Photosynthesis and limiting factors
Photosynthesis is how plants and algae make glucose, using energy transferred from light. Its rate depends on whichever condition is in shortest supply, which is the idea behind limiting factors and greenhouse farming.
Part 1 of 3: Learn it
In short
- carbon dioxide + water → glucose + oxygen, using light absorbed by chlorophyll.
- It takes in energy from light, so it is an endothermic reaction.
- Light intensity, carbon dioxide concentration and temperature can all limit the rate.
Where this is in your specification
Spec points: AQA 4.4.1.1 to 4.4.1.3 and the light intensity required practical (Combined Trilogy 4.4.1), Edexcel Topic 6, OCR B1.4
| Board | Topic: Bioenergetics: photosynthesis and respiration |
|---|---|
| AQA 8461 | 4.4 (8461); 4.4 (8464) |
| Edexcel 1BI0 | Topics 6 and 8 |
| OCR J247 | B1.3 and B1.4 |
| Eduqas C400 | 1.3, 5 |
| Cambridge IGCSE 0610 | 6.1, 6.2, 12.1 to 12.3 |
| National 5 C807 75 | Cell biology: 6 Respiration; Life on Earth: 3 Photosynthesis |
The equation
It happens in the chloroplasts, where chlorophyll absorbs light. Carbon dioxide comes in through the stomata in the leaves; water comes up from the roots in the xylem.
What the plant does with glucose
- Uses it in respiration to release energy.
- Turns it into starch to store it.
- Makes cellulose to build cell walls.
- Makes fats and oils for storage, for example in seeds.
- Combines it with nitrate ions from the soil to make amino acids, then proteins.
Limiting factors
A limiting factor is the condition that is holding the rate back. On a graph of rate against light intensity, the line rises and then levels off: on the rising part light is limiting; on the flat part something else, such as carbon dioxide or temperature, is.
Temperature works through enzymes, so too high a temperature makes the rate fall sharply. Low chlorophyll (for example from a lack of magnesium) also limits the rate.
Light intensity and the practical (Higher for the law)
Place pondweed in water at different distances from a lamp and count the oxygen bubbles per minute, or collect the gas in a measuring cylinder. Light intensity is inversely proportional to the square of the distance: halving the distance makes the light four times as intense.
Keep the temperature the same (a heat shield or a beaker of water between lamp and plant helps) and use the same piece of pondweed each time.
Name the gas plants take in for photosynthesis and the gas they give out.
Show the answer
They take in carbon dioxide and give out oxygen.
Part 2 of 3: See it worked
Worked examples
Example 1
Pondweed releases 2.4 cm³ of oxygen in 5 minutes. Calculate the rate of photosynthesis.
- Rate = volume ÷ time
- 2.4 ÷ 5 = 0.48
Answer: 0.48 cm³ per minute.
Example 2
A lamp is moved from 20 cm to 10 cm away from a plant. By what factor does the light intensity change? (Higher)
- Intensity ∝ 1 ÷ distance²
- The distance is halved, so the intensity changes by 1 ÷ (½)²
- 1 ÷ ¼ = 4
Answer: It becomes 4 times greater.
Common mistakes
- Saying plants get their food from the soil. They make glucose; the soil supplies water and mineral ions.
- Saying light is limiting on the flat part of a rate graph.
- Thinking halving the distance doubles the light intensity. It multiplies it by four.
- Forgetting that plants respire all the time, not only at night.
Give three ways a plant uses the glucose it makes.
Show the answer
Any three of: respiration, starch for storage, cellulose for cell walls, fats and oils, amino acids for proteins.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Name the gas plants take in for photosynthesis and the gas they give out.
They take in carbon dioxide and give out oxygen.
Give three ways a plant uses the glucose it makes.
Any three of: respiration, starch for storage, cellulose for cell walls, fats and oils, amino acids for proteins.
Why might the rate of photosynthesis stop rising even when the light gets brighter?
Another factor, such as carbon dioxide concentration or temperature, has become limiting.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Ces fiches sont en anglais car elles suivent les programmes d'examen britanniques.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses