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7.2.2Topic 7 · Run for your Life

Stages of Respiration — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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7.2.2 Stages of Respiration — Edexcel A Level Biology A (SNAB) 9BI0 notes: Outline the four stages of aerobic respiration, where each occurs, and the inputs, outputs and links between them.Aerobic respiration of glucose has four stages.

  • mitochondrion
  • outer membrane
  • intermembrane space
  • inner membrane
  • Exam-style question 1
  • Exam-style question 2

Section 1 of 8The big picture

Aerobic respiration of glucose has four stages. The first is in the cytoplasm; the other three are in the mitochondrion. Knowing where each stage occurs and what goes in and out of it is a common exam requirement.

StageLocationMain inputsMain outputs
GlycolysisCytoplasmGlucose, 2 ATP, 2 NAD, ADP and Pi2 pyruvate, 2 reduced NAD, net 2 ATP
Link reactionMitochondrial matrix2 pyruvate, NAD, coenzyme A2 acetyl CoA, 2 CO2, 2 reduced NAD
Krebs cycleMitochondrial matrix2 acetyl CoA, NAD, FAD, ADP4 CO2, 6 reduced NAD, 2 reduced FAD, 2 ATP
Oxidative phosphorylationInner mitochondrial membrane (cristae)Reduced NAD and FAD, O2, ADP and PiMany ATP, H2O, NAD and FAD regenerated

Section 2 of 8Mitochondrial structure and function

The mitochondrion is a double-membrane organelle, about 1 to 10 µm long. Its structure is closely linked to its function:

Mitochondrial structure and function
Outer membrane
The outer membrane is permeable to small molecules and separates the organelle from the cytoplasm.
Intermembrane space
The intermembrane space is where protons accumulate during the electron transport chain, forming the gradient used to make ATP.
Inner membrane
The inner membrane is folded into cristae, providing a large surface area that holds the electron carriers and ATP synthase.
  • It is impermeable to H+ ions, which allows the gradient to be maintained.
Matrix
The matrix contains the enzymes of the link reaction and the Krebs cycle, mitochondrial DNA and ribosomes (70S) so the organelle can synthesise some of its own proteins.

Cells with high energy demands such as muscle fibres, sperm tail midpieces and liver cells contain large numbers of mitochondria, with highly folded cristae.

Section 3 of 8Following the carbon

One glucose molecule (6C) is split during glycolysis into two pyruvate (3C each).

1
In the link reaction each pyruvate loses one carbon as CO2, forming a 2C acetyl group.
2
In the Krebs cycle the acetyl group enters and its two carbons are released as two molecules of CO2 over the course of the cycle.
3
After two turns, all six carbons of the original glucose have been released as CO2, which diffuses out of the cell.
4
The hydrogen atoms taken from the substrates are carried by NAD and FAD to the electron transport chain,
  • where they are used to synthesise most of the ATP and are finally combined with oxygen to form water.

Section 4 of 8Where the ATP comes from

For one glucose molecule, direct (substrate-level) ATP formation gives a net of 2 ATP in glycolysis and 2 ATP in the Krebs cycle (one per turn, two turns).

Where the ATP comes from
The 10 reduced NAD and 2 reduced FAD formed overall deliver hydrogen to oxidative phosphorylation, which makes the great majority of the ATP.
Using values of about 2.5 ATP per reduced NAD and 1.5 per reduced FAD, the total is about 30 ATP,
  • although textbooks quote figures from 28 to 32 because some energy is spent transporting molecules into the mitochondrion and because protons can leak across the membrane.

Other respiratory substrates also enter the pathway. Fatty acids are broken into 2C units that form acetyl CoA, giving more ATP per gram than carbohydrate because lipids have a higher proportion of hydrogen. Amino acids can be deaminated and the carbon skeletons enter as pyruvate, acetyl CoA or Krebs cycle intermediates.

Section 5 of 8Linking structure, stage and exam use

A typical exam question gives a diagram of a mitochondrion and asks which letter is the site of a given stage; learn it as: matrix for the link reaction and Krebs cycle, cristae (inner membrane) for the electron transport chain, intermembrane space for the proton gradient, and cytoplasm outside the organelle for glycolysis. The 70S ribosomes and circular DNA support the endosymbiotic origin of mitochondria, so you may be asked to compare them with prokaryotic cells.

When describing the flow of energy, trace the carbon, the hydrogen and the phosphate groups separately.

1
Carbon leaves as CO2 (link reaction and Krebs cycle), hydrogen leaves glucose on NAD and FAD and ends up in water,
  • and phosphate groups are added to ADP in glycolysis, the Krebs cycle and oxidative phosphorylation.
2
Candidates who keep these three flows separate rarely lose marks to confusion between stages.

Section 6 of 8Exam-style questions

  1. Describe how the structure of a mitochondrion is adapted for aerobic respiration. [4]4 marks

  2. Complete a count of the carbon dioxide molecules released from one glucose molecule during aerobic respiration and state where they are released. [3]3 marks

  3. Explain why cells in the liver contain many mitochondria. [2]2 marks

Section 7 of 8Key terms

TermMeaning
CristaeFolds of the inner mitochondrial membrane that increase surface area
MatrixFluid interior of the mitochondrion containing the Krebs cycle enzymes
Intermembrane spaceRegion between the two membranes where protons accumulate
DecarboxylationRemoval of carbon dioxide from a substrate
DehydrogenationRemoval of hydrogen from a substrate
Acetyl coenzyme ATwo-carbon compound that carries an acetyl group into the Krebs cycle
CoenzymeNon-protein molecule that helps an enzyme by carrying chemical groups or electrons

Section 8 of 8Exam technique

1
Putting the Krebs cycle in the cytoplasm: it is in the mitochondrial matrix.
2
Saying that oxygen is used in the Krebs cycle: it is used only at the end of the electron transport chain.
3
Saying that glycolysis happens in the mitochondrion: it happens in the cytoplasm.
4
Counting ATP per glucose as exactly 38: current Edexcel figures are about 30 to 32.
5
Forgetting that everything in the Krebs cycle happens twice per glucose.

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