Bioenergetics
Photosynthesis, respiration, and metabolism.
In this topic
Photosynthesis
Photosynthesis is an endothermic reaction that converts light energy into chemical energy stored in glucose.
Word equation: carbon dioxide + water → glucose + oxygen Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
It takes place in the chloroplasts of plant cells. Chlorophyll absorbs light energy.
Limiting factors — things that can slow down the rate of photosynthesis: - Light intensity - Carbon dioxide concentration - Temperature
Key Points
- Photosynthesis occurs in chloroplasts
- It is an endothermic reaction (takes in energy)
- Glucose is used for: respiration, making cellulose, making amino acids, stored as starch/oils
- Rate increases with light/CO₂ until another factor becomes limiting
Example Questions
2A farmer grows tomatoes in a greenhouse. Suggest two ways they could increase the rate of photosynthesis.
1) Add artificial lighting to increase light intensity. 2) Use a paraffin heater, which releases CO₂ and warmth, increasing CO₂ concentration and temperature.
[2 marks]
2Explain what is meant by a 'limiting factor' in photosynthesis.
A limiting factor is a factor that is in shortest supply and is therefore preventing the rate of photosynthesis from increasing any further. Even if other factors are increased, the rate won't increase until the limiting factor is addressed.
[2 marks]
Respiration
Respiration is an exothermic reaction that releases energy from glucose. It occurs in ALL living cells continuously.
Aerobic respiration (with oxygen): glucose + oxygen → carbon dioxide + water C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O Takes place mainly in the mitochondria.
Anaerobic respiration (without oxygen): In animals: glucose → lactic acid In plants/yeast: glucose → ethanol + carbon dioxide (fermentation)
Anaerobic respiration releases much less energy than aerobic.
Key Points
- Respiration is NOT breathing — it's a chemical reaction in cells
- It happens 24/7 in all living cells
- Energy is used for: muscle contraction, maintaining body temperature, building molecules, active transport
- Oxygen debt builds up during anaerobic respiration and must be 'repaid'
Example Questions
4During intense exercise, muscles switch to anaerobic respiration. Explain why this happens and what the consequence is.
During intense exercise, the body cannot deliver oxygen fast enough to the muscles for aerobic respiration. The muscles switch to anaerobic respiration, which does not require oxygen. This produces lactic acid, which builds up in the muscles and causes fatigue and cramp. An oxygen debt develops that must be repaid after exercise by continued heavy breathing.
[4 marks]