Respiration releases energy from glucose, and the cell uses that energy to make ATP. The aerobic pathway runs in four stages across four locations, ending with the bulk of the ATP made on the inner mitochondrial membrane. When oxygen runs out, only the first stage runs, and the yield collapses.
Read this topic as an analogy.
A coal-fired power station receives lumps of coal from outside. The first yard outside the main building is a breaker yard (glycolysis): each lump of coal is split into two smaller chunks. The yard pays out a few power tokens (a small amount of ATP) and charges up a pair of battery packs (reduced NAD) as it works.
The two chunks are carried inside the main building (mitochondrial matrix), into a sorting hall (link reaction). The sorting hall trims one piece of carbon off each chunk and packages the rest in a wooden crate (coenzyme A) ready for the furnace. The trimmed carbon is vented out as smoke (carbon dioxide). More battery packs are charged at this stage, but no power tokens are paid here.
The main furnace (Krebs cycle) takes each packaged crate and burns through it completely. The carbon in each crate comes out as smoke (two carbon dioxides per turn of the cycle). A handful of power tokens are paid at the furnace, but the real product is more battery packs: many reduced NADs and one reduced FAD per turn. The furnace runs twice for every coal lump (two crates per lump).
The full battery packs are then carried to the generator wall (inner mitochondrial membrane). This is where the bulk of the day's power tokens get made. The wiring on the generator wall is the electron transport chain. Battery charge is taken off the packs and passed through the wiring stage by stage. As it passes, the charge is strong enough to power pumps that lift water up from the floor of the building into a high reservoir behind a dam (protons pumped from the matrix into the intermembrane space, building a proton gradient).
The lifted water itself is not making tokens. What makes the tokens is the water flowing back down through a single narrow channel in the dam, where a waterwheel sits. The waterwheel (ATP synthase) is the only place water can flow back down. As the water flows down through it, the wheel spins a payroll machine that prints power tokens by the thousand. This is chemiosmosis: a controlled flow across an accumulated gradient doing the actual ATP-making work.
At the very end of the wiring there is a cooling air intake (oxygen, the final electron acceptor). Air is pulled in and combined with the spent charge from the wiring to make condensation (water). Without the cooling intake, the wiring backs up: the charge has nowhere to go, the pumps stop, the reservoir slowly drains, and the payroll machine stops too. The whole generator-wall operation needs the cooling intake at the end.
When the air supply fails (oxygen absent), the outside yard keeps working, but only on a smaller scale. The chunks coming out of the yard have nowhere to go (the main building is shut), so the yard dumps the chunks as waste: in an animal plant, the waste is lactate; in a yeast plant, the waste is ethanol and a small puff of carbon dioxide. The trickle of power tokens still paid at the yard is the 2 ATP of anaerobic respiration. Far less than the main generator wall produced, but enough to keep the yard running for a while.
The plant accepts more than coal. Pre-packaged crates (lipids) can be delivered straight to the furnace, bypassing the breaker yard entirely; that is why lipids yield more ATP per gram. Spare structural parts (proteins) only get used when the coal store is empty.
Mapping back to formal vocabulary. Coal is glucose. The breaker yard is glycolysis; the chunks it sends inside are pyruvate. The wooden crates are acetyl coenzyme A; the furnace is the Krebs cycle; the smoke is carbon dioxide. Charged batteries are reduced NAD and reduced FAD. The reservoir behind the dam is the proton gradient across the inner mitochondrial membrane; the waterwheel is ATP synthase; the printed tokens are ATP. The cooling intake is oxygen as the final electron acceptor. Lactate or ethanol are the waste products of anaerobic respiration. Use the formal terms when you write the answer; the power station is for understanding.
Respiration runs in four stages, each in a different cellular location.
Respiration is the catabolic process by which cells release energy from organic substrates, mostly glucose, and use that energy to phosphorylate ADP to ATP. ATP is the universal energy currency of the cell. The aerobic pathway runs in four sequential stages, each in a distinct compartment.
Where each stage runs and what it produces (per glucose).
| Stage | Location | ATP made | Coenzymes reduced |
|---|---|---|---|
| Glycolysis | Cytoplasm | Net 2 ATP | 2 reduced NAD |
| Link reaction | Mitochondrial matrix | 0 ATP | 2 reduced NAD |
| Krebs cycle | Mitochondrial matrix | 2 ATP | 6 reduced NAD, 2 reduced FAD |
| Oxidative phosphorylation | Inner mitochondrial membrane | The great majority | None reduced here (carriers oxidised) |
The earlier stages exist mainly to do two things: release the carbons of glucose as carbon dioxide, and load reduced NAD and reduced FAD with the hydrogens that oxidative phosphorylation will later use. The bulk of ATP made by aerobic respiration comes from the final stage, not the earlier ones. Anaerobic respiration consists of glycolysis alone, yielding only the 2 ATP that glycolysis itself produces.
Write
ATP synthesisedorATP produced. Don't writeenergy is producedormakes energy. AQA rejects energy-as-output language across every year tested; ATP is the product, and "energy" is a vague substitute that scores nothing.
Glycolysis splits glucose into pyruvate and gives a net gain of 2 ATP.
Glycolysis happens in the cytoplasm. It needs no oxygen, so it is the only respiratory stage available when oxygen is absent. The pathway converts one six-carbon glucose into two three-carbon pyruvate molecules through a sequence of enzyme-catalysed reactions.
- Glucose is phosphorylated using ATP, which raises its energy level and traps it inside the cell.
- The phosphorylated six-carbon sugar is split into two molecules of triose phosphate.
- Triose phosphate is oxidised to pyruvate; the hydrogen released in this oxidation reduces NAD to reduced NAD.
- The net gain per glucose is 2 ATP: 4 ATP are made by substrate-level phosphorylation, but 2 ATP are used in the earlier investment phase.
Inputs: 1 glucose, 2 ATP (invested). Outputs: 4 ATP (gross), 2 reduced NAD, 2 pyruvate. Net: 2 ATP, 2 reduced NAD, 2 pyruvate.
The fate of pyruvate depends on oxygen. In aerobic conditions, pyruvate is actively transported from the cytoplasm into the mitochondrial matrix for the link reaction. In anaerobic conditions, pyruvate stays in the cytoplasm and is converted to lactate or ethanol to regenerate NAD. Without that regeneration, glycolysis would halt as the oxidised NAD pool ran out.
Write
net gain of 2 ATP, or4 ATP made and 2 used. Don't write2 ATP producedalone. Thenetqualifier is the mark; without it the ATP point scores zero.
The link reaction and Krebs cycle complete the oxidation of acetate in the matrix.
Pyruvate enters the mitochondrial matrix, where the link reaction joins it to the next stage. The link reaction is two simultaneous events on the same molecule: decarboxylation removes one carbon as carbon dioxide, reducing pyruvate from three carbons to two; oxidation removes hydrogen, which reduces NAD to reduced NAD.
Inputs: 2 pyruvate, 2 NAD, 2 coenzyme A. Outputs: 2 acetyl-CoA, 2 reduced NAD, 2 CO₂. 0 ATP produced directly. The two-carbon acetate group is combined with coenzyme A to form acetyl coenzyme A, the molecule that carries the carbon into the Krebs cycle.
In the Krebs cycle, also in the matrix, the two-carbon acetate from each acetyl-CoA is donated to a four-carbon oxaloacetate already in the matrix, forming a six-carbon citrate. Citrate then undergoes a cycle of oxidation and decarboxylation. By the end, oxaloacetate is regenerated, ready to accept the next acetyl-CoA.
Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP (by substrate-level phosphorylation). The cycle turns twice per glucose, because two acetyl-CoA enter. Per glucose: 4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP.
The function of these two stages is twofold. They release the carbons of glucose as carbon dioxide; this is the CO₂ that aerobic organisms exhale. And they load reduced coenzymes with the hydrogen that oxidative phosphorylation will use next. The reduced NAD and reduced FAD then migrate to the inner mitochondrial membrane.
Write
reduced NAD(orNADH) for the respiration coenzyme. Don't writeNADP,NADPH, orreduced NADP. NADP is the photosynthetic coenzyme; it is not credited in any respiration answer.
Oxidative phosphorylation makes most of the ATP via the electron transport chain and chemiosmosis.
Oxidative phosphorylation is the final and highest-yielding stage of aerobic respiration. It happens on the inner mitochondrial membrane (the cristae), where the electron transport chain proteins and ATP synthase are embedded.
- Reduced NAD and reduced FAD release their hydrogen at the inner membrane. Each hydrogen splits into a proton (released into the matrix) and an electron (entering the chain).
- Electrons pass down the electron transport chain through a series of protein complexes, each with greater affinity for electrons than the previous one.
- The energy released as electrons step down is used to actively pump protons from the matrix across the inner membrane into the intermembrane space, building a proton gradient.
- Protons can return to the matrix only through ATP synthase. As they flow back through the enzyme, ATP synthase phosphorylates ADP to ATP.
- Oxygen accepts the electrons at the end of the chain and combines with protons to form water. Oxygen is the final electron acceptor.
Oxygen is the final electron acceptor. Without it, the chain stalls, no reduced NAD or FAD is reoxidised, and the whole aerobic pathway shuts down.
The proton gradient is a stored form of potential energy known as the proton motive force. Chemiosmosis is the coupling of electron flow to ATP synthesis through that gradient. It produces the great majority of the ATP made in aerobic respiration; the earlier stages exist mainly to feed it with reduced coenzymes.
- Less reduced NAD or reduced FAD is reoxidised at the chain, so less hydrogen is available to feed it.
- Oxygen as the final electron acceptor is no longer needed. The proton gradient collapses, and ATP synthesis by chemiosmosis stops.
Write
oxygen is the final electron acceptorin every electron transport chain answer. And on the Krebs/ETC inhibition or oxygen-absence Explain, the credited chain is exactly two steps. Don't extend into proton-gradient detail, ATP synthase mechanism, or chemiosmosis mechanics; those points do not score on this question type.
Anaerobic respiration regenerates NAD so glycolysis can continue.
When oxygen is absent, the electron transport chain stalls and reduced NAD cannot be reoxidised at the inner membrane. The oxidised NAD pool runs out, and glycolysis would halt because its oxidation steps need oxidised NAD as the hydrogen acceptor. Anaerobic respiration solves this by using pyruvate itself as an alternative hydrogen acceptor, regenerating NAD so glycolysis can keep producing its 2 ATP per glucose.
Pyruvate accepts hydrogen from reduced NAD to form lactate, regenerating NAD. One step, catalysed by lactate dehydrogenase. Lactate accumulates in muscle and is transported later to the liver, where it is oxidised back to pyruvate (lactate loses hydrogen; NAD accepts that hydrogen).
Two steps. First, pyruvate is decarboxylated to ethanal, releasing CO₂. Second, ethanal accepts hydrogen from reduced NAD to form ethanol, regenerating NAD. Ethanol is excreted as a metabolic waste product alongside the carbon dioxide.
When lactate is reconverted to pyruvate in the liver, lactate is
oxidisedand NAD isreduced. Don't reverse the direction. NAD does not "release" anything here, and reduced NAD does not "break down". The redox direction is fixed, and reversing it is a high-impact failure pattern.
The yield is the diagnostic contrast: anaerobic respiration produces only the 2 ATP from glycolysis itself, against the much higher aerobic yield. The substrate is incompletely oxidised; lactate and ethanol still contain energy that aerobic respiration would have released as further ATP. Anaerobic respiration buys time, not output.
Lipids and proteins feed into the respiratory pathway at different points.
Glucose is the standard respiratory substrate, but not the only one. Other organic molecules enter the pathway at different points depending on their chemistry. Under normal nutrition, the cell prioritises carbohydrates first, then lipids, with proteins reserved for starvation.
Disaccharides (sucrose, lactose, maltose) and polysaccharides (starch, glycogen) are first hydrolysed to monosaccharides; the glycosidic bonds between sugar units are broken by the addition of water. The resulting glucose, or close glucose intermediates, then enters glycolysis.
Write
hydrolysedfor the conversion of disaccharides or polysaccharides to monosaccharides. Don't writebroken down. "Broken down" is rejected as imprecise in this specific context; the credited verb is hydrolyse (or hydrolysis).
Triglycerides are hydrolysed to glycerol and three fatty acids. Glycerol is converted to triose phosphate and enters glycolysis. Fatty acids undergo beta-oxidation to two-carbon acetyl units, which combine with coenzyme A to form acetyl-CoA and enter the Krebs cycle directly. Lipids yield more ATP per gram than carbohydrates because they carry more hydrogen per carbon.
Used as a respiratory substrate mainly during starvation, when carbohydrate and lipid stores are gone. Amino acids are first deaminated: the amino group is removed and excreted as urea in mammals. The remaining carbon skeleton enters as pyruvate, acetyl-CoA, or a Krebs cycle intermediate, depending on the amino acid.
Respirometers measure respiration rate by absorbing CO₂ and tracking pressure change.
A respirometer measures respiration rate in small organisms such as germinating seeds, woodlice, or yeast. The apparatus is a sealed tube containing the organism, a CO₂-absorbing chemical (usually potassium hydroxide, KOH), and a manometer (a narrow capillary with coloured liquid) to register pressure changes inside the apparatus. A control tube without the organism, otherwise identical, runs alongside to correct for pressure changes from ambient temperature fluctuation.
- The organisms take up oxygen for aerobic respiration, reducing the volume of oxygen in the sealed gas space.
- The carbon dioxide produced is absorbed by the KOH, removing it from the gas space.
- The net volume of gas inside the apparatus falls, the pressure inside drops below atmospheric, and the manometer liquid moves towards the organism's tube.
Don't write
vacuumornegative pressurefor the pressure change. Writepressure decreaseorthe pressure inside falls below atmospheric. The pressure-consequence step (step 3) is the consistently dropped mark on this question across years; stopping at CO₂ absorption earns only partial credit.
To measure CO₂ production instead of oxygen consumption: (i) remove the KOH so CO₂ is no longer absorbed; (ii) record the distance the liquid moves in this modified run; (iii) subtract this reading from the original (KOH-present) reading to isolate the CO₂ volume. Adding a syringe is explicitly rejected; the modification is by subtraction, not by extra apparatus.
Before readings begin, the apparatus must equilibrate. The sealed system reaches experimental temperature and the internal air pressure stabilises; readings taken too early register thermal expansion, not respiration. Distance moved by the manometer liquid (mm) is converted to volume using V = π r² × d, where r is the radius of the capillary tube (not the diameter), and d is the distance moved.
Halve the diameter before squaring it to get the radius. Substituting the diameter directly into π r² gives a value four times too large; the formula needs the radius. Where a question gives the "bore" of a tube, that is the diameter, not the radius.
State controlled variables specifically: concentration of solution, mass of organisms, species, volume of solution, temperature when temperature is not the independent variable. Variables already named in the question stem cannot score. A controlled variable is one held constant within the main experiment; a control experiment is a parallel run with the organism removed. The two are distinct and must not be confused. For Evaluate stems on training and mitochondrial enzymes, both sides are required: the metabolic chain (For) and the experimental limitations (Against); single-side answers cap at two marks.
Key terms
- ATP
- reduced NAD
- Krebs cycle
- acetyl coenzyme A
- pyruvate
- glucose
- triose phosphate
- oxygen
- oxidation
- anaerobic respiration
- aerobic respiration
- lactate