The shape of the lesson
By the end of the lesson, learners can
- Without oxygen: Go back to the Respiration tab and choose anaerobic in yeast, then anaerobic in muscles.
- Yeast, bread and beer: The yeast in the Yeast tab respires sugar without oxygen, and the carbon dioxide inflates the balloon.
- Sprinting into oxygen debt: Extended.
- Anaerobic: breaks down nutrients to release energy without oxygen; much less energy per glucose.
- Yeast: glucose → alcohol + carbon dioxide (Extended: C6H12O6 → 2C2H5OH + 2CO2).
- Muscles: glucose → lactic acid. No carbon dioxide.
- Oxygen debt (Extended): fast heart rate carries lactic acid to the liver; deep fast breathing supplies oxygen; the liver respires lactic acid aerobically.
One picture every idea returns to
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule than aerobic respiration. In yeast: glucose → alcohol + carbon dioxide. In muscles during vigorous exercise: glucose → lactic acid. Extended: C6H12O6 → 2C2H5OH + 2CO2; lactic acid builds up in muscles and blood, causing an oxygen debt that is repaid after exercise.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, No oxygen, much less energy.. When oxygen runs short, cells can still release a little energy by breaking glucose down only partly. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Without oxygen | Go back to the Respiration tab and choose anaerobic in yeast, then anaerobic in muscles. Project tab 6 of the lab (Anaerobic respiration in yeast and in muscles) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Yeast, bread and beer | The yeast in the Yeast tab respires sugar without oxygen, and the carbon dioxide inflates the balloon. Project tab 7 of the lab (Yeast respiring sugar in a flask) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Sprinting into oxygen debt | Extended. Project tab 8 of the lab (The oxygen debt after a sprint) and let a student drive it while the class predicts. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Sprinting into oxygen debt, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Anaerobic respiration in muscles makes only lactic acid: no carbon dioxide and no alcohol. | Only the yeast equation has carbon dioxide. Never write "glucose → lactic acid + carbon dioxide". |
| The balanced yeast equation has two ethanol and two carbon dioxide, and no oxygen on either side. | Ethanol is C 2 H 5 OH, not CH 3 OH. |
| The oxygen debt is the extra oxygen needed after exercise to remove lactic acid, not a shortage of oxygen in the air. | Lactic acid is broken down in the liver, and it is carried there by the blood. |
Anaerobic respiration: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Describe anaerobic respiration and state the word equation for anaerobic respiration in muscles during vigorous exercise.
State three differences between aerobic respiration and anaerobic respiration in yeast.
Extended. Write the balanced chemical equation for anaerobic respiration in yeast.
Explain how yeast is used in bread-making and in brewing. Name the useful product in each case.
Extended. The table shows the concentration of lactic acid in an athlete's blood before, during and after a 400 m race.
| Time / min | 0 (rest) | 2 (end of race) | 10 | 30 | 60 |
|---|---|---|---|---|---|
| Lactic acid / mmol per dm3 | 1.0 | 12.0 | 9.0 | 4.0 | 1.2 |
(a) Explain the change during the race. (b) Explain how the lactic acid is removed after the race.
Extended. Suggest why a sprinter keeps panting for several minutes after finishing a 100 m race even though they are standing still.
Answer key and marking notes
- The chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
- glucose →
- lactic acid
- Aerobic uses oxygen; anaerobic does not.
- Aerobic produces carbon dioxide and water; anaerobic in yeast produces alcohol (ethanol) and carbon dioxide.
- Aerobic releases much more energy per glucose molecule; anaerobic releases much less (glucose is only partly broken down).
- C6H12O6 → 2C2H5OH + 2CO2
- One mark for correct formulae, one mark for correct balancing.
- Yeast respires sugar anaerobically, producing alcohol (ethanol) and carbon dioxide.
- Bread: the carbon dioxide is the useful product; its bubbles make the dough rise (the alcohol evaporates on baking).
- Brewing: the alcohol is the useful product (the carbon dioxide gives fizz or escapes).
- (a) The muscles need more oxygen than the blood can supply, so they also respire anaerobically, producing lactic acid; it rises from 1.0 to 12.0 mmol per dm3 in the blood (an oxygen debt builds up).
- (b) The heart rate stays fast to transport lactic acid in the blood from the muscles to the liver.
- Breathing stays deeper and faster to supply extra oxygen.
- In the liver lactic acid is respired aerobically, so its concentration falls back towards the resting value by 60 minutes.
- During the race lactic acid built up in the muscles and blood from anaerobic respiration, causing an oxygen debt.
- Deep, fast breathing continues to supply the extra oxygen needed to respire the lactic acid aerobically (in the liver) and repay the debt.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is produced by anaerobic respiration in human muscle cells during vigorous exercise?
- ALactic acid only
- BLactic acid and carbon dioxide
- CAlcohol and carbon dioxide
- DCarbon dioxide and water
Which describes anaerobic respiration?
- ABreaking down nutrient molecules in cells to release energy without using oxygen
- BBreaking down nutrient molecules in cells to release energy using oxygen
- CBreaking down lactic acid in the liver to make glucose and oxygen
- DTaking in air through the mouth when the lungs cannot work properly
Why does anaerobic respiration release much less energy per glucose molecule than aerobic respiration?
- AGlucose is only partly broken down, so the products still contain chemical energy
- BThe energy released is used up straight away in making the oxygen
- CAnaerobic respiration takes place only in mitochondria, which are small
- DGlucose is broken down completely, but the energy escapes as light
Yeast and sugar solution are put in a flask and covered with a layer of oil, and the gas given off is bubbled through limewater. The limewater turns milky. What does this show?
- AYeast releases carbon dioxide when it respires without oxygen
- BYeast releases oxygen when it respires without carbon dioxide
- CYeast releases alcohol vapour, which turns limewater milky
- DYeast respires aerobically using oxygen in the layer of oil
Respiring one mole of glucose releases about 2880 kJ aerobically but only about 150 kJ anaerobically in muscle. About how many times more energy does aerobic respiration release?
- A19
- B2730
- C0.05
- D3030
Extended: the graph shows the lactic acid concentration in a runner's blood. She exercised from 5 to 15 minutes. For how long after the end of exercise did her blood lactic acid stay above its resting level?
- A40 minutes
- B55 minutes
- C50 minutes
- D10 minutes
Extended: how is the lactic acid that builds up during vigorous exercise removed afterwards?
- ABlood carries it to the liver, where it is respired aerobically
- BIt diffuses into the alveoli and is breathed out of the lungs
- CIt is respired anaerobically in the muscles, giving carbon dioxide
- DThe kidneys filter all of it from the blood and excrete it in urine
Extended: yeast respires three molecules of glucose anaerobically. Using the balanced equation, how many molecules of carbon dioxide are produced?
- A6
- B18
- C3
- D2
Quiz key
The word equation in muscles is glucose → lactic acid; no other product is made. Adding carbon dioxide mixes the muscle equation up with the yeast equation. Alcohol and carbon dioxide are made by yeast, not by human muscles. Carbon dioxide and water are the products of aerobic respiration.
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. Using oxygen describes aerobic respiration. The liver respires lactic acid aerobically after exercise, which is part of repaying the oxygen debt, not a definition of anaerobic respiration, and it does not make oxygen. Respiration is not a way of taking in air; that is breathing.
Without oxygen glucose is broken down only partly, to lactic acid or alcohol, and these molecules still contain a lot of chemical energy (alcohol can even be burned as a fuel). Anaerobic respiration does not make oxygen. It takes place in the cytoplasm, not mainly in mitochondria, which are linked with aerobic respiration. Glucose is not broken down completely, and the energy is not lost as light.
The oil stops oxygen from the air dissolving in the solution, so the yeast respires anaerobically; the gas that turns limewater milky is carbon dioxide (glucose → alcohol + carbon dioxide). Respiration does not release oxygen. Limewater tests for carbon dioxide, not alcohol. The oil is there to keep oxygen out, not to supply it.
2880 ÷ 150 = 19.2, so about 19 times more energy is released aerobically. 2730 is the difference (2880 − 150) in kJ, not a ratio. 0.05 is the smaller value divided by the larger one, the wrong way round. 3030 is the sum of the two values.
Exercise ended at 15 minutes and the lactic acid returned to its resting level (about 1 mmol per dm3) at 55 minutes, so it stayed raised for 55 − 15 = 40 minutes while the oxygen debt was repaid. 55 minutes is the time read from the start of the graph. 50 minutes measures from the start of exercise. 10 minutes is the length of the exercise period itself.
After exercise the heart rate stays fast so the blood carries lactic acid from the muscles to the liver, and deep, fast breathing supplies the oxygen for the aerobic respiration of lactic acid there. Lactic acid is not a gas, so it is not breathed out. It is removed using oxygen, not anaerobically, and anaerobic respiration in muscles makes lactic acid rather than removing it. Lactic acid is not removed mainly by the kidneys in urine.
Each glucose gives 2CO2, so three glucose molecules give 3 × 2 = 6. 18 uses the aerobic ratio of 6CO2 per glucose. 3 assumes one carbon dioxide per glucose. 2 is the number for a single glucose molecule.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.