A student measured the rate of photosynthesis of a leaf at five light intensities…
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A student measured the rate of photosynthesis of a leaf at five light intensities. The carbon dioxide stayed at . The rates were , , , and units, at light intensities of , , , and percent of full sunlight. (a) Which factor limits the rate at and percent? (b) Which factor limits it at percent? (c) The student now raises the carbon dioxide to . At which light intensities will the rate change? (d) Which law does this show?
Need a reminder? The key ideaShow
Photorespiration: RuBisCO binds oxygen when carbon dioxide is low and oxygen is high. RuBP and oxygen give one PGA and a 2-carbon compound. It involves the chloroplast, peroxisome and mitochondrion. It makes no sugar, no ATP and no NADPH, and it releases carbon dioxide. C4 plants lack it. Blackman's law of limiting factors: the factor nearest its minimum sets the rate. Light saturates at about 10 percent of full sunlight; carbon dioxide is the major limiting factor; temperature, water also matter.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The rate rises with the light at and percent, so light is limiting there.
Step 2. (b) At percent the rate is flat at units. More light does not help, so some other factor is limiting. Carbon dioxide is the one that was fixed, so carbon dioxide is limiting.
Step 3. (c) Raising the carbon dioxide helps only where it was limiting. The rate will rise at and percent, and also at percent. It will not change at and percent, where light limits.
Step 4. (d) This shows Blackman's law of limiting factors: the factor nearest its minimum sets the rate.
Answer: (a) light; (b) carbon dioxide; (c) at , and percent; (d) Blackman's law of limiting factors.
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