Lesson focus: aerobic respirationRespiration that uses oxygen to release energy from glucose, making carbon dioxide and water. releases energy from glucose using oxygen and occurs continuously in living cells.
Use the aerobic-respiration equation and explain how respiration supports cellular work without describing energy as a material product.
Build on earlier learning
Plant mineralsPlants build glucose and proteins using photosynthesis and mineral ions. Respiration releases energy from glucose in plant, animal and microbial cells.
Practical
Measuring respiration
- Place germinating seeds in a respirometer with soda lime to absorb carbon dioxide.
- As the seeds use oxygen, the coloured liquid moves along the tube.
- Measure the distance moved in ten minutes; compare with a control experiment using boiled seeds.
Record: Distance moved per minute for each set of seeds.
Safety: Soda lime is corrosive: do not touch it.
You will be successful when you can…
- State word and symbol equations.
- Explain the role of mitochondria.
- Give examples of cellular work.
- Interpret respirometer evidence.
Words that carry the science
- aerobic respiration
- A series of enzyme-controlled reactions that releases energy from substrates using oxygen.
- mitochondrion
- An organelle where most aerobic respiration reactions occur in eukaryotic cells.
- ATP
- A molecule that carries energy released by respiration to the cell reactions that need it.
- substrate
- A reactant acted on in a biological or chemical process.
- respirometer
- Apparatus used to estimate respiration by measuring gas-volume or pressure change.
- metabolism
- The complete set of chemical reactions in an organism or cell.
Separate gas exchange, breathing and cellular respiration
01Respiration is a chemical process in cells
The word equation is glucose + oxygen → carbon dioxide + water. A balanced overall equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. The reactions release energy, some of which is used to make ATPA small molecule that carries energy around inside cells, like a rechargeable battery..
Respiration occurs in plants, animals and microorganisms, day and night. In eukaryotes, glycolysis begins in the cytoplasm and most later aerobic stages occur in mitochondrionA tiny part of a cell where aerobic respiration happens, releasing energy from glucose..
02Released energy supports cellular work
Cells use energy released by respiration for active transport, muscle contraction, biosynthesis, cell division and maintaining temperature gradients in endotherms. Some energy ultimately disperses to the surroundings as heating.
Energy is not “made” by respiration and is not a material released alongside carbon dioxide. Chemical energy stores decrease as transfers occur.
03Respirometers measure a proxy
Germinating seeds consume oxygen. If carbon dioxide is absorbed by soda lime or another appropriate absorbent, total gas volume falls and a marker fluid moves. A control experiment containing inert beads helps identify changes caused by temperature or pressure.
Keep organism mass, temperature and time controlled. Living material must not contact corrosive absorbent. Movement provides evidence of oxygen uptake, but calibration is needed to convert distance into volume.
Aerobic respiration and exercise
Muscles respire faster during exercise. The heart beats faster and you breathe faster and deeper, to bring more oxygen and glucose and remove carbon dioxide.
Controls
Values are illustrative. Energy is released, not made. Try 80% effort, then rest.
Calculate respiration rate
0.84 cm³ of oxygen is taken up by 2.0 g of germinating seeds in 12 minutes. Calculate rate per gram.
- Calculate rate for the sample: 0.84 ÷ 12 = 0.070 cm³ min⁻¹.
- Divide by mass: 0.070 ÷ 2.0.
- Calculate 0.035.
- State the unit: cm³ g⁻¹ min⁻¹.
How to make the reasoning visible: Normalising by mass allows samples of different size to be compared more fairly.
Interpret respirometer evidence
The values are illustrative oxygen uptakes for equal seed masses.
| Temperature / °C | Oxygen uptake / cm³ in 10 min | Rate / cm³ min⁻¹ | Interpretation |
|---|---|---|---|
| 10 | 0.18 | 0.018 | Low measured rate |
| 20 | 0.36 | 0.036 | Rate doubles |
| 30 | 0.61 | 0.061 | Highest measured rate |
| 40 | 0.29 | 0.029 | Rate falls |
| Control at 30 | 0.02 | 0.002 | Small non-biological change |
Why subtract or account for the movement in the control experiment?
Why can the 40 °C result be lower than the 30 °C result?
Check your understanding
Answer from memory, then check against the suggested answer.
State the word equation for aerobic respiration.
Where does most aerobic respiration occur in a eukaryotic cell?
Name one use of energy released by respiration.
Explain why a plant respires in darkness.
Explain why oxygen uptake may not exactly match carbon dioxide output.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A respirometer is used to compare respiration in germinating seeds at two temperatures.
Explain why the coloured liquid in the respirometer moves.
(3)Describe how to make the comparison valid.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., connect apparatus features to the biological quantity they allow you to infer.