5a. Why is the design of the photosynthesis apparatus not ideal for comparing irradiance levels of two different types of leaves?
Recording the amount of light actually striking the leaf surface would be the most useful measurement when comparing photosynthesis for different leaves; however, this is not easily accomplished because the light sensor and the leaf would have to be in the same place at the same time. As a way around this problem, our apparatus measures "transmitted" light--the amount of light that passes through the leaf. For a given leaf, do you think that it is reasonable to assume that transmitted light is proportional to irradiance?
If the amount of transmitted light is the same for two different leaves, can we assume that the irradiance levels are also the same? No, since the amount of light passing through the leaves will also depend upon such properties as leaf thickness and amounts of photosynthetic and nonphotosynthetic pigments.
Measuring the amount of light reflecting off the leaf surface may circumvent some of these problems, but would be subject to others. Can you think of reasons why measuring 'reflective light' also also might not yield an accurate comparison of irradiance for different types of leaves?
One way to directly measure irradiance might be to design an apparatus with a light sensor and leaf chamber that can be alternatively rotated into place. Can you think of some disadvantages to this design?
To measure light absorption, it will be necessary to determine how much light is actually being absorbed by the leaf itself. This is a difficult to measure directly, but can be estimated as the difference in 'incident light level'-- the amount of light striking the leaf -- and 'transmitted light' -- the amount of light passing through the leaf. Do you think that this is actually an accurate measure of the amount of light being absorbed by the photosynthetic pigments? Why not?
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