Revision notes for Edexcel A A Level Biology Aerobic and anaerobic respiration. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Aerobic and anaerobic respiration
What you'll learn
- How cells transfer energy from glucose into ATP.
- The roles of glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation.
- Why oxygen is essential for high ATP yield.
- How anaerobic respiration supports intense exercise and leads to recovery oxygen uptake.
Why cells respire
Cells need a constant supply of ATP: adenosine triphosphate, the immediate energy-transfer molecule used for processes such as muscle contraction, active transport and biosynthesis.
ATP releases energy when it is hydrolysed to ADP and inorganic phosphate, often written as:
ATP+H2O→ADP+Pi+energy\mathrm{ATP} + \mathrm{H_2O} \to \mathrm{ADP} + \mathrm{P}_{i} + \text{energy}ATP+H2O→ADP+Pi+energyA respiratory substrate is an organic molecule broken down in respiration. Glucose is the main one you meet at A-Level, although lipids and proteins can also be respired.
Definition
Respiration
Respiration is the enzyme-controlled process in cells that transfers energy from respiratory substrates into ATP.
Aerobic respiration: the big picture
Aerobic respiration is respiration that uses oxygen. In eukaryotic cells, it starts in the cytoplasm and continues in mitochondria.
The overall equation for aerobic respiration of glucose is:
C6H12O6+6O2→6CO2+6H2O\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6} + 6\mathrm{O}_{2} \to 6\mathrm{CO}_{2} + 6\mathrm{H}_{2}\mathrm{O}C6H12O6+6O2→6CO2+6H2OEnergy is not “made”; it is transferred from glucose into ATP, with some energy released as heat.
Key Idea
Why aerobic respiration is efficient
Aerobic respiration fully oxidises glucose, so it releases much more ATP per glucose molecule than anaerobic respiration.
Useful redox language
A lot of respiration is about moving hydrogen atoms and electrons.
Definition
Oxidation and reduction
Oxidation is loss of electrons or hydrogen. Reduction is gain of electrons or hydrogen. A coenzyme is a small non-protein molecule that helps an enzyme work; in respiration, NAD and FAD carry hydrogen and electrons.
When NAD gains hydrogen, it becomes reduced NAD. When FAD gains hydrogen, it becomes reduced FAD. These reduced coenzymes carry hydrogen/electrons to oxidative phosphorylation.
Common Mistake
Oxygen is not used in every stage
Oxygen is not directly used in glycolysis, the link reaction or the Krebs cycle. Its key role is at the end of oxidative phosphorylation, where it acts as the final electron acceptor.
Stage 1: glycolysis
Glycolysis means “glucose splitting”. It happens in the cytoplasm, the fluid part of the cell outside the nucleus and organelles.
In glycolysis:
- glucose, a 6-carbon molecule, is phosphorylated using ATP
- it is split into two 3-carbon molecules
- these are converted into two molecules of pyruvate
- ATP is made by substrate-level phosphorylation, where phosphate is transferred directly from an intermediate molecule to ADP
- NAD is reduced to reduced NAD
Per glucose molecule, glycolysis gives:
- 2 pyruvate
- net 2 ATP
- 2 reduced NAD
Stage 2: the link reaction
If oxygen is available, pyruvate enters the mitochondrion. The mitochondrial matrix is the fluid inside the inner membrane.
The link reaction converts pyruvate into acetyl coenzyme A, often shortened to acetyl CoA. This reaction involves:
- decarboxylation: removal of carbon dioxide
- dehydrogenation: removal of hydrogen
- reduction of NAD to reduced NAD
Per glucose molecule, the link reaction gives:
- 2 acetyl CoA
- 2 carbon dioxide
- 2 reduced NAD
Stage 3: the Krebs cycle
The Krebs cycle happens in the mitochondrial matrix. Acetyl CoA enters the cycle and combines with a 4-carbon molecule. Through a series of enzyme-controlled reactions, the 4-carbon molecule is regenerated.
Per glucose molecule, the Krebs cycle gives:
- 4 carbon dioxide
- 2 ATP
- 6 reduced NAD
- 2 reduced FAD
Example
Tracking carbon atoms in aerobic respiration
- Start with one glucose molecule, which has 6 carbon atoms.
- Glycolysis splits glucose into two pyruvate molecules. No carbon dioxide is released here, so all 6 carbon atoms are still present.
- In the link reaction, each pyruvate loses one carbon dioxide. Two pyruvate molecules therefore release 2 carbon dioxide molecules.
- The remaining 4 carbon atoms enter the Krebs cycle as two acetyl CoA molecules. The Krebs cycle releases these as 4 carbon dioxide molecules.
- Total carbon dioxide released is 2 from the link reaction plus 4 from the Krebs cycle, giving 6 carbon dioxide molecules per glucose.
Stage 4: oxidative phosphorylation
Oxidative phosphorylation happens on the inner mitochondrial membrane, which is folded into cristae to give a large surface area. The space between the inner and outer membranes is the intermembrane space.
Reduced NAD and reduced FAD donate hydrogen/electrons to the electron transport chain, a series of carrier proteins in the inner mitochondrial membrane.
As electrons move along the chain, energy is used to pump protons into the intermembrane space. This creates an electrochemical gradient, meaning a difference in proton concentration and charge across the membrane.
Protons then flow back into the matrix through ATP synthase, an enzyme that makes ATP from ADP and inorganic phosphate. This movement of protons down their gradient is called chemiosmosis.
Finally, oxygen accepts electrons and protons to form water.
Key Idea
The role of oxygen
Without oxygen, the electron transport chain cannot keep accepting electrons. Reduced NAD and reduced FAD cannot be reoxidised efficiently, so the link reaction and Krebs cycle slow or stop.
Anaerobic respiration in muscle cells
Anaerobic respiration is respiration without oxygen. In mammalian muscle cells, pyruvate is converted into lactate in the cytoplasm.
This does not produce extra ATP directly. Its main purpose is to regenerate oxidised NAD, so glycolysis can continue producing its net 2 ATP per glucose.
Common Mistake
Anaerobic respiration still makes ATP
Do not say anaerobic respiration makes no ATP. It makes a small amount: the net 2 ATP from glycolysis.
Anaerobic respiration is useful during intense exercise because ATP demand can rise faster than oxygen delivery. However, it cannot continue indefinitely. Lactate and associated hydrogen ions can contribute to a fall in pH, which affects enzyme and muscle protein function.
During recovery, lactate can be transported in the blood to the liver, where it may be converted back to glucose, or it can be oxidised by well-oxygenated tissues.
In yeast, anaerobic respiration produces ethanol and carbon dioxide rather than lactate. This is important in brewing and bread-making.
Oxygen deficit and recovery
At the start of intense exercise, your oxygen demand increases immediately, but oxygen uptake takes time to rise. The gap is called an oxygen deficit and is partly covered by anaerobic respiration.
After exercise, oxygen uptake stays above resting level for a while. This is called excess post-exercise oxygen consumption, or EPOC. It helps remove lactate and restore ATP and phosphocreatine stores.
Example
Explaining lactate production during a sprint
- During a sprint, ATP demand in muscle cells rises very rapidly because myosin heads need ATP for repeated contraction cycles.
- Oxygen delivery by ventilation and circulation cannot increase instantly enough to match this demand.
- Because oxygen is limited, oxidative phosphorylation cannot supply all the ATP required.
- Muscle cells therefore rely more on glycolysis, and pyruvate is converted to lactate to regenerate oxidised NAD.
- This allows glycolysis to continue temporarily, but only with a low ATP yield per glucose.
Measuring respiration rate
A respirometer can estimate aerobic respiration rate by measuring oxygen uptake. A carbon dioxide absorbent, such as soda lime, removes carbon dioxide, so any fall in gas volume is mainly due to oxygen being used.
Good practical design includes:
- keeping temperature constant, because enzymes are temperature-sensitive
- using a control tube with inert material of equal volume
- allowing organisms time to acclimatise before taking readings
- repeating measurements and calculating a mean
- expressing rate per unit mass if organisms differ in size
Example
Calculating specific oxygen uptake rate
A sample of organisms has a mass of 0.020 kg. In 300 s, it uses 1.8 × 10⁻⁶ m³ of oxygen. Calculate the specific oxygen uptake rate.
- Use the rate formula, including mass because the question asks for a specific rate:
- Substitute the values with units:
- Calculate the denominator:
- Divide oxygen volume by the denominator:
Exam technique
In the exam
- Link each stage to its location: glycolysis in cytoplasm, link reaction and Krebs cycle in the matrix, oxidative phosphorylation on the inner mitochondrial membrane.
- If asked why oxygen is needed, say it is the final electron acceptor and allows reduced NAD and reduced FAD to be reoxidised.
- For anaerobic respiration, focus on regeneration of oxidised NAD, not just “making lactate”.
- In rate calculations, write the formula, substitute values with units, then give the final answer with units.
Self review
Check yourself
- Why does oxidative phosphorylation stop if oxygen is unavailable?
- What is the purpose of converting pyruvate to lactate in muscle cells?
- Where are ATP, reduced NAD and reduced FAD produced during aerobic respiration?
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