Anaerobic Respiration

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.36 — Differences between aerobic and anaerobic respiration
• 2.38 — Word equations for anaerobic respiration in plants and in animals
• 2.39 — Practical: investigate the evolution of carbon dioxide and heat from respiring seeds or other organisms (required practical)

Specification section 2(e) Nutrition — also taught in Photosynthesis, What Limits Photosynthesis, The Leaf: Structure and Gas Exchange, A Balanced Diet.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

You can sprint far harder than you can run, but only for about twenty seconds. That limit is not willpower and it is not fuel — there is plenty of glucose in the muscle. It is oxygen. Your lungs, heart and blood can only deliver so much per second, and a sprinting muscle demands more than that ceiling allows. What happens next is the interesting part: the muscle does not stop and wait. It switches to a way of releasing energy from glucose that needs no oxygen at all.

The trade anaerobic respiration makes

1. It needs NO oxygen, so it can run at whatever rate the muscle demands without waiting for delivery.
2. But it breaks glucose down only PARTIALLY, so it releases far less energy — a small fraction of what the same molecule yields with oxygen.
3. Because the breakdown is incomplete, it leaves behind a product that still holds most of the glucose's energy.
4. In animals that product is lactic acid, and it causes problems of its own.
5. So anaerobic respiration buys speed and independence from oxygen, and pays for it in efficiency and in what it leaves behind.

Lactic acid builds up in the muscle, and as it does the muscle becomes fatigued and painful and eventually stops working properly. It has to be removed, and removing it requires oxygen — which is why you go on breathing hard for minutes after you have stopped running. That extra oxygen, taken in after the exercise to break down the lactic acid that accumulated during it, is the oxygen debt. You did not have the oxygen at the time, so you borrowed against it and repaid it afterwards.

Plants and yeast start from the same glucose and get a different product: ethanol and carbon dioxide instead of lactic acid. This version is called fermentation, and two everyday things depend on it. Bread rises because yeast in the dough releases carbon dioxide, trapped as bubbles — and the ethanol simply evaporates in the oven. Beer and wine exist because of the other product, the ethanol, and there the carbon dioxide is what escapes through the airlock. One reaction; which product matters depends entirely on what you are making.

Why so much less energy — worth understanding rather than memorising

1. Aerobic respiration breaks glucose ALL the way down, to carbon dioxide and water. Neither product holds any usable chemical energy, so almost everything the glucose contained has been released.
2. Anaerobic respiration stops part way. Lactic acid and ethanol are still substantial molecules with most of their bonds intact.
3. Those bonds still hold chemical energy — which is exactly why ethanol burns, and why it is sold as a fuel.
4. So the energy was never destroyed or wasted. It is still sitting in the product, unreleased, because there was no oxygen available to finish the job.

Think of it like a log fire that keeps going out

Burn a log completely and you are left with a little ash — nothing in it will burn again, because everything the wood stored has been released. Now imagine the fire smothered halfway: you are left with charcoal, and charcoal is famously good fuel, because most of the wood's energy is still locked inside it. Aerobic respiration is the complete burn; anaerobic respiration is the smothered one. Lactic acid and ethanol are the charcoal — products that still hold most of the energy, left behind because there was no oxygen to finish. The proof is that ethanol is literally sold as fuel: a product you can run a car on is not an empty molecule.

One prediction to test yourself with, because the practical on this page turns on it. If respiration releases energy, some of that energy must end up as heat — so a flask of germinating seeds should be measurably warmer than a flask of boiled ones, and the living flask should turn limewater milky while the dead flask does not. Notice what the boiled seeds are for: they are not decoration, they are the proof that it is LIFE doing the warming. Every difference between the two flasks — the temperature, the carbon dioxide — has only one remaining explanation, because everything else about them is identical. And notice why every batch is rinsed in disinfectant first: seeds carry microbes on their surfaces, microbes respire too, and an experiment that measures the wrong organism's respiration measures nothing.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Anaerobic respiration

The release of energy from the incomplete breakdown of glucose in the absence of oxygen. It releases much less energy per glucose molecule than aerobic respiration.

Learn this definition · Oxygen debt

The extra oxygen taken in after exercise, used to break down the lactic acid that built up in the muscles while anaerobic respiration was taking place.

glucose → lactic acid

Conditions: anaerobic respiration in animals. Note there is no carbon dioxide and no water among the products.

glucose → ethanol + carbon dioxide

Conditions: anaerobic respiration in plants and in yeast, also called fermentation.

The three-way comparison — be able to write this table out

During and after a sprint — the sequence to write

1. Demand for energy rises above the rate at which oxygen can be delivered to the muscles.
2. The muscles respire anaerobically as well as aerobically, releasing energy without oxygen.
3. Lactic acid is produced and builds up, causing muscle fatigue and pain.
4. After exercise, breathing and heart rate stay high to take in extra oxygen — the oxygen debt.
5. That oxygen is used to break down the accumulated lactic acid, and breathing returns to normal.

Required practical 2.39 — carbon dioxide and heat from respiring seeds

1. Rinse two batches of seeds in disinfectant, so microbes on their surfaces cannot respire and affect the results.
2. Boil one batch and allow it to cool. These dead seeds are the control.
3. Place each batch in its own vacuum flask with a thermometer, and plug the neck with cotton wool so air can enter but heat is not easily lost.
4. Record the temperature in each flask at the same time each day for several days.
5. For carbon dioxide, draw air slowly through each flask and bubble it through limewater; limewater turning milky shows carbon dioxide is being released.

Variables

Independent (changed) — whether the seeds are alive or have been boiled
Dependent (measured) — temperature rise, and whether the limewater turns milky

Sources of error

Model answer [4 marks]

Explain why a runner continues to breathe deeply for several minutes after finishing a sprint. [4]

During the sprint the muscles could not be supplied with oxygen fast enough, so they also respired anaerobically. This produced lactic acid, which built up in the muscles. Breaking that lactic acid down requires oxygen, so extra oxygen must be taken in after the exercise — the oxygen debt. Breathing therefore stays deep and rapid until the lactic acid has been removed.

Model answer [3 marks]

Explain why anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. [3]

In aerobic respiration glucose is broken down completely, to carbon dioxide and water, so nearly all the energy stored in it is released. In anaerobic respiration the glucose is only partly broken down. The products, lactic acid or ethanol, still contain a large amount of chemical energy in their bonds, and that energy is not released.

Mark-losing trap. Anaerobic respiration in animals produces lactic acid ONLY — no carbon dioxide and no water. Adding either loses the mark.

Mark-losing trap. Yeast and plants make ethanol AND carbon dioxide. Animals make neither, so the two equations are never interchangeable.

Mark-losing trap. Anaerobic respiration is less efficient, not wasteful. The energy is still in the lactic acid or ethanol, simply unreleased.

Mark-losing trap. The oxygen debt is repaid AFTER exercise. It is extra oxygen taken in later, not oxygen that went missing at the time.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Name the product of anaerobic respiration in human muscle cells.
2. Grade 7 · Identify [1 mark] — Which word equation shows anaerobic respiration in yeast?
3. Grade 8 · Explain [4 marks] — A sprinter's legs ache during a 200 m race, and she goes on breathing hard for several minutes after crossing the line. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — Aerobic respiration releases about 2900 kJ per mole of glucose. Anaerobic respiration in animals releases about 120 kJ per mole of glucose. Calculate the energy released anaerobically as a percentage of that released aerobically, to one decimal place.
5. 9+ · Analyse [5 marks] — Yeast is added to a sealed flask of glucose solution fitted with a gas syringe. Gas is produced steadily for several hours, then the rate slows and finally stops — even though tests show plenty of glucose is still present in the flask. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who proved fermentation is life without air, and the one who read organisms by what they leave behind. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Louis Pasteur — the one who proved fermentation is life without air

Pasteur established that fermentation is the work of living yeast, and then found something stranger: yeast ferments FASTER when the air is removed, and switches back to using oxygen when air returns. That switch between two ways of releasing energy from the same glucose is exactly what this lesson describes, and he found it decades before anyone could explain it.

• “How did you show that fermentation needed living yeast rather than happening on its own?”
• “What did you notice when you took the air away from a fermenting vessel?”
• “Why would an organism keep two different ways of getting energy from sugar?”
• “What is it that finally stops a fermentation from continuing?”

Alexander Fleming — the one who read organisms by what they leave behind

Fleming's whole working method was to grow microorganisms under controlled conditions and identify them by what appeared around them — a cleared ring, a colour change, a substance they had released. This lesson asks you to do the same thing in miniature: limewater turning milky and a flask warming up are products by which respiration is detected rather than seen.

• “How can you tell what an organism is doing without ever watching it work?”
• “Why does a control plate matter so much when you are growing microbes?”
• “What could make you think an organism had produced something when it had not?”
• “How do you know a change on a plate was caused by the organism itself?”

Carry on through the course

• Previous lesson: Respiration and ATP
• Next lesson: The Thorax and Ventilation
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification