Hi Tara. Topic 12 is small and it is worth being honest about that: there is less to learn here than in any other topic you have done. Sub-topic 12.1 is what respiration is for — seven uses of energy, and one experiment with yeast that Cambridge has been setting in one form or another for thirty years. Sub-topic 12.2 is aerobic respiration: one word equation, one balanced equation, and the fact that it happens in every living cell of every living organism, all the time. Sub-topic 12.3 is anaerobic respiration, which splits into two — yeast makes alcohol, muscle makes lactic acid, and the two are never mixed up in the same organism. Then the oxygen debt, which is three sentences the syllabus gives you almost word for word. Finish with 12.4, the checklist.
One warning before you start, and it is the only one you really need. Respiration releases energy. It does not produce it, make it or create it. The energy was already there, stored in the chemical bonds of the glucose; respiration is the controlled series of reactions that lets the cell get at it. Mark schemes for this topic are written around that verb, and “respiration produces energy” is a sentence that has cost more marks in 0610 than any other single sentence in the syllabus. Say releases. Say it every time.
The Definition You Already Own
You met respiration in Topic 1, in MRS GREN, and the definition Cambridge gave you there is still the definition it wants now:
Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
Three parts, and every one of them is doing work. In cells — not in the lungs, not in the blood, in the cytoplasm and mitochondria of every living cell you have. Break down nutrient molecules — usually glucose, and “break down” is why energy comes out. Release energy — not produce it.
Respiration is a chemical reaction inside cells. Ventilation (breathing) is the movement of air into and out of the lungs. Gas exchange is diffusion across the alveolus. They are three different things happening in three different places, and a mark scheme will not accept one for another.
Test yourself on this sentence: “a plant respires.” True — every cell of it, every second, day and night. “A plant breathes.” False. It has no lungs, no ribs and nothing to ventilate.
Where the Energy Goes
Cambridge names seven uses of the energy released by respiration, and a three-mark question wants three of them. The two people always forget are active transport and maintaining a constant body temperature, so learn those two first — everyone remembers muscles.
| Use of energy | Where you have already met it |
|---|---|
| Muscle contraction | Topic 9 — the heart muscle contracting; Topic 11 — the intercostal muscles and the diaphragm |
| Protein synthesis | Topic 4 — proteins are built from amino acids, and joining them costs energy |
| Cell division | Topic 2 — new cells are produced by division of existing cells |
| Active transport | Topic 3 — movement against a concentration gradient, using energy from respiration. Ion uptake by root hair cells is the example the syllabus names |
| Growth | Topic 1 — a permanent increase in size and dry mass. You cannot build new material out of nothing |
| Passage of nerve impulses | Neurones conduct electrical impulses (Topic 2) and it costs energy to keep doing it |
| Maintenance of a constant body temperature | Some of the energy released is transferred to the surroundings as thermal energy, which is how a mammal stays warmer than the air around it |
If a question gives you a root hair cell taking up nitrate ions from soil in which the nitrate concentration is lower than inside the cell, the chain is: that is uptake against the gradient, so it is active transport, so it needs energy from respiration, so anything that stops respiration — no oxygen in waterlogged soil, cyanide, very low temperature — stops the uptake. That single chain has appeared on more papers than the whole of the rest of 12.1 put together.
The Yeast Experiment
Cambridge names one investigation in 12.1: the effect of temperature on respiration in yeast. Yeast is chosen because it is a single-celled organism you can keep alive in a test tube, it respires fast when it is given sugar, and — the useful part — it gives off carbon dioxide, which is easy to measure. You never measure “respiration” directly. You measure a gas.
Independent variable: the temperature of the water bath.
Dependent variable: the rate of carbon dioxide production — cm³ per minute, or bubbles per minute.
Control variables: the volume and concentration of the yeast suspension; the volume and concentration of the glucose solution; the same yeast, from the same batch; the same length of time in the water bath before readings start, so the mixture actually reaches the set temperature; the same apparatus and the same person counting.
Repeats: three at each temperature, then a mean, and any obvious anomaly identified and left out of the mean rather than quietly averaged in.
Leave the tube in the water bath for five minutes before you start timing. If you do not, the yeast is not yet at the temperature written on the label and every reading is wrong in the same direction.
The thin layer of oil on the surface stops oxygen dissolving in from the air, so you know the yeast is respiring anaerobically. If a question asks why the oil is there, that is the answer — not “to stop evaporation”.
The Curve, and Why It Is Not Symmetrical
Plot rate against temperature and you get a shape you have seen before — in Topic 5, for enzymes. That is not a coincidence: respiration is a series of reactions, and every one of them is controlled by an enzyme. So the respiration curve is an enzyme curve wearing a different label.
An enzyme is a molecule. Molecules are not alive, so they cannot be killed. At high temperature the enzyme molecule changes shape, the active site no longer fits the substrate, and the enzyme cannot form enzyme–substrate complexes. That is denaturation, and it is permanent.
You may say the yeast cells are killed at high temperature — they are — but the mark is for the enzymes being denatured. Write both if you have room; write the enzyme one first.
The volume changed by 21.0 − 6.0 = 15.0 cm³. The time it took was 10 − 4 = 6 minutes. So the rate is 15.0 ÷ 6 = 2.5 cm³ per minute. The unit is part of the answer and is often the second mark.
21 ÷ 10 = 2.1 answers a different question — the mean rate over the whole ten minutes, including the first four. You were asked about the interval between two readings, so both the volume and the time must be differences.
21 cm³ is a total, not a rate, and a total tells you nothing on its own because it depends on how long you waited. Left for an hour, a very slow reaction would also reach 21 cm³. To compare two temperatures you must divide by time in both cases — and you must have collected for the same length of time, or divide, or you are comparing nothing at all.
The optimum is at or near 40 °C from these data — that part is a fair reading. Say so first; an evaluation that only attacks is half an answer.
If glucose were running out, the rate would fall at every temperature and it would fall with time, not with temperature. All five tubes had the same glucose, and the tubes at 20 and 30 °C did not slow down. The fall from 92 to 22 to 0 as the temperature rises is caused by the enzymes being denatured — their active sites change shape, so the substrate no longer fits.
The true optimum is not necessarily exactly 40 °C. The readings are ten degrees apart, so all you can honestly say is that it lies between 30 and 50 °C. To narrow it down you would repeat at 2 °C intervals across that range. Saying so is nearly always a mark on an evaluate or improve question.
The Definition, and the One Word That Carries It
Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
The only thing that separates this from the general definition in 12.1 is the phrase use oxygen. Leave it out and you have defined respiration, not aerobic respiration, and the mark goes.
Where It Happens
Aerobic respiration happens in the mitochondria, the organelles you met in Topic 2.1. That is all 0610 asks of you about them: they are the site of aerobic respiration. You do not need to know anything about their internal structure and you will never be asked.
What is worth noticing is the pattern: cells that need a great deal of energy contain a great many mitochondria. Muscle cells, and the cells lining the small intestine that carry out active transport, are packed with them. If a question shows you an electron micrograph or a table of mitochondria per cell and asks you to suggest what the cell does, the answer is always “something that requires a lot of energy”, and then you name the process from the context you have been given.
Aerobic respiration is not something that happens in special respiring organs. It happens in every living cell of every living organism — your brain cells, the cells of an onion, the cells of a bacterium, the cells of an oak leaf at midday. When a question says “where does respiration take place?” the answer “in the cells” is worth more than any organ you could name.
The Mirror of Photosynthesis — and Why It Is Not the Opposite
Put the two equations one above the other and the resemblance is obvious:
| Photosynthesis (Topic 6) | Aerobic respiration (Topic 12) | |
|---|---|---|
| Equation | carbon dioxide + water → glucose + oxygen | glucose + oxygen → carbon dioxide + water |
| Energy | light energy is transferred to chemical energy in glucose | chemical energy in glucose is released for the cell to use |
| Needs light? | Yes — it stops completely in the dark | No — it continues day and night |
| Which cells? | Only cells containing chloroplasts | Every living cell |
| Which organisms? | Plants (and some other organisms not on this syllabus) | All living organisms |
“Respiration is the opposite of photosynthesis.” The equations are reverses of each other, and saying that is fine. But the processes are not opposites in any useful sense: they happen in different organelles, they are controlled by different enzymes, one needs light and one does not, and — the point that matters — a plant does both at the same time in daylight. A plant does not “switch” from one to the other at dusk.
What changes at dusk is the net gas exchange. In bright light photosynthesis is much faster than respiration, so the leaf takes in carbon dioxide overall and gives out oxygen. In darkness only respiration is happening, so the leaf takes in oxygen and gives out carbon dioxide. At dawn and at dusk there is a moment when the two rates are equal and there is no net exchange at all — which is a lovely graph question and a favourite one.
Detecting Respiration: What You Can Actually Measure
You cannot see respiration. You detect it in one of four ways, and a good data question will use one of them and expect you to know what it proves.
| What you measure | How | What it shows |
|---|---|---|
| Carbon dioxide given out | Limewater turns cloudy / milky; or hydrogencarbonate indicator turns from red to yellow | Carbon dioxide is being released. Note the limewater test is qualitative — it tells you whether, not how much |
| Oxygen taken in | A respirometer — a sealed tube of organisms with the carbon dioxide absorbed, so the volume of gas falls as oxygen is used | The volume of oxygen used per unit time, which is the rate of aerobic respiration |
| Temperature rise | A vacuum flask of germinating seeds warms up compared with a flask of dead ones | Energy is being released and some of it is transferred to the surroundings as thermal energy |
| Mass loss | Dry mass of a store of seeds or a potato falls slowly in storage | Nutrient molecules are being broken down |
The respirometer is the one that produces the numbers, so it is worth seeing how it works. The apparatus below is described fully whenever it appears in a question, so you are never expected to have memorised it — but if you understand it once, those questions become straightforward.
Volume = cross-sectional area × distance moved = 1.5 × 48 = 72 mm³. That is the volume of oxygen the peas removed from the sealed air, because the soda lime absorbed every molecule of carbon dioxide they gave out.
Per gram: 72 ÷ 5.0 = 14.4 mm³ per gram in 20 minutes. Per hour: 20 minutes is a third of an hour, so multiply by 3: 14.4 × 3 = 43.2 mm³ per gram per hour.
Do the two conversions separately and write each one down. Trying to do them in one step is where the arithmetic goes wrong, and if you show the intermediate value you keep the method mark even if you slip.
Should the answer per hour be bigger or smaller than the answer per 20 minutes? Bigger — there is three times as much time for oxygen to be used. If you had divided by 3 and got 4.8 you would have a number that means the peas respire more slowly the longer you leave them, which is nonsense you can catch in five seconds.
Purple means the carbon dioxide concentration has fallen. Yellow means it has risen. Neither colour tells you that a process has stopped — it tells you which way the balance has tipped.
In tube 1 the plant is photosynthesising and respiring at the same time. Photosynthesis is much faster, so it uses carbon dioxide faster than respiration releases it, and the net effect is a fall — purple. Respiration has not stopped for an instant.
In tube 2 there is no light, so there is no photosynthesis. Only respiration is happening, so carbon dioxide accumulates and the indicator turns yellow. This tube is the one that gives you direct evidence of respiration.
A third tube containing indicator and no plant should be set up as a control, to show that any colour change is caused by the plant and not by handling, breathing on the tube or a change in temperature.
What It Is, and the One Thing It Is Not
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
And the sentence that goes with it, which is worth a mark of its own: anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.
The most common wrong answer in 12.3 is that anaerobic respiration releases no energy. If it released no energy no organism would ever bother with it, and a sprinter would collapse the moment she ran out of oxygen. It releases much less — the glucose is only partly broken down, so most of the energy stays locked up in the alcohol or the lactic acid.
That last clause is the explanation Cambridge wants when it asks why less energy is released: the glucose molecule is not completely broken down, so the products still contain a great deal of chemical energy.
Two Organisms, Two Different Equations
If a question gives you 2880 kJ per mole for aerobic and 150 kJ per mole for anaerobic in muscle, and asks how many times more energy aerobic respiration releases, the answer is 2880 ÷ 150 = 19.2 times. Note what the question asked for: “how many times more” wants a ratio, and a ratio has no unit. If it asks for the percentage of the energy that is released anaerobically, that is 150 ÷ 2880 × 100 = 5.2 %. Read which way round the division goes before you touch the calculator.
Why a Muscle Ever Bothers
If anaerobic respiration is so wasteful, why does a muscle ever use it? Because during vigorous exercise the muscle cannot get oxygen delivered fast enough. The heart is beating as fast as it can, the breathing is as deep as it can be, and still the demand outruns the supply. At that point the muscle has two choices: stop, or carry on respiring without oxygen. It carries on.
So anaerobic respiration in muscle is not an alternative to aerobic respiration — it happens as well as, on top of, the aerobic respiration that is already going flat out. That is why an exam answer should say the muscles respire “anaerobically as well” rather than “instead”.
The Oxygen Debt
During vigorous exercise lactic acid builds up in the muscles and in the blood. This build-up is what causes the oxygen debt — the extra oxygen the body will have to take in afterwards in order to deal with the lactic acid.
Cambridge then limits the removal of the debt to exactly three things. Learn them as three, and give all three:
1. The heart rate stays fast after the exercise has stopped, so that the lactic acid is transported in the blood from the muscles to the liver.
2. Breathing stays deeper and faster after the exercise has stopped, so that extra oxygen is supplied for the aerobic respiration of the lactic acid.
3. The lactic acid is respired aerobically in the liver.
Three sentences, three marks, and the word that must appear in all of them is liver.
The build-up of lactic acid also explains why a muscle stops working properly during a hard sprint. The acid causes muscle fatigue — the muscle cannot go on contracting efficiently. It is temporary, and it goes as soon as the lactic acid has been removed.
It does not cause the aching you feel two days after unaccustomed exercise. Blood lactate is back to normal within about an hour, as the graph below shows, so it cannot be responsible for something that peaks 48 hours later. If a question offers you that as an option, it is a distractor.
It is also not “a poison” or “a toxin” in the sense of something the body cannot handle. It is an ordinary molecule with a great deal of chemical energy left in it, which is precisely why the liver bothers to respire it rather than excrete it.
At rest the concentration is 1.0 arbitrary units. The peak is 9.2. The increase is 9.2 − 1.0 = 8.2 units. Always quote both numbers as well as the difference — “describe with figures” is very often a mark in itself.
Lactic acid is produced in the muscles, not in the blood. It takes time to diffuse out of the muscle cells and be carried away in the blood, so the blood concentration goes on rising for a few minutes after the muscles have stopped producing any more. The graph is measuring the blood, and the blood is downstream.
The graph shows lactic acid back at the resting level within about an hour. A substance that has gone cannot be causing pain 48 hours later, so the data do not support the claim. Note the shape of the answer: you are not just disagreeing, you are quoting the feature of the graph that makes the claim impossible.
Shortfall = 3.6 − 3.0 = 0.6 dm³ per minute. Over 8 minutes that is 0.6 × 8 = 4.8 dm³ of oxygen she needed and did not get. That is the oxygen debt.
During recovery she is taking in 0.9 but only needs 0.3 to stay alive at rest, so the extra oxygen going into repaying the debt is 0.9 − 0.3 = 0.6 dm³ per minute. Time = debt ÷ extra per minute = 4.8 ÷ 0.6 = 8 minutes.
The trap here is dividing 4.8 by 0.9, which gives 5.3 minutes. That would only be right if her whole intake went into the debt, and it does not — she still has a body to run in the meantime.
Her heart rate stays high, so that the lactic acid is carried in the blood from the muscles to the liver. Her breathing stays deeper and faster, so that extra oxygen continues to be taken in and delivered for the aerobic respiration of the lactic acid in the liver. Each change needs its purpose attached; a bare list of the two changes scores two out of four.
The One Verb That Decides Marks
Not produces. Not makes. Not creates. Not generates. Not gives off.
The reason is not fussiness, it is physics: energy cannot be created. The energy was already in the glucose, stored in its chemical bonds, put there by photosynthesis in a leaf months or years ago. Respiration is the controlled series of reactions that releases that stored energy in a form the cell can use.
Cambridge writes the syllabus objective itself using this verb — “break down nutrient molecules to release energy” — and mark schemes follow. In a topic this small, the examiner has to discriminate somehow, and this is the sentence used to do it. Read back over any answer you write on this topic and check the verb before you move on. It takes three seconds.
The Nine Errors That Cost the Most Marks
Every one of these is a sentence that sounds sensible, gets written by thousands of candidates every year, and earns nothing.
| Never write | Write instead | Why |
|---|---|---|
| “Respiration produces energy” | “Respiration releases energy” | Energy cannot be created. It was already stored in the glucose |
| “Respiration is breathing” | “Respiration is a chemical reaction in cells; breathing is ventilation” | Three separate words — respiration, ventilation, gas exchange — for three separate processes in three separate places |
| “Plants respire at night and photosynthesise in the day” | “Plants respire all the time; in daylight they photosynthesise as well, and faster” | What changes with the light is which process is faster, not which one is happening |
| “Anaerobic respiration produces no energy” | “It releases much less energy per glucose molecule” | The glucose is only partly broken down, so energy remains in the alcohol or lactic acid |
| “Yeast produces lactic acid” | “Yeast produces alcohol and carbon dioxide” | Lactic acid is muscle. Alcohol is yeast. Never both in one organism |
| “Muscles produce lactic acid and carbon dioxide” | “glucose → lactic acid” | Lactic acid is the only product. Adding carbon dioxide imports it from the yeast equation |
| “The lactic acid is broken down in the muscles” | “It is carried in the blood to the liver and respired aerobically there” | The word liver is the mark. This is the most expensive single omission in 12.3 |
| “The enzymes are killed at 60 °C” | “The enzymes are denatured — the shape of the active site changes” | A molecule cannot be killed, because it was never alive |
| “glucose + oxygen → carbon dioxide + water + energy” | “glucose + oxygen → carbon dioxide + water”, then “energy is released” underneath | Energy is not a substance, so it does not belong in a chemical equation |
Reading the Command Word
State / Name — one word or one short phrase. “State the word equation” wants the equation and nothing else.
Describe — say what happens. For the yeast curve this means the shape and the figures: rises to a maximum at about 40 °C, then falls steeply to zero by 60 °C.
Explain — say why. Every mark needs a because. This is where “denatured”, “active site”, “not completely broken down” and “transported to the liver” live.
Compare — every sentence must mention both. “Aerobic respiration needs oxygen” is not a comparison; “aerobic respiration needs oxygen whereas anaerobic respiration does not” is.
Suggest — apply what you know to an organism or experiment you have not met. The mark is for the reasoning, not for having seen it before.
Calculate — show the working, and put the unit on the answer. In this topic the unit is nearly always something “per minute” or “per gram per hour”.
Evaluate — say what the data do support, what they do not, and reach a judgement. An answer that only agrees is half an answer.
How to Attack a Topic 12 Data Question
Because the recall in this topic is so small, almost every mark above the first two is a data mark. Work through them in the same order every time.
1. Read the heading of the dependent variable and its unit. “Volume of carbon dioxide / cm³” and “rate of carbon dioxide production / cm³ per minute” are different quantities and behave differently on a graph.
2. Decide whether you have been given a total or a rate. If it is a total, you will almost certainly have to divide by time before you can compare anything.
3. Describe with figures. Quoting two numbers from the table is very often a mark on its own and costs five seconds.
4. For every temperature graph, split the curve at the optimum. The left side is kinetic energy and collisions. The right side is denaturation. They are two different explanations and a question that gives three marks usually wants both plus the optimum.
5. Check where a substance was made against where it was measured. Lactic acid is made in muscle and measured in blood, which is why the peak comes late. Carbon dioxide is made in a cell and measured in a syringe, which is why there is a lag at the start.
6. Do not claim more than the data allow. Readings ten degrees apart cannot locate an optimum to the nearest degree. One person on one afternoon cannot establish a general rule. Saying so is almost always the last mark on an evaluate question.
Three Scenarios to Test Yourself On
“Produces energy” should be releases energy. The energy was already stored in the glucose; respiration lets the cell get at it.
The muscles do not run out of oxygen and swap over. They go on respiring aerobically at whatever rate the delivered oxygen allows, and respire anaerobically as well to make up the shortfall. Write “in addition”, not “instead”.
Anaerobic respiration in muscle gives lactic acid only. No carbon dioxide. That product has been imported from the yeast equation, and it is the single most common contamination between the two halves of 12.3.
The lactic acid is transported in the blood to the liver and respired aerobically there. Nothing happens to it in the muscle. Because the student put it in the wrong organ, the mark for “liver” is lost even though the rest of the idea was right.
62 at 20 °C sits far above the pattern set by the other five readings — it is between the values for 30 and 40 °C. That makes it an anomalous result, and the word “anomalous” is worth writing.
Repeat the reading at 20 °C. If the repeat agrees with 21, the 62 is discarded and left out of the mean. What you must not do is quietly average it in, because a single wild value drags a mean a long way and hides the real pattern.
Either the water bath was not at 20 °C — perhaps the tube was put in before the bath had cooled, or the thermometer was misread — or the bubbles were miscounted, most likely by counting for less than a full minute and scaling up, or by a different person counting at a different speed.
Both causes are about method, and both suggest their own improvement: allow the bath to stabilise and check it with the thermometer at the start and end, and have the same person count for a full timed minute.
There is a real difference in the means: 8.4 against 6.9, a fall of 1.5 units, or about 18 %. That is consistent with the altitude-trained group respiring anaerobically less during the test, which would happen if they could deliver oxygen to their muscles faster. Quote the figures — that is a mark.
“Removes the oxygen debt” is far too strong. A lactate of 6.9 is still well above a resting value, so a debt was still built up — it was smaller, not absent. The sample is also tiny (eight in each group), we are given means with no spread, and the two groups may have differed in fitness, age or training volume before the study began. This is a correlation, and the study as described did not control for anything else.
The evidence is consistent with altitude training reducing how much anaerobic respiration is needed, and there is a plausible mechanism for it. But the claim as written overstates the result: a smaller debt is not no debt, and eight runners cannot establish a general rule. Notice the judgement is not “the article is wrong” — it is “the article has overstated what these data can show”.
The Night-Before Checklist
The definition of respiration, in three parts. Why the verb is “releases”. All seven uses of energy — including active transport and maintaining body temperature. The yeast experiment: what you measure, the independent, dependent and control variables, why the tube sits in the bath before you start, why there is oil on top. The shape of the temperature curve, the optimum, and two different explanations for the two sides of it. Why the fall is steeper than the rise. The definition of aerobic respiration, with the words “use oxygen” in it. The word equation. The balanced equation, with six of everything except the glucose. Mitochondria as the site. The four ways of detecting respiration, and what soda lime is for in a respirometer. The definition of anaerobic respiration, with “without using oxygen” in it. Much less energy per glucose molecule, and why. The yeast word equation and its balanced version. The muscle word equation, with nothing else on the right-hand side. Lactic acid building up in the muscles and the blood, causing an oxygen debt. The three ways the debt is removed — fast heart rate, deeper and faster breathing, aerobic respiration of lactic acid in the liver. That a lactate peak comes after the exercise. That lactic acid is gone within about an hour.
That list is the entire topic. If you can produce it out loud in four minutes, you are ready.