Respiration (A-level only)
AQA
A-Level
Respiration transfers energy from organic molecules to ATP. ATP is synthesised by substrate-level phosphorylation in glycolysis and the Krebs cycle, and by oxidative phosphorylation. In oxidative phosphorylation, electrons from reduced coenzymes pass along the electron transfer chain, releasing energy to actively transport protons. The resulting proton gradient drives ATP synthase to phosphorylate ADP. ATP is the immediate energy source for cells. It is hydrolysed by ATP hydrolase to ADP and Pi, releasing manageable energy for processes such as active transport, muscle contraction and protein synthesis. Glycolysis initially requires ATP to phosphorylate glucose, so respiration both consumes and produces ATP. Phosphorylation makes glucose more reactive and less stable, allowing it to be split more readily; this is accepted as lowering the activation energy of the reaction.
25
Objectives
19
Exam Tips
33
Pitfalls
38
Key Terms
39
Mark Points
Subtopics in this area
Respiration produces ATP.
Glycolysis is the first stage of anaerobic and aerobic respiration. It occurs in the cytoplasm and is an anaerobic process.
Glycolysis involves the following stages: phosphorylation of glucose to glucose phosphate, using ATP production of triose phosphate oxidation of triose phosphate to pyruvate with a net gain of ATP and reduced NAD.
If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD.
The oxidised NAD produced in this way can be used in further glycolysis.
If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport.
Aerobic respiration in such detail as to show that: pyruvate is oxidised to acetate, producing reduced NAD in the process acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle in a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory) other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.
Required practical 9: Investigation into the effect of a named variable on the rate of respiration of cultures of single-celled organisms.
Respiration (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Describe how ATP is synthesised during glycolysis, the Krebs cycle and oxidative phosphorylation.
- Explain why ATP is described as the immediate energy source of a cell.
- Identify cellular processes that require the hydrolysis of ATP, including the initial stages of glycolysis.
- State where glycolysis occurs and explain why it can proceed without oxygen.
- Explain how a cell can continue to make ATP when its oxygen supply fails, naming the stage responsible and where it happens.
- Predict which stages of respiration continue in a cell whose mitochondria cannot function.
- Describe the stages of glycolysis in order, naming glucose phosphate, triose phosphate and pyruvate.
- Explain why glucose must be phosphorylated before it can be broken down.
- Calculate the net ATP yield of glycolysis from the numbers of ATP used and produced.
- Describe the conversion of pyruvate to lactate and to ethanol, naming the specific hydrogen acceptor in each case.
- Explain why anaerobic respiration yields far less ATP per glucose molecule than aerobic respiration.
- Explain the importance of regenerating oxidised NAD for the continuation of glycolysis.
- Explain why the conversion of pyruvate to lactate or to ethanol allows glycolysis to continue.
- Describe the fate of reduced NAD under aerobic and under anaerobic conditions.
- Predict what happens to ATP production in a cell whose supply of oxidised NAD is blocked.
- State how pyruvate crosses the inner mitochondrial membrane.
- Explain why glucose itself does not enter the mitochondrion.
- Suggest why cells with a high rate of respiration contain large numbers of mitochondria.
- Complete a diagram of the link reaction showing pyruvate, carbon dioxide, reduced NAD and acetylcoenzyme A.
- Describe one turn of the Krebs cycle in terms of carbon number, decarboxylation and the coenzymes reduced.
- Explain how the oxidation of reduced NAD leads to ATP synthesis via chemiosmosis and the formation of water.
- State where the breakdown products of lipids and of amino acids enter aerobic respiration.
- Describe how to measure the rate of respiration in a yeast culture using gas volume or a redox indicator.
- Explain the purpose of potassium hydroxide and of a boiled yeast control in a respirometer investigation.
- Calculate a rate of respiration from a volume of gas collected over a measured time.
Marking Points
Key points examiners look for in your answers
- State that ATP is produced via substrate-level phosphorylation in glycolysis and the Krebs cycle.
- Explain that oxidative phosphorylation produces ATP using energy from the electron transfer chain and a proton gradient driving ATP synthase.
- Identify that ATP is hydrolysed to ADP and inorganic phosphate by ATP hydrolase to release energy.
- Provide a named use of ATP, such as active transport, muscle contraction, DNA replication or protein synthesis.
- Note that glycolysis requires an initial investment of ATP to phosphorylate glucose, making glucose more reactive and less stable, which is accepted as lowering the activation energy of the reaction.
- one mark for locating glycolysis in the cytoplasm
- one mark for stating that glycolysis does not require oxygen, so it is an anaerobic process
- one mark for identifying it as the first stage of both aerobic and anaerobic respiration
- one mark for explaining that a cell short of oxygen can still make ATP because glycolysis continues
- Stating that glucose is phosphorylated to glucose phosphate using ATP to make it more reactive.
- Identifying that glucose phosphate is split into two molecules of triose phosphate.
- Describing the oxidation of triose phosphate to pyruvate.
- Explaining that NAD accepts hydrogen to form reduced NAD during the oxidation stage.
- Calculating a net gain of two ATP per glucose molecule, as four are produced and two are used.
- Naming the ATP synthesis at this stage as substrate-level phosphorylation.
- Stating that in animals, pyruvate acts as the hydrogen acceptor from reduced NAD to form lactate.
- Stating that in plants and yeast, pyruvate is decarboxylated to form ethanal and carbon dioxide.
- Identifying that ethanal acts as the hydrogen acceptor from reduced NAD to form ethanol.
- Explaining that these reactions regenerate oxidised NAD, allowing glycolysis to continue.
- Stating that anaerobic respiration produces no further ATP beyond the net two molecules from glycolysis.
- one mark for stating that NAD is regenerated in its oxidised form when reduced NAD gives up its hydrogen
- one mark for the oxidised NAD accepting hydrogen from triose phosphate in further glycolysis
- one mark for explaining that glycolysis, and so ATP production, would stop if NAD were not regenerated
- one mark for identifying the small size of the cell's NAD pool as the reason recycling is necessary
- Pyruvate is transported into the mitochondrial matrix by active transport.
- A specific carrier protein in the inner mitochondrial membrane is required for pyruvate transport.
- Pyruvate, not glucose, is the molecule that enters the mitochondrion after glycolysis.
- Pyruvate is used in the link reaction within the mitochondrial matrix, linking glycolysis to the Krebs cycle.
- one mark for pyruvate being oxidised to acetate, producing reduced NAD and releasing carbon dioxide
- one mark for the Krebs cycle generating reduced coenzymes, losing carbon dioxide and producing ATP by substrate-level phosphorylation
- one mark for protons diffusing across the inner membrane through ATP synthase to synthesise ATP (chemiosmosis)
- one mark for oxygen acting as the final electron acceptor to form water
- one mark for stating that breakdown products of lipids and amino acids can enter the Krebs cycle
- One mark for a valid measure of rate, such as volume of gas produced per unit time or the reciprocal of the time taken to decolourise the indicator.
- One mark for naming controlled variables, including yeast concentration, substrate volume, pH and temperature where it is not the independent variable.
- One mark for using a water bath and allowing the apparatus to equilibrate before readings are taken.
- One mark for a control such as boiled or dead yeast, to show the change is caused by living cells.
- One mark for repeating readings and calculating a mean.
- One mark for stating that in a respirometer, carbon dioxide is absorbed by potassium hydroxide or soda lime so that liquid movement measures oxygen uptake alone.
Examiner Tips
Expert advice for maximising your marks
- 💡Always name the specific process ATP powers (e.g., active transport or muscle contraction) rather than just stating it is used for energy.
- 💡Remember to distinguish between substrate-level phosphorylation (glycolysis and Krebs cycle) and oxidative phosphorylation when describing ATP synthesis.
- 💡Give both facts when asked where and how glycolysis occurs: in the cytoplasm, and with no oxygen needed.
- 💡If a question describes a cell with damaged mitochondria, glycolysis is the stage that still runs.
- 💡Use the words 'net gain' when quoting ATP from glycolysis, because ATP is spent before any is made.
- 💡Track the carbons: six in glucose, then two lots of three in triose phosphate, then two pyruvate.
- 💡Always use the word 'net' when quoting the ATP yield, and give both the four made and the two used when the question is worth two marks or more.
- 💡Always specify the hydrogen acceptor accurately: pyruvate for animals, ethanal for plants and yeast.
- 💡When explaining why anaerobic respiration is necessary, focus on the regeneration of oxidised NAD to keep glycolysis running, rather than just the end products.
- 💡When asked why lactate or ethanol is made at all, answer in terms of regenerating NAD, not of disposing of pyruvate.
- 💡Use the words oxidised and reduced precisely, because examiners look for the correct direction of hydrogen transfer.
- 💡Finish the chain of reasoning: NAD regenerated, glycolysis continues, ATP still produced.
- 💡Always specify active transport and mention a carrier protein; 'moves into the mitochondria' is insufficient.
- 💡Check the molecule before writing: glycolysis ends at pyruvate, so pyruvate is what crosses the inner membrane.
- 💡Count the carbons through every step, because that catches most Krebs cycle errors before you write them down.
- 💡Clearly distinguish between substrate-level phosphorylation in the Krebs cycle and oxidative phosphorylation via ATP synthase.
- 💡Say how you will calculate the rate before you describe the method, because the units tell the examiner what you measured.
- 💡Name the control and say what it controls for; a boiled yeast tube is the usual answer.
- 💡If asked to improve the investigation, offer more repeats, narrower intervals between values of the independent variable, and a colorimeter in place of judging colour by eye.
Common Mistakes
Pitfalls to avoid in your exam answers
- Stating respiration 'produces energy' instead of transferring energy to ATP; energy cannot be created or destroyed.
- Claiming respiration does not use ATP, forgetting that glycolysis requires an initial investment of ATP to phosphorylate glucose.
- Treating ATP as a long-term energy store rather than an immediate energy source that is continuously synthesised.
- Naming 'energy' as the use of ATP rather than specifying a cellular process like active transport.
- Rejecting 'lowers activation energy' as a description of glucose phosphorylation; this is accepted as equivalent to making glucose more reactive and less stable.
- placing glycolysis in the mitochondrial matrix, which is where the link reaction and the Krebs cycle occur
- describing glycolysis as aerobic simply because it happens when oxygen is present
- claiming glycolysis produces no ATP because ATP is used at the start, when there is a net gain
- forgetting that prokaryotes respire despite having no mitochondria
- treating anaerobic respiration as a wholly different process rather than the same first stage with a different ending
- Quoting four ATP as the yield of glycolysis without subtracting the two used in phosphorylation; correction: always state the net gain is two ATP.
- Writing that NAD is oxidised in glycolysis; correction: NAD is reduced by accepting hydrogen.
- Saying glucose is split straight into two pyruvate molecules; correction: include the intermediate triose phosphate stage.
- Using reduced NADP instead of reduced NAD; correction: reduced NADP is used in photosynthesis, whereas reduced NAD is used in respiration.
- Stating that pyruvate is the hydrogen acceptor in both pathways; correction: pyruvate is the acceptor in lactate fermentation, but ethanal is the acceptor in ethanol fermentation.
- Conflating lactate with lactic acid; correction: lactate is the ionised form (C₃H₅O₃⁻), while lactic acid is the protonated form (C₃H₆O₃). Also, avoid stating that lactate directly causes muscle fatigue as an absolute fact, as this is a contested theoretical model.
- Stating that anaerobic respiration produces ATP directly at the pyruvate conversion step; correction: the only ATP produced anaerobically is the net gain from glycolysis.
- Saying NAD is reduced during fermentation; correction: reduced NAD is being oxidised to regenerate NAD.
- saying NAD is used up in respiration rather than continuously recycled
- confusing the direction of the change, so that reduced NAD is described as being reduced again
- crediting the anaerobic pathway with producing ATP, when its purpose is to regenerate NAD
- assuming a fresh supply of NAD is synthesised for every glucose molecule respired
- Writing that glucose enters the mitochondrion. Correction: glucose is broken down to pyruvate in the cytoplasm; pyruvate enters the mitochondrion.
- Stating that pyruvate simply diffuses into the matrix. Correction: pyruvate crosses the inner mitochondrial membrane by active transport via a carrier protein.
- Claiming pyruvate crosses both mitochondrial membranes by active transport. Correction: the outer membrane is highly permeable; only the inner membrane requires active transport.
- writing reduced NADP instead of reduced NAD in a respiration answer, which examiners reject
- omitting water as the product formed when oxygen accepts electrons and protons at the end of the chain
- describing the Krebs cycle ATP as coming from oxidative phosphorylation instead of substrate-level phosphorylation
- Taking a single reading with no repeats, so that no mean can be calculated; correction: repeat each condition and calculate a mean.
- Starting the timer before the apparatus has reached the temperature of the water bath; correction: allow equilibration in the water bath before taking readings.
- Leaving potassium hydroxide out of a respirometer, so that carbon dioxide released masks the oxygen taken up; correction: include potassium hydroxide or soda lime to absorb carbon dioxide so liquid movement measures oxygen uptake alone.
- Judging a colour change by eye with no colour standard, then treating the end point as precise; correction: use a colorimeter or a colour standard to make the end point objective.
- Treating the decolourisation of methylene blue as proof of aerobic respiration specifically, when it shows dehydrogenase activity; correction: describe it as a measure of dehydrogenase activity, not proof of aerobic respiration.