Respiration

• One line to carry into the exam: respiration releases energy from nutrient molecules inside cells — with oxygen it releases relatively more and produces carbon dioxide and water; without oxygen it releases much less per glucose molecule and produces lactic acid in muscle or alcohol and carbon dioxide in yeast.
• If you are a Core candidate, this is the whole topic. Ten questions, eight statements. The Extended check that follows is not yours, and nothing in it is needed to answer anything above.
• The pair to carry into the exam hall: muscle makes lactic acid; yeast makes alcohol and carbon dioxide. Recite them together, always in that order, and the most expensive mistake in this topic becomes impossible.

• Define respiration as the chemical reactions in cells that break down nutrient molecules and release energy, and state that it is enzyme-controlled and happens in all living cells. Core 1.1.1
• Distinguish clearly between breathing (ventilation), gas exchange and respiration, and say where each happens. Core 1.1.1
• State the uses of energy in living organisms: muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and the maintenance of a constant body temperature — and give a biological example of each. Core 12.1.1
• Describe aerobic respiration as the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Core 12.2.1
• State the word equation for aerobic respiration as glucose + oxygen → carbon dioxide + water. Core 12.2.2
• Describe anaerobic respiration as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. Core 12.3.1
• State that anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, and explain why. Core 12.3.2
• State the word equation for anaerobic respiration in yeast as glucose → alcohol + carbon dioxide. Core 12.3.3
• State the word equation for anaerobic respiration in muscles during vigorous exercise as glucose → lactic acid, and never confuse the two sets of products. Core 12.3.4
• Compare aerobic and anaerobic respiration with matched comparative statements. Core 12.3.2
• Investigate and describe the effect of temperature on respiration in yeast, naming the independent, dependent and controlled variables and a valid control experiment. Core 12.1.2
• Describe the shape of the temperature–rate curve, referring to an optimum temperature and to denaturation above it. Core 5.1.5
• Calculate rates, means and percentage changes from respiration data, identify anomalous results, and separate a result from a trend, a conclusion, an explanation, a limitation and an improvement. Practical skill — Papers 5 and 6
• State the balanced chemical equation for aerobic respiration as \(\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}\), and check that it balances. Supplement 12.2.3
• State the balanced chemical equation for anaerobic respiration in yeast as \(\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2}\). Supplement 12.3.5
• State that lactic acid builds up in the muscles and in the blood during vigorous exercise, causing an oxygen debt, and explain as a causal chain why it builds up. Supplement 12.3.6
• Outline how the oxygen debt is removed after exercise: fast heart rate carrying lactic acid from the muscles to the liver, deeper and faster breathing supplying oxygen, and aerobic respiration of the lactic acid in the liver — and read all of this off an oxygen-consumption recovery graph. Supplement 12.3.7
• Explain the temperature–rate curve in terms of kinetic energy, the frequency of effective collisions and enzyme–substrate complexes. Supplement 5.1.8

Why the root hair cell is the example worth learning. Soil water is a very dilute solution of mineral ions, and the ions must be moved into the root hair cell against the concentration gradient. That is active transport, and active transport needs energy released by respiration — which is why root hair cells are packed with mitochondria. The link runs: respiration → energy released → active transport → ion uptake.

• One thing this topic does not ask for. No 0610 statement requires a number for the energy released — not a count of ATP molecules, not a value in kilojoules. The comparison is always the syllabus’s own: aerobic releases much more energy per glucose molecule than anaerobic. Inventing a figure is a way of being wrong on purpose.
• 1. Answering “respiration” with a description of breathing Core 1.1.1 Respiration happens in cells and breaks down nutrient molecules. If your answer mentions the diaphragm, the ribs or air, you are answering a different question. Repair Every definition of respiration must contain three things: chemical reactions, in cells, breaking down nutrient molecules to release energy.
• 2. Putting energy into the equation as if it were a substance Core 12.2.2 Writing “glucose + oxygen → carbon dioxide + water + energy” and then trying to balance the energy is a category error. Atoms balance; energy is released. Repair Write the equation with chemical substances only, and say in words that energy is released.
• 3. Swapping the anaerobic products between muscle and yeast Core 12.3.3 Core 12.3.4 Muscle during vigorous exercise gives lactic acid. Yeast gives alcohol and carbon dioxide. A yeast answer containing lactic acid, or a muscle answer containing alcohol, scores zero for that mark however good the rest of the sentence is. Repair Memorise them as a pair, together, so that recalling one always drags the other into view.
• 4. Confusing a controlled variable with a control experiment Core 12.1.2 Keeping glucose concentration the same in every tube is a controlled variable. Running a tube of boiled yeast is a control experiment. Practical questions ask for one and are frequently given the other. Repair Controlled variable = something held constant. Control = an extra run that tests whether the thing you claim is responsible really is.
• 5. Defining the oxygen debt as lactic acid, or as breathlessness Supplement 12.3.6 The oxygen debt is a quantity of oxygen — the extra oxygen needed after exercise, above the resting requirement. Lactic acid is a substance, and it is what causes the debt. Breathlessness is a sensation, and it is evidence of the debt being repaid. Repair “Lactic acid builds up in the muscles and blood, causing an oxygen debt; the debt is removed by aerobic respiration of that lactic acid in the liver.”
• Trap. “Respiration happens in the lungs.” It happens in the cells of the lungs, exactly as it happens in the cells of your toe, your liver and a dandelion leaf — but the lungs are not where the body respires. They are where gas exchange happens.
• Three ways this equation gets written wrongly. Oxygen on the right (that is photosynthesis); carbon dioxide on the left (that is photosynthesis again); or “+ energy” added to the products and then balanced. All three lose the mark outright.
• Careful with fatigue. It is safe to write that lactic acid builds up in the muscles and blood during vigorous exercise and that it must be removed afterwards. It is not safe to write that lactic acid is the proven cause of every ache, cramp or feeling of tiredness — the evidence for that is far less settled than older textbooks suggest. Say what accumulates and what happens to it.
• A gas leak is a systematic error. It does not scatter the results randomly — it makes every reading too low, in the same direction, so repeating the experiment will not reveal it and taking a mean will not remove it. Check for leaks before you start; that is what “gas-tight” means in a method.
• 1. “Respiration means breathing.” Breathing is the movement of air into and out of the lungs. Respiration is a set of chemical reactions in cells. They are not synonyms, and an organism with no lungs still respires. Write Respiration is the chemical reactions in cells that break down nutrient molecules and release energy.
• 2. “Respiration happens in the lungs.” Gas exchange happens in the lungs. Respiration happens inside cells — including the cells of the lungs, but no more there than anywhere else. Write Respiration takes place inside every living cell — in the cytoplasm, and in the mitochondria of plant and animal cells.
• 3. “Plants do not respire — they photosynthesise.” Plants are made of living cells, and living cells respire. Photosynthesis is an extra process, not a substitute. Write Plant cells respire continuously; those with chlorophyll also photosynthesise when there is suitable light.
• 4. “Plants only respire at night.” Respiration does not switch off in daylight. What changes is the net gas exchange, because photosynthesis is also happening and is usually faster. Write Plants respire day and night; in bright light photosynthesis is faster, so overall the leaf takes in carbon dioxide and releases oxygen.
• 5. “Aerobic respiration produces oxygen.” Oxygen is a reactant, on the left of the arrow. Producing oxygen is photosynthesis. Write glucose + oxygen → carbon dioxide + water.
• 6. “Carbon dioxide is a reactant in aerobic respiration.” Carbon dioxide is a product. Carbon dioxide as a reactant, again, is photosynthesis. Write Carbon dioxide and water are the products; glucose and oxygen are the reactants.
• 7. “Respiration produces energy.” Energy cannot be produced or created. It is already present in the glucose, and respiration makes it available. Write Respiration releases energy from glucose, and that energy is transferred to the cell’s activities.
• 8. “Energy must balance in the equation like the atoms do.” Supplement 12.2.3 A chemical equation balances atoms. Energy is not made of atoms and has no place in the balance. Write the equation with substances only, then state separately that energy is released.
• 9. “Anaerobic respiration releases no energy.” Then it would be pointless. It releases relatively less energy than aerobic respiration, because glucose is only partly broken down. Write Anaerobic respiration releases a relatively small amount of energy from each glucose molecule.
• 10. “Anaerobic means without carbon dioxide.” It means without using oxygen. Yeast respiring anaerobically produces carbon dioxide as one of its two products. Write Anaerobic respiration releases energy from glucose without using oxygen.
• 11. “Muscles produce alcohol when they respire anaerobically.” Alcohol is the yeast product. Muscles during vigorous exercise produce lactic acid. Write glucose → lactic acid (muscles during vigorous exercise).
• 12. “Yeast produces lactic acid.” The mirror image of the previous error, and equally fatal. Write glucose → alcohol + carbon dioxide (yeast).
• 13. “Anaerobic respiration starts when all the oxygen has gone.” It starts when oxygen cannot be delivered fast enough for the rate of aerobic respiration required. Aerobic respiration carries on alongside it. Write When oxygen delivery is insufficient for the rate required, some cells respire anaerobically as well.
• 14. “The oxygen debt is the lactic acid in the muscles.” Supplement 12.3.6 Lactic acid is a substance, and it is what causes the debt. The oxygen debt is a quantity of oxygen. Write Lactic acid builds up in the muscles and blood, causing an oxygen debt; the debt is the extra oxygen needed afterwards, above the resting requirement.
• 15. “The oxygen debt is just being out of breath.” Supplement 12.3.7 Deep, fast breathing is the mechanism by which the extra oxygen is obtained — evidence of the debt being repaid, not its definition. Breathlessness is a sensation. Write Deeper and faster breathing continues to supply oxygen for aerobic respiration of lactic acid.
• 16. “Cold temperatures denature enzymes.” Cold slows enzymes down; it does not change the shape of the active site. Warm them again and they work. Write At low temperature the rate is low, but the enzymes are not denatured. Extended candidates may add: molecules have less kinetic energy, so effective enzyme–substrate collisions are less frequent.
• 17. “The enzymes were killed by the heat.” Enzymes are molecules, not organisms. Cells can be killed; enzymes are denatured. Write Above the optimum the enzymes denature: the active site changes shape and the substrate no longer fits. At high enough temperatures the yeast cells may die.
• 18. “The foam was twice as high, so respiration was twice as fast.” Foam height is not a measure of gas volume, and bubble counts are only approximate because bubbles differ in size. One run at one temperature also proves nothing about a universal optimum. Write Collect the gas in a gas syringe and measure its volume in a fixed time; repeat and take a mean.
• 19. “Anaerobic respiration in yeast is C6H12O6 → C2H5OH + CO2.” Supplement 12.3.5 It does not balance, and the missing piece is the same in both products: one glucose molecule gives two of each. Count the carbon and you find six on the left and three on the right. Write \(\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2}\) — and remember that \(\mathrm{C_2H_5OH}\) has six hydrogen atoms, not five.

• Where the tiering bites in Topic 12. In one specific place. Both routes learn that yeast respiring anaerobically produces alcohol and carbon dioxide; only Extended candidates write it as \(\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2}\). Both routes learn that muscles produce lactic acid; only Extended candidates go on to what happens to that lactic acid afterwards. If you are unsure which route a question is on, look at whether it asks for a symbol equation or for anything after the exercise has stopped.
• Read the tier column before you read anything else. Eight of these twelve statements are Core, and they are not grouped: Core and Supplement alternate three times down the list. A Core candidate who works straight through the chapter without looking at the labels will spend time on two symbol equations and a whole recovery sequence that no Core paper can ask for.
• Command words that change the answer. State wants the fact alone — and eight of this topic’s twelve statements say state. Describe wants what happens, in order, with no causes. Outline — used once, for the removal of the oxygen debt — wants the main steps of a process without the fine detail. Investigate means the method itself is assessable. Compare wants matched statements about both things in the same sentence.
• The oxygen count is the one people get wrong. Glucose already contains six oxygen atoms of its own. Add the twelve from the six oxygen molecules and you have eighteen — which is exactly the twelve in \(\mathrm{6CO_2}\) plus the six in \(\mathrm{6H_2O}\). If your count comes to twelve, you forgot the oxygen inside the glucose.
• The hydrogen count is the one to rehearse. \(\mathrm{C_2H_5OH}\) looks as though it has five hydrogen atoms. It has six — the one in the \(\mathrm{OH}\) group is easy to miss, and missing it makes the equation look unbalanced when it is not.
• Writing a matched comparison. Weak: “Aerobic respiration releases a lot of energy. Anaerobic respiration is used in exercise.” Those two sentences are about different features, so they do not compare anything. Strong: “Aerobic respiration releases a relatively large amount of energy per glucose molecule, whereas anaerobic respiration releases much less.” One feature, both processes, one sentence.
• Every part carries its purpose. Look at how 12.3.7 is written: each lettered part names a change and what it is for. “Heart rate stays fast” earns nothing on its own; “heart rate stays fast to transport lactic acid in the blood from the muscles to the liver” is the statement. Write the purpose every time, and use the word aerobic respiration for what happens to the lactic acid — “broken down” alone is vaguer than the syllabus is.
• One experiment does not establish a universal optimum. These results give the optimum for this strain of yeast, at this glucose concentration and pH, measured at ten-degree intervals. A different yeast, or narrower intervals, would give a different answer. Write “the optimum for this yeast under these conditions was about 40 °C”, not “the optimum temperature for respiration is 40 °C”.
• Kinetic energy explains the rise, not the fall. Molecules above the optimum have more kinetic energy, not less — so a fall explained by “less kinetic energy” is self-contradictory. Above the optimum the cause is the shape of the active site, and nothing else.

• Topic 12 is short — twelve numbered statements. Every route meets it, and its statements fall into three recognisable shapes.
• Describe respiration. State a word equation. Name the products of anaerobic respiration in yeast. These are one- and two-mark items on Papers 1 and 3 (Core) and Papers 2 and 4 (Extended), where the only risk is imprecise wording, so learn the statements verbatim. Extended candidates add the two balanced symbol equations.
• Why does lactic acid build up during a sprint? How is the oxygen debt removed? These are Supplement, so they belong to Papers 2 and 4. They want an ordered causal chain, one linked step per mark. A list of true facts in the wrong order scores badly.
• Yeast results in a table or on a graph: calculate a rate, spot the anomaly, describe the trend, name a controlled variable, suggest an improvement. This appears in the theory papers and in Paper 5 (Practical Test) or Paper 6 (Alternative to Practical), which both routes sit. One statement, 12.1.2, carries all of it, and it is the only statement in the topic whose verb is investigate.
• Command words that change the answer. State wants the fact alone — and eight of this topic’s twelve statements say state. Describe wants what happens, in order, with no causes. Outline — used once, for the removal of the oxygen debt — wants the main steps of a process without the fine detail. Investigate means the method itself is assessable. Compare wants matched statements about both things in the same sentence.
• One thing this topic does not ask for. No 0610 statement requires a number for the energy released — not a count of ATP molecules, not a value in kilojoules. The comparison is always the syllabus’s own: aerobic releases much more energy per glucose molecule than anaerobic. Inventing a figure is a way of being wrong on purpose.