The shape of the lesson
By the end of the lesson, learners can
- When oxygen runs out: Pick lactate (mammals), then ethanol (yeast), and follow the carbon atoms and the NAD.
- Rice in flooded fields: In the same tab, pick rice.
- Measuring the rate of respiration: With temperature selected, press Add methylene blue and time each tube.
- Lactate: pyruvate + reduced NAD → lactate + NAD (mammals).
- Ethanol: pyruvate → ethanal + CO2 → ethanol (yeast, rice roots).
- Yield: 2 ATP anaerobic, about 30 or more aerobic.
- Redox indicator decolourises when reduced; rate = 1 ÷ time.
One picture every idea returns to
In anaerobic conditions there is no final electron acceptor, so oxidative phosphorylation, the Krebs cycle and the link reaction stop. Glycolysis can continue only if reduced NAD is reoxidised another way. In mammals, pyruvate accepts the hydrogen and becomes lactate (lactate fermentation); in yeast and plant roots, pyruvate is decarboxylated to ethanal, which accepts the hydrogen and becomes ethanol (ethanol fermentation). Either way only the 2 ATP from glycolysis are made per glucose, against about 30 or more aerobically, because most of the energy stays in lactate or ethanol. Rice copes with flooded roots through aerenchyma, ethanol fermentation and fast stem growth. Redox indicators such as methylene blue and DCPIP are decolourised as they accept hydrogen from respiring yeast, so the time taken measures the rate of respiration.
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, keep glycolysis going.. A sprinting muscle, a vat of yeast and a rice root standing in mud all face the same problem: no oxygen to take the hydrogen. 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: When oxygen runs out | Pick lactate (mammals), then ethanol (yeast), and follow the carbon atoms and the NAD. Project tab 7 of the lab (Energy and respiration lab, anaerobic respiration and rice) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Rice in flooded fields | In the same tab, pick rice. | Pairs |
| 21 to 30 min | Teach and check: Measuring the rate of respiration | With temperature selected, press Add methylene blue and time each tube. Project tab 8 of the lab (Energy and respiration lab, redox indicators) and let a student drive it while the class predicts. | Cooperative Concept Mapping |
| 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, 19 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 Measuring the rate of respiration, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 does not "make lactate to release energy". | The ATP comes only from glycolysis; making lactate or ethanol just reoxidises NAD. Say so in your answer. |
| Aerenchyma lets oxygen diffuse to the roots; it is not a tissue that makes oxygen, and rice roots do not take oxygen from the water by active transport. | Return to Rice in flooded fields: ask a pair to state the correct version and show it on the lab before moving on. |
| A decolourised indicator has been reduced, not oxidised: it has gained hydrogen. | And use a comparison of rates (1 ÷ time), not times, when you describe the effect of temperature. |
Anaerobic respiration, rice and measuring respiration rate: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Outline lactate fermentation in a mammalian muscle cell.
Describe how yeast regenerates NAD when respiring in anaerobic conditions.
Explain why the energy yield from respiration in aerobic conditions is much greater than in anaerobic conditions.
Explain how rice is adapted to grow with its roots submerged in water.
Yeast suspensions with methylene blue were kept at different temperatures. Times to decolourise: 15 °C 480 s; 25 °C 240 s; 35 °C 125 s; 45 °C 150 s. Calculate the rate (1000 ÷ time, in s−1) at each temperature and describe the trend.
Plan an investigation, using a simple respirometer, into the effect of temperature on the rate of respiration of germinating seeds.
Answer key and marking notes
- Glucose is converted to pyruvate in glycolysis, producing 2 ATP and reduced NAD.
- Pyruvate accepts hydrogen from reduced NAD and is reduced to lactate, catalysed by lactate dehydrogenase.
- This regenerates NAD, so glycolysis can continue; lactate is later taken to the liver and converted back.
- Pyruvate is decarboxylated to ethanal, releasing carbon dioxide (pyruvate decarboxylase).
- Ethanal accepts hydrogen from reduced NAD and is reduced to ethanol (alcohol dehydrogenase).
- The NAD released can accept hydrogen in glycolysis again, so glycolysis continues to make 2 ATP per glucose.
- In anaerobic conditions only glycolysis produces ATP: a net 2 ATP per glucose.
- In aerobic conditions pyruvate is completely oxidised in the link reaction and Krebs cycle, producing many reduced NAD and FAD.
- These drive oxidative phosphorylation, which makes most of the ATP; anaerobically most of the energy stays in lactate or ethanol.
- Aerenchyma in the stems and roots has large air spaces through which oxygen diffuses down to the roots.
- Root cells carry out ethanol fermentation, and their high levels of alcohol dehydrogenase stop toxic ethanal and ethanol building up, so they tolerate ethanol.
- Stems grow faster when flooded, keeping leaves above the water for gas exchange and photosynthesis.
- Rates: 15 °C 2.1; 25 °C 4.2; 35 °C 8.0; 45 °C 6.7 (all × 10−3 s−1).
- The rate roughly doubles for each 10 °C rise from 15 to 35 °C, as enzymes and substrates gain kinetic energy and collide more often.
- The rate falls at 45 °C, as dehydrogenase enzymes begin to denature.
- Place a known mass of germinating seeds on a gauze above potassium hydroxide (to absorb CO2) in a respirometer, with a control tube of glass beads of the same volume.
- Stand the apparatus in a thermostatically controlled water bath at one temperature, for example 10, 20, 30 and 40 °C, and leave it to equilibrate.
- Measure the distance moved by the coloured liquid in a fixed time; calculate volume of O2 = πr2h and the rate per gram per minute.
- Repeat three times at each temperature, calculate means and plot rate against temperature.
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.
Which products are formed from pyruvate by yeast cells in anaerobic conditions?
- Aethanol and carbon dioxide
- Blactate and carbon dioxide
- Cethanol and water
- Dlactate only
After intense exercise, what happens to the lactate produced in muscle once oxygen is available again?
- Ait is carried in the blood to the liver, where it is converted back to pyruvate and glucose
- Bit is excreted unchanged by the kidneys straight away
- Cit is converted to ethanol in the muscle cells
- Dit is used directly by ATP synthase in the muscle to make ATP
A mutant yeast lacks the enzyme pyruvate decarboxylase. What will happen to it in anaerobic conditions?
- Ait cannot regenerate NAD, so glycolysis stops and no ATP is made
- Bit makes lactate instead of ethanol and continues to grow normally
- Cit converts pyruvate directly to ethanol using alcohol dehydrogenase
- Dit makes more ATP, because pyruvate builds up and is respired
DCPIP was added to a suspension of respiring yeast. Which observation shows that the yeast is respiring?
- Athe DCPIP changes from blue to colourless as it is reduced
- Bthe DCPIP changes from colourless to blue as it is oxidised
- Cthe DCPIP changes from blue to colourless as it is oxidised
- Dthe DCPIP turns pink as carbon dioxide is released
When a paddy field floods, the stems of rice plants grow faster. How does this help the plant?
- Ait keeps the leaves above the water, so they can exchange gases with the air and photosynthesise
- Bit increases the amount of ethanol the roots can produce
- Cit allows the stems to absorb oxygen from the water by active transport
- Dit removes the need for aerenchyma in the roots
Methylene blue was used to measure the rate of respiration of yeast at different glucose concentrations, with all other variables controlled. Why does the rate level off above about 4% glucose?
- Athe dehydrogenase enzymes are working at their maximum rate, so the amount of enzyme now limits the rate
- Bhigh glucose concentrations denature the dehydrogenase enzymes
- Cthe methylene blue is used up before it can be reduced
- Dat high glucose concentrations the yeast respires anaerobically and stops reducing the indicator
A yeast and methylene blue mixture decolourised in 320 s at 25 °C and in 150 s at 35 °C. By what factor did the rate of respiration increase?
- A2.1
- B0.47
- C170
- D1.4
Yeast growing anaerobically used glucose about 15 times faster than yeast growing at the same rate aerobically. Which explanation is best?
- Aanaerobic respiration yields only 2 ATP per glucose, compared with about 30 or more aerobically
- Bethanol inhibits ATP synthase, so more glucose is needed
- Canaerobic yeast must use extra glucose to make ethanal
- Dglycolysis runs more slowly without oxygen, so more glucose is wasted
Quiz key
Pyruvate is decarboxylated to ethanal, releasing carbon dioxide, and ethanal is reduced to ethanol. Lactate is the product in mammals, and lactate fermentation releases no carbon dioxide. Water is formed only when oxygen accepts electrons in aerobic respiration. Lactate only describes mammalian muscle, not yeast.
Lactate is taken to the liver and oxidised back to pyruvate, which can be respired or converted to glucose; the extra oxygen needed is the oxygen debt. Lactate is a useful energy source, so it is not simply excreted. Mammals have no pathway to make ethanol. ATP synthase is driven by a proton gradient, not by lactate.
Without pyruvate decarboxylase, no ethanal is made, so reduced NAD cannot be reoxidised; NAD runs out, glycolysis stops and the cell makes no ATP without oxygen. Yeast does not have the lactate pathway. Alcohol dehydrogenase reduces ethanal, not pyruvate. Pyruvate cannot be respired further without oxygen, so building up gives no extra ATP.
DCPIP accepts hydrogen removed by dehydrogenase enzymes, so it is reduced, and reduced DCPIP is colourless. It starts blue, so it cannot turn blue as the yeast respires. Gaining hydrogen is reduction, not oxidation. DCPIP is a redox indicator, not a pH or carbon dioxide indicator, so it does not turn pink.
Faster stem growth keeps the leaves above the water surface, where gases diffuse quickly, so the plant can photosynthesise and oxygen can pass down the aerenchyma to the roots. It does not increase ethanol production, which the plant tolerates rather than needs. Oxygen is not taken up from water by active transport. Aerenchyma is still needed to carry oxygen from the shoots to the roots.
As glucose concentration rises, more enzyme active sites are occupied and the rate rises; once nearly all active sites are in use, extra glucose has no effect, so the quantity of enzyme (yeast) limits the rate. Glucose does not denature enzymes; the rate would fall, not level off. The same volume of indicator is used in every tube, so it is not the limiting factor that changes. Anaerobic respiration still involves dehydrogenation in glycolysis, so the indicator would still be reduced.
Rate ratio = (1 ÷ 150) ÷ (1 ÷ 320) = 320 ÷ 150 = 2.1, so the rate roughly doubled for a 10 °C rise. 0.47 is 150 ÷ 320, the ratio of the times, which is upside down for rates. 170 is the difference in seconds, not a factor. 1.4 compares the temperatures, 35 ÷ 25, not the rates.
Anaerobically only glycolysis makes ATP, 2 per glucose, while aerobic respiration gives about 30 or more, roughly 15 times as many, so anaerobic yeast must respire about 15 times as much glucose for the same ATP. Ethanol does not inhibit ATP synthase; oxidative phosphorylation stops because there is no oxygen. Ethanal is made from pyruvate, not from extra glucose. Glycolysis actually speeds up in anaerobic conditions; it does not slow down.
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.