscle pH and fatigue, although fatigue has several causes. During recovery, lactate can be transported and metabolised, including conversion in the liver.
02Yeast can carry out alcoholic fermentation
In yeast the simplified equation is glucose → ethanol + carbon dioxide. Carbon dioxide makes bread dough rise, while ethanol is important in brewing and can be used as a biofuel feedstock.
fermentationWhen yeast or bacteria respire without oxygen, such as yeast making bread rise. releases less energy per glucose than aerobic respiration, so more substrate is needed for the same energy demand. Yeast can also respire aerobically when oxygen is available.
03A fermentation investigation needs sealed but safe conditions
Mix equal yeast and glucose suspensions at controlled temperatures and measure carbon dioxide volume over time. An oil layer can reduce oxygen entry, but vessels must not be rigidly sealed because gas pressure can build.
Keep yeast concentration, glucose concentration, pH, volume and equilibration time controlled. Gas volume is a useful proxy, but dissolved carbon dioxide and leaks can cause underestimation.
Anaerobic respiration and oxygen debt
When muscles cannot get oxygen fast enough they also respire anaerobically, making lactic acid. After exercise you keep breathing hard to break it down: the oxygen debt.
Lactic acid in the muscle
Controls
Values are illustrative. Anaerobic respiration releases much less energy than aerobic respiration from the same glucose.
Compare gas-production rates
Yeast produces 45 cm³ of carbon dioxide in 15 minutes at 30 °C.
- Use rate = volume ÷ time.
- Substitute 45 ÷ 15.
- Calculate 3.0 cm³ min⁻¹.
- Describe it as measured carbon-dioxide production, a proxy for fermentation rate.
How to make the reasoning visible: The number is meaningful only with the yeast mass, substrate concentration and other conditions stated.
Compare respiratory pathways
Complete each comparison using products, oxygen use and relative energy transfer.
| Pathway | Oxygen used? | Products | Relative energy released per glucose | Context |
|---|---|---|---|---|
| Aerobic respiration | Yes | Carbon dioxide + water | Higher | Most sustained cellular activity |
| Animal anaerobic | No | Lactate | Lower | Short periods of high muscle demand |
| Yeast fermentation | No | Ethanol + carbon dioxide | Lower | Bread, brewing and biotechnology |
| Recovery metabolism | Usually yes | Lactate is transported and processed | Uses additional oxygen | After intense exercise |
Why does bread dough expand during yeast fermentation?
Why is “lactic acid causes all fatigue” an oversimplification?
Check your understanding
Answer from memory, then check against the suggested answer.
What product forms in human muscle during anaerobic respiration?
What two products form in alcoholic fermentation?
Which pathway releases more energy per glucose?
Explain why a sprinter may continue breathing rapidly after finishing.
Explain why fermentation rate may fall at very high sugar concentration.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Human cells can respire aerobically or anaerobically.
Compare aerobic respiration with anaerobic respiration in muscle cells.
(5)Total for this question: 5 marks
Command word: For compareA command word: say how two or more things are similar and how they are different., write paired similarities and differences rather than two separate descriptions.