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5.2Energy Transfers In and Between Organisms (A Level only)

Respiration (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 11 parts Lesson 6 of 30 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Respiration: ATP, Glycolysis, Link Reaction and Krebs Cycle — AQA A LEVEL BIOLOGY 7402 notes: Describe glycolysis, the link reaction and the Krebs cycle and state their products.Respiration is a chemical process inside living cells that releases energy from organic molecules.

  • acetyl CoA
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Decarboxylation
  • Forgetting that the link reaction and Krebs cycle happen twice per glucose

Section 1 of 11Respiration and energy

Respiration is a chemical process inside living cells that releases energy from organic molecules. The chemical energy stored in nutrients such as carbohydrates is transferred to ATP, and energy can then be released easily by ATP hydrolysis. Glucose is the main respiratory substrate. Aerobic respiration is the breakdown of a respiratory substrate to produce ATP using oxygen.

Section 2 of 11Glycolysis

Glycolysis is the first stage of respiration and takes place in the cytoplasm.

1
Oxygen is not required, so it is an anaerobic process that occurs in both aerobic and anaerobic respiration.
2
A glucose molecule (6C) is phosphorylated using 2 ATP,
  • which makes the stable glucose more reactive,
  • then split into two triose phosphate molecules which are oxidised to two pyruvate (3C).
3
Glycolysis produces pyruvate, ATP and reduced NAD (NADH).
4
Two ATP are used and 4 are made (by substrate-linked phosphorylation), so there is a net gain of 2 ATP per glucose, plus 2 reduced NAD and 2 pyruvate.

Section 3 of 11The link reaction

When oxygen is available, pyruvate enters the mitochondria by active transport across the double membrane into the matrix, against a concentration gradient.

Acetyl CoA
In the link reaction pyruvate is oxidised (hydrogen removed, to form reduced NAD) and decarboxylated (CO2 lost),
  • and the resulting acetate combines with coenzyme A to form acetyl CoA.
Two pyruvate are produced per glucose, so the link reaction happens twice per glucose.

Section 4 of 11The Krebs cycle

The Krebs (citric acid) cycle occurs in the matrix and is a series of enzyme-controlled reactions.

1
Acetyl CoA releases a two-carbon acetate group, which reacts with a four-carbon molecule (oxaloacetate) to produce a six-carbon molecule.
2
This is then decarboxylated and dehydrogenated in steps, regenerating oxaloacetate.
3
For each acetyl CoA the cycle produces: 3 reduced NAD, 1 reduced FAD, 1 ATP (by substrate-level phosphorylation) and 2 CO2.
4
Other respiratory substrates may enter the Krebs cycle, such as the breakdown products of lipids and amino acids.
5
You should learn the products, not the names of the 4C and 6C compounds.
StageLocationProducts per glucose
GlycolysisCytoplasm2 pyruvate, net 2 ATP, 2 reduced NAD
Link reactionMatrix2 acetyl CoA, 2 CO2, 2 reduced NAD
Krebs cycleMatrix4 CO2, 6 reduced NAD, 2 reduced FAD, 2 ATP

Section 5 of 11Worked examples

  1. State the net ATP gain from glycolysis and explain why ATP is first used. [3]3 marks

  2. Describe what happens to pyruvate in the link reaction. [3]3 marks

  3. State the products of one turn of the Krebs cycle. [3]3 marks

Section 6 of 11Oxidative phosphorylation

Oxidative phosphorylation is the final stage of aerobic respiration, on the cristae of the inner mitochondrial membrane.

Reduced NAD and reduced FAD from earlier stages are oxidised, releasing hydrogen atoms (protons and electrons).
The electrons pass along a chain of electron carriers in the inner membrane, releasing energy at each step.
The released energy is used to actively transport protons across the inner mitochondrial membrane from the matrix into the intermembrane space, creating a proton gradient.
Chemiosmosis
The protons move down their concentration gradient back into the matrix by facilitated diffusion through proton channels associated with ATP synthase.
  • This movement is chemiosmosis.
ATP synthase catalyses the formation of ATP from ADP and Pi.
At the end of the chain electrons pass to oxygen, the final electron acceptor, which combines with protons to form water.

Oxygen's role as the final electron acceptor allows the electron transport chain to continue functioning.

Section 7 of 11Anaerobic respiration

When oxygen is absent or in short supply, cells may switch to anaerobic respiration because there is no final electron acceptor for the electron transport chain, so oxidative phosphorylation stops producing ATP and reduced NAD and FAD cannot be oxidised. Anaerobic respiration regenerates NAD so glycolysis can continue making a small amount of ATP.

Lactate fermentation
(animals, some bacteria): pyruvate accepts hydrogen from reduced NAD and is reduced to lactate, regenerating NAD.
Ethanol fermentation
(yeast, plants): pyruvate is decarboxylated to ethanal,
  • which accepts hydrogen from reduced NAD to form ethanol, regenerating NAD.

A common misconception is that fermentation produces ATP; it does not. Its purpose is to regenerate NAD, which allows glycolysis to continue producing ATP.

Section 8 of 11Required practical: respiration rate in yeast

Factors affecting respiration in yeast can be investigated using a redox indicator, a substance that changes colour depending on whether it is reduced or oxidised (such as methylene blue, which goes from blue to colourless as it is reduced by hydrogen from respiration).

1
Prepare tubes of yeast suspension and glucose with the redox indicator.
2
Add a set volume of buffer solution to each tube to maintain a constant pH.
3
Place three tubes into each water bath at a range of temperatures, for example 10, 20, 30, 40 and 50 °C.
4
Leave in the baths for at least ten minutes, then time how long it takes for the solution to become colourless.

Plot temperature against time taken for colour change. As temperature increases the rate of respiration should increase, so the time to become colourless decreases until enzymes denature. Alternatives: investigate substrate type (different sugars) or other variables.

Section 9 of 11Worked examples

  1. Explain the role of oxygen in aerobic respiration. [2]2 marks

  2. Explain why lactate fermentation does not produce ATP but allows ATP production to continue. [3]3 marks

  3. Explain how the proton gradient is used to make ATP. [3]3 marks

Section 10 of 11Key terms

TermMeaning
GlycolysisBreakdown of glucose to pyruvate in the cytoplasm
Link reactionConversion of pyruvate to acetyl CoA
Krebs cycleCycle in the mitochondrial matrix releasing CO2 and reduced coenzymes
Acetyl CoACompound that carries acetate into the Krebs cycle
DecarboxylationRemoval of carbon dioxide
OxidationLoss of electrons or hydrogen
TermMeaning
Oxidative phosphorylationATP synthesis using energy from the electron transport chain and oxygen
ChemiosmosisMovement of protons through ATP synthase
Final electron acceptorOxygen at the end of the chain
FermentationAnaerobic regeneration of NAD from reduced NAD
Redox indicatorDye that changes colour when reduced or oxidised
CristaeFolds of the inner mitochondrial membrane

Section 11 of 11Exam technique

Avoid these mistakes
Forgetting that the link reaction and Krebs cycle happen twice per glucose.
Saying glycolysis needs oxygen.
Saying glycolysis makes four ATP net.
Avoid these mistakes
Saying fermentation makes ATP.
Writing that electrons end up in carbon dioxide - oxygen is the final acceptor.
Forgetting that protons are pumped from the matrix into the intermembrane space.

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