Respiration

• One line to carry into the exam: respiration releases energy from glucose inside cells — with oxygen it releases relatively more and produces carbon dioxide and water; without oxygen it releases relatively less and produces lactic acid in human muscle or ethanol and carbon dioxide in yeast.
• The pair to carry into the exam hall: muscle makes lactic acid; yeast makes ethanol 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.
• Distinguish clearly between breathing (ventilation), gas exchange and respiration, and say where each happens.
• State the uses of the energy released by respiration: muscle contraction, protein synthesis, cell division, growth, active transport, the passage of electrical impulses along neurones, and maintenance of a constant body temperature — and give a biological example of each.
• Define aerobic respiration and state both the word equation and the balanced symbol equation.
• Check the balanced equation atom by atom and explain why energy does not appear as a balanced term.
• Define anaerobic respiration and explain why it releases relatively less energy per glucose molecule than aerobic respiration.
• State the word equation for anaerobic respiration in human muscle and in yeast, and never confuse the two sets of products.
• Compare aerobic and anaerobic respiration with matched comparative statements.
• Explain that plant cells respire continuously, by day and by night, and that photosynthesis and respiration are separate processes.
• Explain, as a causal chain, why lactic acid accumulates during vigorous exercise.
• Define EPOC (oxygen debt) and describe the recovery sequence after vigorous exercise in the correct order.
• Interpret an oxygen-consumption recovery graph and identify the EPOC region.
• Plan the investigation of the effect of temperature on respiration in yeast, naming the independent, dependent and controlled variables and a valid control experiment.
• Explain the shape of the temperature–rate curve in terms of kinetic energy, effective collisions, an optimum and enzyme denaturation.
• 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.

Why the root hair cell is the favourite example. 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.

Cellular Respiration

Cellular respiration is the enzyme-controlled breakdown of nutrient molecules, principally glucose, inside living cells, releasing energy that the cell transfers to processes such as muscle contraction, protein synthesis, cell division, active transport, nerve impulse conduction and the maintenance of body temperature. It occurs in the cytoplasm and mitochondria, continues day and night in plants as well as animals, and does not require the organism to be moving.

Energy Transfer in Cells

The energy released when glucose is broken down in respiration is transferred to the work a cell must do: contraction of muscle, synthesis of proteins from amino acids, cell division, growth, active transport of substances against a concentration gradient, conduction of electrical impulses along neurones, and the maintenance of a constant body temperature in mammals and birds. Energy is released and transferred, never created or destroyed, so a cell that stops respiring quickly stops doing all of these things.

Balanced Equation for Aerobic Respiration

The balanced symbol equation for aerobic respiration is C6H12O6 + 6O2 produces 6CO2 + 6H2O. It balances with six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms on each side: the glucose molecule supplies six carbon, twelve hydrogen and six oxygen atoms, and the six oxygen molecules supply a further twelve oxygen atoms. Energy is released during the reaction but is not a chemical substance, so it is never included as a term to be balanced.

Lactic Acid

Lactic acid is the product of anaerobic respiration in human muscle cells. It forms when the rate at which the blood delivers oxygen to a muscle is not sufficient for the rate of aerobic respiration required, so some glucose is broken down anaerobically instead. Because the glucose is only partly broken down, the lactic acid still contains chemical energy. It accumulates in the muscles and in the blood during vigorous exercise and is transported in the blood to the liver, where it is broken down using oxygen during recovery.

Fermentation in Yeast

Fermentation is anaerobic respiration in yeast: glucose is broken down without using oxygen to produce ethanol and carbon dioxide, releasing a relatively small amount of energy. Yeast is a single-celled fungus, and because one of its products is a gas, the volume of carbon dioxide released in a fixed time can be measured and used as an indicator of the rate of respiration. Yeast does not produce lactic acid; lactic acid is the product of anaerobic respiration in human muscle.

Anaerobic Respiration

Anaerobic respiration is the chemical reactions in cells that release energy from glucose without using oxygen. Because the glucose is only partly broken down, a relatively small amount of energy is released compared with aerobic respiration of the same amount of glucose. The products depend on the organism: human muscle cells produce lactic acid, while yeast produces ethanol and carbon dioxide. The word anaerobic means without using oxygen; it does not mean without carbon dioxide.

Respiration in Plants

Plant cells respire continuously, by day and by night, in every living cell including roots, stems, leaves and flowers. Respiration and photosynthesis are different processes: photosynthesis requires suitable light and occurs only in cells containing chlorophyll, whereas respiration is enzyme-controlled and does not stop when photosynthesis begins. What changes with light is the net gas exchange of the plant: in darkness only respiration occurs so carbon dioxide is released overall, while in bright light photosynthesis is faster than respiration so oxygen is released overall.

Oxygen Consumption Recovery Graph

A recovery graph plots the rate of oxygen consumption against time before, during and after vigorous exercise. It shows a steady resting rate, a rise to a higher rate during exercise, and, after exercise stops, a rate that stays above the resting value and falls only gradually back toward it. The shaded area between the curve and the resting rate after exercise represents excess post-exercise oxygen consumption, the extra oxygen used in recovery, chiefly for the breakdown of lactic acid in the liver.

EPOC (Oxygen Debt)

EPOC stands for excess post-exercise oxygen consumption and is commonly called oxygen debt at this level. It is the extra oxygen consumed after vigorous exercise has finished, over and above the amount that would have been used at rest. It arises because lactic acid has accumulated during exercise: heart rate remains elevated so that blood transports lactic acid from the muscles to the liver, breathing remains deeper and faster so that extra oxygen enters the blood, and oxygen is then used in the liver in the breakdown of the lactic acid. EPOC is a quantity of oxygen, not a substance and not a sensation.

Rate of Respiration

The rate of respiration is how much respiration occurs per unit time. In the yeast investigation it is measured as the volume of carbon dioxide produced divided by the time taken, giving units such as cubic centimetres per minute. Carbon dioxide is used because it is a gas produced by yeast in anaerobic respiration, so it can be collected in a gas syringe and its volume measured quantitatively; foam height and bubble counts are weaker measures because bubbles differ in volume and foam is not a measure of gas volume.

• 1. Answering “respiration” with a description of breathing Respiration happens in cells and involves glucose. 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, releasing energy from glucose.
• 2. Putting energy into the equation as if it were a substance 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 humans and yeast Human muscle gives lactic acid. Yeast gives ethanol and carbon dioxide. A yeast answer containing lactic acid, or a human answer containing ethanol, 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. Defining oxygen debt as lactic acid, or as breathlessness EPOC is a quantity of oxygen — the extra oxygen consumed after exercise, above the resting rate. Lactic acid is a substance. Breathlessness is a sensation. Repair “The extra oxygen taken in after exercise, used in the liver in the breakdown of lactic acid.”
• 5. Confusing a controlled variable with a control experiment 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.
• 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 accumulates in muscle 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 in the cytoplasm and mitochondria of every living cell.
• 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.” 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. “Humans produce ethanol when they respire anaerobically.” Ethanol is the yeast product. Human muscle produces lactic acid. Write glucose → lactic acid (human muscle).
• 12. “Yeast produces lactic acid.” The mirror image of the previous error, and equally fatal. Write glucose → ethanol + 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. “Oxygen debt is the lactic acid in the muscles.” Lactic acid is a substance. EPOC is a quantity of oxygen. Write EPOC is the extra oxygen consumed after exercise, above the resting requirement, used in the liver in the breakdown of lactic acid.
• 15. “Oxygen debt is just being out of breath.” Deep, fast breathing is the mechanism by which the extra oxygen is obtained — evidence of EPOC, not its definition. Breathlessness is a sensation. Write Breathing remains deeper and faster so that extra oxygen enters the blood.
• 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 molecules have less kinetic energy, so effective enzyme–substrate collisions are less frequent and the rate is low.
• 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.

• Command words that change the answer. State wants the fact alone. Describe wants what happens, in order, with no causes. Explain wants the causes — expect to write “because” or “so that” at least once per mark. 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.
• 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 from each glucose molecule, whereas anaerobic respiration releases a relatively small amount.” One feature, both processes, one sentence.
• Why order matters. A question worth four marks is marked on the links, not the vocabulary. “Heart rate stays high” earns nothing on its own; “heart rate stays high so that the blood can carry lactic acid from the muscles to the liver” earns the mark. Attach a purpose to every step.
• 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”.

• Respiration is short in the syllabus and generous in the papers. It appears in three recognisable shapes.
• Define respiration. State a word equation. Complete a balanced symbol equation. Name the products of anaerobic respiration in yeast. These are one- and two-mark items where the only risk is imprecise wording, so learn the definitions verbatim.
• Why does lactic acid build up during a sprint? Why does breathing stay deep after the race has finished? These 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, explain it with enzymes, name a controlled variable, suggest an improvement. This is where the largest single block of marks in the topic sits.
• Command words that change the answer. State wants the fact alone. Describe wants what happens, in order, with no causes. Explain wants the causes — expect to write “because” or “so that” at least once per mark. Compare wants matched statements about both things in the same sentence.