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5.2.2Energy Transfers In and Between Organisms (A Level only)

The Stages of Aerobic Respiration — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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5.2.2 The Stages of Aerobic Respiration — AQA A Level Biology 7402 notes: Give an overview of the four stages of aerobic respiration and show how they link, including where reduced coenzymes, CO2, water and ATP are produced.Aerobic respiration of glucose:C6H12O6 + 6O2 → 6CO2 + 6H…

  • Glycolysis
  • two pyruvate
  • Link reaction
  • decarboxylated and dehydrogenated
  • acetyl coenzyme A
  • Krebs cycle

Section 1 of 8Stage by stage

1
Glycolysis
(cytoplasm): one 6C glucose is phosphorylated using 2 ATP and split into two 3C triose phosphates, which are oxidised to two pyruvate.
  • Net gain2 ATP and 2 reduced NAD.
  • No oxygen is needed.
2
Link reaction
(matrix): pyruvate enters the mitochondrion by active transport, is decarboxylated and dehydrogenated,
  • and combines with coenzyme A to give acetyl coenzyme A.
  • CO2 and reduced NAD are made.
3
Krebs cycle
(matrix): the 2C acetyl group joins a 4C compound,
  • producing 6C citrate,
  • which is decarboxylated and dehydrogenated back to the 4C compound.
  • Products per turn include CO2, reduced NAD, reduced FAD and ATP.
    • (by substrate-level phosphorylation)
4
Oxidative phosphorylation
(inner membrane): the hydrogen from reduced NAD and FAD is used, via the electron transport chain and chemiosmosis, to make most of the ATP.
  • Oxygen is the final electron acceptor and forms water.

Section 2 of 8Totals per glucose

StageReduced NADReduced FADATP (direct)CO2
Glycolysis202 (net)0
Link reaction (x2)2002
Krebs cycle (x2)6224
Total10246

The Krebs cycle and link reaction must be counted twice, as each glucose gives two pyruvate. Each reduced NAD gives about 2.5 ATP and each reduced FAD gives about 1.5 ATP in oxidative phosphorylation.

Section 3 of 8Mitochondrial structure and function

Mitochondria are adapted to the job:

Mitochondrial structure and function
Double membrane
The double membrane separates the contents from the cytoplasm, and the intermembrane space holds the high concentration of protons.
Inner membrane is folded into cristae
The inner membrane is folded into cristae, giving a large surface area for electron carriers and ATP synthase.
Matrix
The matrix contains enzymes for the link reaction and the Krebs cycle, mitochondrial DNA and ribosomes.

Cells with high demand for ATP (muscle, liver, sperm tails) have many mitochondria.

Section 4 of 8Key points to link

CO2 is made only in the link reaction and the Krebs cycle (not glycolysis). Water is made only at the end, from oxygen. Most of the ATP is made in oxidative phosphorylation, so without oxygen the reduced coenzymes cannot be reoxidised and the later stages stop.

Section 5 of 8Mitochondria and respiration in other cells

Be able to work with a diagram of the mitochondrion: outer membrane, inner membrane, cristae, matrix and intermembrane space.

Mitochondria and respiration in other cells
When asked to calculate magnification, remember to use the scale bar and give the unit.
Mitochondria are about 1 to 10 micrometres long,
  • so they can be seen with an electron microscope in detail but are barely visible with a light microscope.

Respiration of other substrates. Fatty acids are broken down in the matrix to acetyl CoA, which enters the Krebs cycle; amino acids are deaminated, and the carbon skeletons enter at the link reaction or Krebs cycle.

Mitochondria and respiration in other cells
Lipids yield more ATP per gram than carbohydrates because they contain more hydrogen atoms per molecule, so more reduced coenzymes are made.
This is why they are used as long-term energy stores.

The respiratory quotient (RQ = CO2 produced / O2 used) is about 1.0 for carbohydrate, 0.7 for lipid and 0.9 for protein, and it is used to find out which substrate is being respired. A value above 1.0 suggests that some anaerobic respiration is occurring.

Compare the stages

  • only glycolysis is common to aerobic and anaerobic respiration, only the Krebs cycle forms reduced FAD, and only oxidative phosphorylation makes large amounts of ATP.
  • Use this to answer questions on what happens when one stage is blocked, for example by a poison such as cyanide, which blocks the final electron carrier so that the whole sequence stops.

In examination questions on the ATP yield, remember that the figures are approximate and that some ATP is used to move pyruvate and reduced NAD across the mitochondrial membranes.

Section 6 of 8Exam-style questions

  1. State the total number of CO2 molecules released when one glucose molecule is respired aerobically, and where they are made. [2]2 marks

  2. Explain why oxygen is needed in aerobic respiration even though it is not used until the last stage. [3]3 marks

  3. Describe how the structure of a mitochondrion allows it to make ATP efficiently. [3]3 marks

Section 7 of 8Key terms

TermMeaning
Aerobic respirationRespiration that uses oxygen as final electron acceptor.
DecarboxylationRemoval of carbon dioxide from a molecule.
PyruvateThe 3-carbon product of glycolysis.
Acetyl coenzyme AThe 2-carbon acetyl group carried by coenzyme A into the Krebs cycle.
CristaeFolds of the inner mitochondrial membrane.
MatrixThe inner fluid of the mitochondrion, site of the link reaction and Krebs cycle.
OxidationLoss of electrons or hydrogen, or gain of oxygen.

Section 8 of 8Exam technique

1
Saying that glycolysis needs oxygen.
2
Forgetting to double the link reaction and Krebs cycle counts for one glucose.
3
Stating that ATP is produced mainly in the Krebs cycle. Most is made in oxidative phosphorylation.
4
Placing oxidative phosphorylation in the matrix.

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