Limiting factors in photosynthesisAQA A-Level Biology: Subtopic test

1. 1
   A grower measures the rate of photosynthesis of tomato plants in a glasshouse at a constant 20 °C, which is close to the optimum for the plants' enzymes, and a carbon dioxide concentration of 0.04%. As the light intensity is increased the rate rises steadily, then levels off at high light intensities.

   (a)

   Which environmental factor is most likely to be limiting the rate of photosynthesis where the graph has levelled off?

   [1 mark]

   • ACarbon dioxide concentration
   • BLight intensity
   • CTemperature
   • DOxygen concentration

   (b)

   At the lowest light intensities, which compound would be expected to accumulate in the leaf cells?

   [1 mark]

   • ARuBP
   • BTriose phosphate
   • CGlucose
   • DGP

   (c)

   Explain why the rate of photosynthesis at a high light intensity increases when the glasshouse carbon dioxide concentration is raised to 0.1%.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The rate of photosynthesis of pondweed was measured at a range of temperatures, with light intensity and carbon dioxide concentration both kept high. The rate roughly doubled between 10 °C and 20 °C, was greatest at about 35 °C, and fell to almost zero at 55 °C.

   (a)

   Which is the best explanation for the increase in rate between 10 °C and 35 °C?

   [1 mark]

   • AEnzymes are progressively denatured
   • BStomata open wider as the temperature rises
   • CChlorophyll absorbs more light at higher temperatures
   • DMolecules have more kinetic energy, so more enzyme-substrate complexes form per second

   (b)

   Which is the best explanation for the fall in rate above 40 °C?

   [1 mark]

   • AEnzymes such as rubisco are denatured as their tertiary structure changes
   • BEnzyme active sites become more complementary to their substrates
   • CThe activation energy of the reactions rises
   • DThe light intensity falls as the temperature rises

   (c)

   Explain why light intensity and carbon dioxide concentration were both kept high throughout the experiment.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A grower compares lettuce yields in three glasshouses of the same size over one growing season. Glasshouse X has no extra heating, lighting or carbon dioxide. Glasshouse Y is heated to 25 °C and enriched with carbon dioxide. Glasshouse Z has the same heating and carbon dioxide enrichment as Y, plus supplementary lighting. The mean yields were 4.0 tonnes (X), 6.2 tonnes (Y) and 7.1 tonnes (Z). The extra running costs compared with X were £1500 for Y and £3800 for Z. Lettuce sells for £1200 per tonne.

   (a)

   Calculate the net financial gain per season of heating and carbon dioxide enrichment (Glasshouse Y) compared with Glasshouse X.

   [3 marks]

   (b)

   Evaluate whether the grower should add supplementary lighting to the heating and carbon dioxide enrichment.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A tomato grower is deciding how to spend money in a glasshouse where light is already strong in summer. A trial measured the rate of photosynthesis as a percentage of the rate under normal conditions (0.04% carbon dioxide, 20 °C, strong light = 100), with the annual cost per square metre. Raising carbon dioxide to 0.10%: rate 130, cost £2. Raising temperature to 30 °C: rate 105, cost £9. Increasing light intensity by a further 50%: rate 102, cost £15. Raising both carbon dioxide to 0.10% and temperature to 30 °C: rate 150, cost £11.

   (a)

   Explain how a grower can use light, temperature and carbon dioxide concentration to increase the rate of photosynthesis in a glasshouse, and why increasing one factor can sometimes have no effect.

   [6 marks]

   (b)

   Evaluate the trial data to advise the grower on how to spend money to increase the rate of photosynthesis.

   [6 marks]

   Total for question 4: 12 marks

End of questions