Growers often adjust the conditions inside a greenhouse to increase the rate of photosynthesis and raise crop yields. A grower investigated how the concentr...
Question 1 Report
Growers often adjust the conditions inside a greenhouse to increase the rate of photosynthesis and raise crop yields. A grower investigated how the concentration of carbon dioxide in the air affects the rate of photosynthesis of tomato plants. The rate was measured at a series of increasing carbon dioxide concentrations, with the other conditions kept constant. The measurements were then repeated at a low light intensity and at a high light intensity, so that the effects of light and carbon dioxide together could be compared. The results are shown on the graph.
(a) Using the graph, state the rate of photosynthesis at 0.04% carbon dioxide in high light. [1]
(b) Describe the effect of increasing the carbon dioxide concentration on the rate in high light. [3]
(c) Explain why, in low light, the rate levels off above 0.20% carbon dioxide. [3]
(d) Calculate the percentage increase in the high-light rate when the carbon dioxide concentration rises from 0.04% to 0.20%. [2]
(e) Explain why raising the carbon dioxide concentration above the normal atmospheric level benefits the grower. [2]
(f) Suggest two other conditions the grower should control in the greenhouse. [2]
Answer Details
(a) Rate at 0.04% CO\(_2\) in high light [1]. Reading up from \(0.04\%\) on the horizontal axis to the high-light line and across to the vertical axis gives a rate of 10 arbitrary units.
(b) Effect of increasing CO\(_2\) in high light [3]. As the carbon dioxide concentration rises the rate of photosynthesis increases; it climbs from about \(10\) units up to about \(38\) units; then it levels off (plateaus) above about \(0.20\%\). Award the three marks for: rate increases; supported with figures; then plateaus. The plateau shows a different factor has become limiting.
(c) Why the low-light line levels off above 0.20% [3]. In low light the light intensity itself is the limiting factor; once carbon dioxide is plentiful, adding still more cannot raise the rate any further because there is not enough light to use it; so the rate is now controlled by the light, not by carbon dioxide. All three linked points are needed.
(d) Percentage increase from 0.04% to 0.20% in high light [2]. The rate rises from \(10\) to \(38\) units. Percentage increase \( = \dfrac{38 - 10}{10} \times 100 = \dfrac{28}{10} \times 100 = 280\% \). One mark for the correct method, one for the answer of \(280\%\).
(e) Why extra CO\(_2\) benefits the grower [2]. Carbon dioxide is a raw material for photosynthesis; raising its concentration above the normal atmospheric level speeds up the rate of photosynthesis, so the plants grow faster and the crop yield increases. One mark for CO\(_2\) being a raw material, one for the higher rate or yield.
(f) Two other conditions to control [2]. Any two of: temperature; light intensity; water supply; mineral or nitrate supply. One mark each.
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