By the end of this lesson
- Explain the effect of increased carbon dioxide on crop yield.
- Explain the effect of increased temperature on crop yield.
- Interpret graphs of photosynthesis rate under different conditions.
BIG QUESTION
If extra carbon dioxide increases yield, why not keep adding more?
Above a certain concentration another factor limits photosynthesis, so extra CO₂ costs money without benefit.
Increased carbon dioxide
Carbon dioxide is a raw material of photosynthesis. In a closed glasshouse on a sunny day, plants can lower the CO₂ concentration so that it becomes limiting factorThe factor in shortest supply, such as light intensity, carbon dioxide concentration or temperature, that stops the rate of a process from increasing..
Adding carbon dioxide raises the rate of photosynthesis, so more glucose is made for growth and yield rises. Above a certain concentration the rate levels off because light or temperature becomes the limiting factor.
Increased temperature
Photosynthesis is controlled by enzymeA protein that acts as a biological catalyst, speeding up a chemical reaction without being used up.. Raising temperature towards the optimum increases kinetic energy and collisions, increasing the rate.
Above the optimum, enzymes begin to denatureTo permanently change the shape of the active site of an enzyme, for example by high temperature or extreme pH, so the substrate no longer fits and the enzyme stops working. and the rate falls. High temperatures also increase water loss by transpiration. Growers therefore keep temperature near, not above, the optimum for the crop.
Interacting factors and economics
Increasing CO₂ is most effective when light and temperature are also high. Graphs with several curves at different CO₂ or light levels show which factor limits the rate at each point.
Growers balance the cost of fuel and CO₂ against the extra yield. Illustrative example: if raising CO₂ adds 15% to yield but costs more than the value of that crop, it is not economic.
CO₂ and temperature in glasshouses
Each factor helps until something else limits.
| Factor raised | Why it can increase yield | Limit |
|---|---|---|
| Carbon dioxide | More raw material for photosynthesis. | Rate plateaus when light or temperature limits. |
| Temperature | Faster enzyme-controlled reactions. | Enzymes denature above optimum; more water loss. |
| Both together | Each removes a limiting factor. | Light, water or cost may become limiting. |
Language that earns precision
- limiting factor
- The factor in shortest supply that restricts the rate of a process.
- optimum temperature
- The temperature at which an enzyme-controlled process is fastest.
- denature
- A permanent change in enzyme shape so that it no longer works.
Common misconceptions
Claim: The hotter the glasshouse, the better the yield.
Correction: Above the optimum, enzymes denature and the rate of photosynthesis falls.
Claim: Adding CO₂ always increases yield.
Correction: Only while CO₂ is the limiting factor.
Claim: Enzymes are killed by heat.
Correction: Enzymes are not alive; they are denatured.
Match the answer to the command word
Where a curve plateaus, state that another factor is now limiting and name it if data allow.
Link temperature to enzymes and CO₂ to raw material for photosynthesis.
Recall, apply, then justify
Explain why increasing the carbon dioxide concentration in a glasshouse can increase crop yield.
Hint
Raw material, rate, glucose.
Suggested marking points
- Carbon dioxide is needed for photosynthesis / is a limiting factor.
- The rate of photosynthesis increases.
- More glucose is made for growth, so yield increases.
Explain why a grower does not heat a glasshouse to 45 °C.
Hint
Enzymes.
Suggested marking points
- This is above the optimum temperature.
- Enzymes for photosynthesis denature, so the rate falls (and yield decreases).
A graph shows photosynthesis rate against light intensity at 0.04% and 0.1% CO₂ (illustrative). Both curves are the same at low light but the 0.1% curve plateaus higher. Explain the pattern.
Planning hint
Which factor limits at low and at high light?
What a strong design includes
- At low light intensity, light is the limiting factor.
- So changing CO₂ makes no difference to the rate.
- As light increases, the rate increases.
- At high light, CO₂ becomes the limiting factor.
- At 0.04% CO₂ the curve plateaus at a lower rate.
- With 0.1% CO₂ the rate can rise further until another factor limits.