6.1.6Plant Nutrition
Environmental Factors and the Rate of Photosynthesis — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 7 of 11
Section 1 of 6Factors that change the rate
Rate of photosynthesis
Light intensity
- Light intensity Light provides the energy for photosynthesis, so increasing light intensity increases the rate, until another factor becomes limiting.
Carbon dioxide concentration
- Carbon dioxide concentration Carbon dioxide is a raw material, so a higher concentration increases the rate, until something else limits it.
Temperature
- Temperature Photosynthesis is controlled by enzymes.
- Warming increases the rate because molecules have more kinetic energy and collide more often.
Too hot
- Too hot Above the optimum temperature the enzymes are denatured and the rate falls.
Section 2 of 6Method: counting bubbles from pondweed
1
Place a piece of pondweed such as Elodea or Cabomba in a beaker of water, with the cut end upwards.
2
Set up a lamp at a measured distance from the beaker and allow the plant to adjust for a few minutes.
3
Count the bubbles of oxygen released from the cut stem in one minute.
4
Repeat the count to calculate a mean, then change the independent variable and repeat.
5
For greater accuracy, collect the oxygen in a gas syringe or an inverted tube over a longer time and measure its volume.
Section 3 of 6Varying each factor
| Factor | How it is varied | Control needed |
|---|---|---|
| Light intensity | Change the distance of the lamp from the beaker | Place a glass tank of water between lamp and plant to absorb heat so the temperature does not change |
| Temperature | Use a water bath set to different temperatures | Same lamp distance and same carbon dioxide level |
| Carbon dioxide concentration | Dissolve different amounts of sodium hydrogen carbonate in the water | Same temperature and light |
Section 4 of 6Interpreting the graphs
1Rate rises steeply
- Rate rises steeply At first the factor being increased is limiting, so the rate increases with it.
2Line begins to level
- Line begins to level Another factor becomes limiting.
3Plateau
- Plateau Adding more of the first factor makes no difference;
- the rate is controlled by a different factor, such as temperature or carbon dioxide.
4Temperature curve falls
- Temperature curve falls At high temperature the enzymes denature, so the rate decreases.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Rate of photosynthesis | How quickly a plant makes glucose, measured here by oxygen released per unit time. |
| Limiting factor | The factor in shortest supply, which limits the rate of a process. |
| Light intensity | A measure of how bright the light is; it decreases as distance from the source increases. |
| Sodium hydrogen carbonate | A compound that dissolves in water to provide carbon dioxide for the plant. |
| Aquatic plant | A plant that grows in water, such as Elodea, used because its oxygen bubbles can be seen. |
Section 6 of 6Exam technique
1
Saying that raising temperature always increases photosynthesis; beyond the optimum, enzymes denature and the rate drops.
2
Not controlling temperature when changing light distance, because the lamp also heats the water.
3
Counting bubbles for too short a time or only once, which gives unreliable results.
4
Assuming the graph levels off because the plant has 'run out of light', when another factor has become limiting.
5
Forgetting to explain that bubbles of gas are oxygen, a product of photosynthesis.
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