Photosynthesis
Photosynthesis is the endothermic reaction in which plants and algae use energy transferred from light by chlorophyll to convert carbon dioxide and water into glucose and oxygen, following the equation 6CO2 + 6H2O -> C6H12O6 + 6O2. Its rate is controlled by limiting factors, light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll, with whichever factor is in shortest supply limiting the rate.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn both equations and be able to state that the reaction is endothermic because energy is transferred from the surroundings, from light, into the chemical store of the products.
- Learn the four limiting factors, and answer every graph question the same way: describe the part where the line rises (that factor is limiting), then the plateau (that factor is no longer limiting, and something else is), then name a plausible other factor.
- Learn the five uses of the glucose made: for respiration, converted into insoluble starch for storage, used to make cellulose to strengthen cell walls, used to make amino acids for protein synthesis (with nitrate ions taken up from the soil), and used with other nutrients to make lipids for storage in seeds.
- For light intensity investigations, remember that light intensity is inversely proportional to the square of the distance from the lamp, so doubling the distance reduces the intensity to a quarter. Calculate 1 / d squared when a question gives distances rather than intensities.
- For the required practical, use pondweed and a lamp, keep temperature constant with a water bath or heat shield, allow the plant to acclimatise for a minute after each move, and measure the oxygen produced by counting bubbles or, better, collecting the gas in a measuring cylinder or gas syringe.
- For questions about greenhouses, explain the economics as well as the biology: increasing temperature and carbon dioxide concentration raises the rate and so the yield, but each costs money to supply, so a grower increases a factor only while the extra yield is worth more than the extra cost.
Worked example
A lamp is placed 10 cm from pondweed, and then moved to 20 cm. Using the inverse square law, calculate how the light intensity at 20 cm compares with that at 10 cm, and predict the effect on the rate of photosynthesis if light is the limiting factor.
- Write the relationship: light intensity is proportional to 1 / d squared, where d is the distance from the lamp.
- At 10 cm: 1 / 10 squared = 1 / 100 = 0.01 arbitrary units.
- At 20 cm: 1 / 20 squared = 1 / 400 = 0.0025 arbitrary units.
- Compare: 0.0025 / 0.01 = 0.25, so the intensity at 20 cm is one quarter of the intensity at 10 cm.
- Predict: if light intensity is the limiting factor, the rate of photosynthesis falls to about a quarter of its previous value.
- Add the condition: this holds only while light is the limiting factor. If carbon dioxide concentration or temperature is limiting instead, reducing the light will have a smaller effect than predicted.
Practice questions
Try each question, then tap to reveal the answer.
Q1Write the word equation for photosynthesis.Show answer
Answer: Carbon dioxide + water, using energy from light, gives glucose + oxygen.
Q2Why is photosynthesis described as endothermic?Show answer
Answer: Energy is taken in from the surroundings, transferred from light by chlorophyll.
Q3Name four limiting factors of photosynthesis.Show answer
Answer: Light intensity, carbon dioxide concentration, temperature, and the amount of chlorophyll.
Q4Give three uses of the glucose produced in photosynthesis.Show answer
Answer: Any three of: respiration, converted to starch for storage, making cellulose for cell walls, making amino acids (with nitrate ions), making lipids for storage in seeds.
Q5Why is glucose converted into starch for storage?Show answer
Answer: Starch is insoluble, so it does not affect the water potential of the cell and does not cause water to move in by osmosis.
Q6A lamp is moved from 15 cm to 30 cm away. What happens to the light intensity?Show answer
Answer: It falls to one quarter, because intensity is proportional to 1 / distance squared and the distance has doubled.
Q7Why does the rate of photosynthesis fall at very high temperatures?Show answer
Answer: The enzymes controlling the reactions are denatured, so their active sites change shape and the reactions stop.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A graph shows the rate of photosynthesis against light intensity at 20 degrees Celsius. The line rises steeply and then levels off. Explain the shape of the graph.
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Explain why a plant kept in the dark for several days uses up its starch stores.
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A tomato grower is deciding whether to install a paraffin heater in a greenhouse. The heater raises the temperature and releases carbon dioxide. Evaluate this decision.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the photosynthesis worksheet pack - 18 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 19 of GCSE Biology Workbook, the whole course as one free printable PDF.
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