Photosynthesis: Question 10
Syllabus 13.2
A student investigates the rate of photosynthesis of an aquatic plant at a constant temperature of 25°C, using two different concentrations of dissolved carbon dioxide: 0.04% (found in normal, air-saturated water) and 0.4% (a carbon-dioxide-enriched solution). At each of several light intensities, the student measures the rate of photosynthesis as the volume of oxygen produced per minute. The results are shown in the table.
| Light intensity (arbitrary units) | Rate at 0.04% CO₂ (mm³ O₂ min⁻¹) | Rate at 0.4% CO₂ (mm³ O₂ min⁻¹) |
|---|---|---|
| 0 | 0 | 0 |
| 10 | 8 | 8 |
| 20 | 15 | 15 |
| 30 | 15 | 21 |
| 40 | 15 | 26 |
| 50 | 15 | 26 |
(a) Explain why the rate of photosynthesis is the same at both carbon dioxide concentrations when light intensity is 10 units. [2]
(b) Explain why the rate of photosynthesis at 0.04% CO₂ stops increasing once light intensity reaches 20 units, whereas the rate at 0.4% CO₂ continues to increase up to a light intensity of 40 units. [3]
(c) At a light intensity of 50 units and 0.4% CO₂, the rate of photosynthesis has levelled off at 26 mm³ O₂ min⁻¹. Predict, with a reason, the effect on this rate of raising the temperature from 25°C to 45°C, keeping light intensity and carbon dioxide concentration the same. [2]
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Worked solution
Part (a): Why the rates match at low light intensity
At a light intensity of 10 units, both cultures show the same rate of 8 mm³ O₂ min⁻¹. At this low light intensity, the light-dependent stage can only supply ATP and reduced NADP relatively slowly, whatever the carbon dioxide concentration, so light intensity is the limiting factor in both cases. Since carbon dioxide is already present in more than sufficient amounts to match this modest demand at both 0.04% and 0.4%, having ten times more carbon dioxide available in the enriched solution makes no practical difference to the rate. The two curves therefore overlap at low light intensities.
Part (b): Why the two curves plateau at different light intensities
At 0.04% CO₂, the rate rises with light intensity up to 20 units, then stays constant at 15 mm³ O₂ min⁻¹ even as light intensity is increased further to 30, 40 and 50 units. Because the rate no longer responds to more light, light intensity can no longer be the limiting factor beyond this point. As carbon dioxide concentration is the only variable that differs between the two curves, it must be the low (0.04%) carbon dioxide concentration that is now limiting the rate. There simply is not enough dissolved carbon dioxide reaching rubisco to be fixed any faster, however much ATP and reduced NADP the (now under-used) light-dependent stage could supply.
At 0.4% CO₂, ten times as much carbon dioxide is available. This is enough to allow carbon fixation to keep pace with the light-dependent stage for longer, so light intensity remains the limiting factor up to a higher value (the rate keeps rising until light intensity reaches 40 units, reaching a higher plateau of 26 mm³ O₂ min⁻¹. Beyond 40 units, the rate levels off again; since light and (given the tenfold increase) carbon dioxide have both been effectively ruled out as the cause of this second plateau, some other factor) most likely temperature, or the total amount of enzyme present, must now be limiting the rate instead.
Part (c): Predicting the effect of raising temperature to 45°C
At a light intensity of 50 units and 0.4% CO₂, the rate has already plateaued at 26 mm³ O₂ min⁻¹, showing that neither light intensity nor carbon dioxide concentration is limiting the rate any further at this point. The factor holding the rate back here at the fixed temperature of 25°C is most likely temperature itself, acting via its effect on the rate of enzyme-catalysed reactions such as those catalysed by rubisco in the Calvin cycle.
Raising the temperature from 25°C to 45°C is likely to take these enzymes above their optimum temperature. Rather than increasing the rate further, this would be expected to disrupt the hydrogen and ionic bonds maintaining the enzymes’ tertiary structure, denaturing them and altering the shape of their active sites. The predicted effect is therefore a decrease in the rate of photosynthesis, not a further increase, even though light and carbon dioxide remain unchanged and plentiful.
Final answers
- (a) Light intensity limits the rate at both CO₂ concentrations when light is low, so the extra CO₂ in the enriched solution makes no difference, the rates match.
- (b) At 0.04% CO₂, carbon dioxide becomes limiting once light intensity exceeds 20 units, capping the rate; the tenfold higher CO₂ supply in the 0.4% solution keeps carbon dioxide from limiting the rate until a higher light intensity (40 units), giving a higher plateau.
- (c) The plateau at 50 units/0.4% CO₂ is most likely due to temperature; raising temperature to 45°C would likely exceed the optimum for photosynthetic enzymes such as rubisco, causing denaturation and a decrease, not an increase, in rate.