Photosynthesis and Plant Nutrition: Question 4
Syllabus 6.1
A hobbyist grows basil in a sealed indoor grow tent fitted with adjustable LED lights and a small canister that can enrich the air inside with extra carbon dioxide. With temperature held constant at the basil's optimum throughout, the hobbyist records the rate of photosynthesis of the basil (in arbitrary units) at several LED light intensities, both with the normal (atmospheric) carbon dioxide level and with the canister topping up the air to a raised carbon dioxide level:
| LED light intensity / units | Rate at normal CO2 level | Rate at raised CO2 level |
|---|---|---|
| 5 | 6 | 6 |
| 15 | 13 | 14 |
| 25 | 17 | 22 |
| 35 | 18 | 28 |
| 45 | 18 | 33 |
(a) Complete the balanced chemical equation for photosynthesis below by writing in the missing formula and the two missing balancing numbers:
___CO₂ + ___H₂O → C₆H₁₂O₆ + ___O₂ [2]
(b) Describe how the rate of photosynthesis at the normal carbon dioxide level changes as the LED light intensity increases from 5 to 45 units. [2]
(c) Explain why raising the carbon dioxide level makes almost no difference to the rate of photosynthesis at a light intensity of 5 units, but makes a large difference at a light intensity of 45 units. [3]
(d) Suggest one factor, other than light intensity, carbon dioxide concentration or temperature, that could still limit the rate of photosynthesis in this basil plant even under otherwise ideal grow-tent conditions. [1]
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Worked solution
Part (a): The balanced chemical equation
Photosynthesis converts 6 molecules of carbon dioxide and 6 molecules of water into 1 molecule of glucose and 6 molecules of oxygen, giving equal numbers of each type of atom on both sides:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Part (b): Describing the rate at normal carbon dioxide level
Reading the “Rate at normal CO2 level” column as light intensity rises:
- from 5 to 35 units, the rate climbs steadily: 6, 13, 17, 18
- from 35 to 45 units, the rate stays at 18. It has levelled off (plateaued)
So the rate of photosynthesis increases with light intensity up to about 35 units, then stops increasing (plateaus) even though the light intensity keeps rising to 45 units.
Part (c): Explaining the effect of raised carbon dioxide at each light intensity
At a light intensity of 5 units, the rate is 6 whether the carbon dioxide level is normal or raised. Since extra carbon dioxide makes no difference here, carbon dioxide cannot be what is holding the rate back. It must be the light intensity that is in short supply and limiting the rate.
At a light intensity of 45 units, there is plenty of light available, so light intensity is no longer the limiting factor. Under the normal carbon dioxide level, the rate has already plateaued at 18, showing that the carbon dioxide concentration has become the limiting factor instead. Once extra carbon dioxide is supplied, this limit is removed, allowing more carbon dioxide to be fixed into glucose and the rate to rise further, up to 33.
This is why whichever factor is in shortest supply (light intensity at low light, carbon dioxide at high light) is the one that limits the rate, and only increasing that particular factor can raise the rate further.
Part (d): Another possible limiting factor
Even with ideal light, carbon dioxide and temperature, the basil’s own biology could still cap the rate. For example, the amount of chlorophyll in its leaves (or the number of chloroplasts) limits how much light energy can be absorbed and used, and the surface area of its leaves limits how quickly carbon dioxide can diffuse in to reach the photosynthesising cells.
Final answers
- (a) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- (b) The rate rises steadily from 6 to 18 as light intensity increases from 5 to 35 units, then plateaus at 18 from 35 to 45 units.
- (c) At 5 units, light intensity is limiting (extra CO2 has no effect); at 45 units, light is no longer limiting but carbon dioxide is (extra CO2 raises the rate further).
- (d) Any sensible factor such as the amount of chlorophyll/chloroplasts or the leaf surface area available for light absorption and gas exchange.