Aerobic respiration overviewEdexcel A-Level Biology B: Revision notes
Section 1
ATP and why cells respire
All cells need a constant supply of energy for metabolic reactions such as muscle contraction, active transport and synthesis. Cellular respiration oxidises organic molecules such as glucose and transfers the energy released to ATP (adenosine triphosphate).
When ATP is hydrolysed to ADP and inorganic phosphate (Pᵢ), energy is released in a small, useful amount close to where it is needed:
ATP + H₂O → ADP + Pᵢ
ATP is an immediate energy source, not a long-term store; it is continually made and used. Respiration is not 100% efficient, so part of the energy released from glucose is released as heat. The use of ATP also releases heat. This explains why the body temperature of organisms rises with increased activity.
Do not say respiration 'makes energy'. Energy is transferred from glucose to ATP (and heat).
Section 2
The four stages of aerobic respiration
Aerobic respiration has four linked stages. The overall equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
- Glycolysis: in the cytoplasm. Glucose is converted to pyruvate.
- Link reaction: in the mitochondrial matrix. Pyruvate is converted to a 2-carbon compound.
- Krebs cycle: in the mitochondrial matrix.
- Oxidative phosphorylation: on the inner mitochondrial membrane. Most ATP is made here.
Glycolysis does not need oxygen, but the later stages need oxygen, because oxidative phosphorylation depends on it.
Learn the location of each stage: cytoplasm, matrix, matrix, inner membrane.
Section 3
What each stage produces
- Glycolysis: pyruvate, a small net yield of ATP, and reduced NAD (NADH).
- Link reaction: carbon dioxide, NADH, and a 2-carbon compound that enters the Krebs cycle.
- Krebs cycle: carbon dioxide, NADH and other reduced coenzymes, and a small amount of ATP.
- Oxidative phosphorylation: most of the ATP, and water.
Carbon is lost as carbon dioxide (decarboxylation), and hydrogen is removed (dehydrogenation) to reduce coenzymes such as NAD. These reduced coenzymes carry the hydrogen to oxidative phosphorylation.
Section 4
Mitochondria and where it happens
The first stage happens in the cytoplasm, and the other three in the mitochondria. Mitochondria have an outer membrane, a folded inner membrane (cristae) and a fluid matrix.
Cells with a high demand for ATP, such as muscle and sperm cells, contain many mitochondria, so that ATP can be produced at a rate to match its use.
Must know
- Respiration transfers energy from glucose to ATP, and some is released as heat.
- Glycolysis: cytoplasm. Link reaction and Krebs cycle: matrix. Oxidative phosphorylation: inner membrane.
- CO₂ comes from the link reaction and the Krebs cycle.
- Most ATP is made in oxidative phosphorylation.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Aerobic respiration overview
- A hummingbird hovers in front of a flower, beating its wings about 50 times each second. Its flight muscle cells contain very large numbers of mitochondria, and the bird feeds on nectar that is rich in sugar.Explain why the flight muscle cells contain very large numbers of mitochondria.2 marks
- A student places 50 germinating pea seeds in an insulated flask in a dark cupboard and records the temperature each day. A second, identical flask contains boiled peas that cannot respire. After three days the temperature in the first flask has risen from 20.0 °C to 26.5 °C, while the second flask stays at 20.0 °C.Explain why the temperature in the first flask rose, using ideas about the transfer of energy in respiration.2 marks
- A researcher breaks open liver cells and separates the contents into two fractions: the cytoplasm, and the mitochondria. She sets up two tubes in aerated solution. Tube 1 contains only the cytoplasm fraction, glucose, ADP, phosphate and NAD. Tube 2 contains only the mitochondria fraction, pyruvate, ADP, phosphate and NAD.Predict and explain what products will be formed in tube 1 after incubation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).