A Sealed Chamber, a Colour Filter and a CO2 Series
A student places a weighed shoot of an aquatic plant in a sealed 500 mL chamber at 25 °C with a dissolved-oxygen electrode. Under equal photon flux, the net oxygen change over 30 minutes is: white light +90 units, red +86, blue +64, green +18. In complete darkness the same shoot gives −12 units. In a second run at saturating white light, the CO2 concentration is varied: 0.04% gives +90, 0.10% gives +124, 0.50% gives +126, 1.00% gives +126. When the temperature is raised to 40 °C at 0.10% CO2, the net value falls to +40. The student writes three conclusions: (i) green light is not absorbed, so photosynthesis is zero in green light; (ii) at 0.04% CO2 the plant cannot photosynthesise; (iii) doubling CO2 from 0.50% to 1.00% should double the rate. The teacher says all three are wrong or incomplete.
1. Calculate gross photosynthesis in white light from the data and explain the difference between the gross and net values.
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In white light the net oxygen change is +90 units. In darkness the shoot uses oxygen at 12 units, which represents respiration. Assuming respiration continues at about the same rate in light, gross photosynthesis equals net photosynthesis plus respiration: 90 + 12 = 102 units.
The net value is what the electrode actually records: oxygen produced by photosynthesis minus oxygen consumed by respiration. The gross value is the total oxygen produced before respiratory consumption is subtracted. This is why a positive net value does not mean respiration has stopped; it means photosynthesis is producing more oxygen than respiration uses. The same logic applies to the green-light data: a net value of +18 corresponds to a gross value of about 30 units.
2. Evaluate conclusions (i) and (ii) using the data. In each case state the limiting factor and the evidence.
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Conclusion (i) is wrong. In green light the net change is +18, which is above the dark value of −12. The gross value is about 30 units, so photosynthesis is occurring, not zero. Green light is poorly absorbed by chlorophyll compared with red and blue, but it is not absorbed at all in this experiment, and accessory pigments also capture some green light. The limiting factor is light quality, specifically the fraction absorbed, not the complete absence of absorption.
Conclusion (ii) is also wrong. At 0.04% CO2 the net value is +90, so photosynthesis is clearly occurring. What the data show is that CO2 is limiting at 0.04%: raising CO2 to 0.10% increases the net value to +124. The plant can photosynthesise at atmospheric CO2 concentration; it simply does so at a lower rate than when CO2 is raised. The evidence is the increase, not a zero rate.
3. Evaluate conclusion (iii) and explain the fall at 40 °C.
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Conclusion (iii) is wrong because the CO2 response has saturated. Between 0.10% and 0.50% there is almost no change, and between 0.50% and 1.00% there is no change at all. CO2 is therefore no longer the limiting factor at these concentrations. Some other factor, such as light intensity, temperature or the capacity of the Calvin cycle enzymes, is limiting. Doubling CO2 cannot double the rate when another factor controls the rate.
At 40 °C the net value falls to +40. High temperature can denature proteins, including RuBisCO and components of the photosystems, and it increases respiration. It also favours photorespiration relative to photosynthesis in C3 plants. The result is a lower net oxygen output even though CO2 is still at 0.10%. Temperature has become the limiting factor, and the fall is not caused by CO2 shortage.