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Inquiry Questions
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Discovery of the light-independent reactions. Blackman measured photosynthesis rates under differing light intensities, CO2 concentrations, and temperatures. As this graph shows, light is the limiting factor at low light intensities, while temperature and CO2 concentration are the limiting factors at higher light intensities.
Blackman found that increasing light intensity above 2000 foot-candles did not lead to any further increase in the rate of photosynthesis. Can you suggest a hypothesis that would explain why?

Answer: At 2000 foot-candles, all of the available chlorophyll molecules are in use, at full bore. Adding more photons can't enlist any more chlorophyll molecules, and so it has no effect on the rate.

Emerson and Arnold's experiment. When photosynthetic saturation is achieved, further increases in intensity cause no increase in output.
Under what experimental conditions would you expect the saturation levels for a given number of chlorophyll molecules to be higher?

Answer: Increasing the temperature or CO2 concentration should increase the output of individual photocenters and so should raise the rate at which saturation is achieved.

FIGURE 10.13
The enhancement effect. The rate of photosynthesis when red and far-red light are provided together is greater than the sum of the rates when each wavelength is provided individually. This result baffled researchers in the 1950's. Today, it provides the key evidence that photosynthesis is carried out by two photochemical systems with slightly different wavelength optima.
What would you conclude if "both lights on" did not change the relative rate of photosynthesis?

Answer: If exposure to both lights did not increase the rate of photosynthesis, you might conclude that the two photosystems do not act in series, or that there is only one photosystem that absorbs light of either wavelength.

Self Test

1). Within chloroplasts, the semiliquid matrix in which the Calvin cycle occurs is called:
    a). stroma.
    b). thylakoids.
    c). grana.
    d). photosystem.

2). Visible light occupies what part of the electromagnetic spectrum?
    a). the entire spectrum
    b). the entire upper half (with longer wavelengths)
    c). a small portion in the middle
    d). the entire lower half (with shorter wavelengths)

3). The colors of light that are most effective for photosynthesis are
    a). red, blue, and violet.
    b). green, yellow, and orange
    c). infrared and ultraviolet
    d). All colors of light are equally effective.

4). A photosystem consists of
    a). a group of chlorophyll molecules, all of which contribute excited electrons to the synthesis of ATP.
    b). a pair of chlorophyll a molecules.
    c). a group of chlorophyll molecules held together by proteins.
    d). a group of chlorophyll molecules that funnels light energy toward a single chlorophyll b molecule.

5). Which photosystem is believed to have evolved first?
    a). photosystem I
    b). photosystem II
    c). cyclic photophosphorylation
    d). All photosystems evolved at the same time, but in different organisms.

6). Oxygen is produced during photosynthesis when
    a). the carbon is removed from carbon dioxide to make carbohydrates.
    b). hydrogen from water is added to carbon dioxide to make carbohydrates.
    c). water molecules are split to provide electrons for photosystem I.
    d). water molecules are split to provide electrons for photosystem II.

7). During photosynthesis, ATP molecules are generated by
    a). the Calvin cycle.
    b). chemiosmosis.
    c). the electron transport chain.
    d). light striking the chlorophyll molecules.

8). The overall purpose of the Calvin cycle is to:
    a). generate molecules of ATP.
    b). generate NADPH.
    c). give off oxygen for animal use.
    d). build organic (carbon) molecules.

9). The final product of the Calvin cycle is
    a). RuBP.
    b). G3P.
    c). glucose.
    d). PGA.

10). C4 photosynthesis is an adaptation to hot, dry conditions in which
    a). CO2 is fixed and stored in the leaf.
    b). water is stored in the stem.
    c). oxygen is stored in the root.
    d). light energy is stored in chloroplasts.

Test Your Visual Understanding

1). Match the following labels with their appropriate location on the figure.
a). b6-f complex
b). H20
c). NADP reductase
e). Photosystem I
f). Photosystem II
g). Water-splitting enzyme

1). a--2, b--6, c--4, d--5, e--3, f--1, g--7

Apply Your Knowledge
1). To reduce six molecules of carbon dioxide to glucose via photosynthesis, how many molecules of NADPH and ATP are required?
Answer: For every three molecules of CO2 that enters the Calvin cycle, one molecule of the three carbon glyceraldehyde 3-phosphate (G3P) is produced. Two molecules of G3P are needed to produce one molecule of glucose. Therefore, the Calvin cycle needs to make a total of 6 turns to produce two molecules of G3P. One turn of the Calvin cycle requires 3 molecules of ATP and 2 molecules of NADPH so for 6 turns:

3 ATP x 6 = 18 ATPs
2 NADPH x 6 = 12 NADPHs

2). What is the advantage of having many pigment molecules in each photosystem but only one reaction center chlorophyll? In other words, why not couple every pigment molecule directly to an electron acceptor?
Answer: If every pigment molecule were coupled to an electron acceptor, there would need to be hundreds of electron acceptors present in the photosystem and hundreds of electron transport chains. Maintaining several electron transport chains would consume a lot of energy. It is more energy economical to have the energy from the sun funneling into one electron acceptor and electron transport chain.

3). The two photosystems, P680 and P700, of cyanobacteria, algae, and plants yield an oxidant capable of cleaving water. How might the subsequent evolution of cellular respiration have been different if the two photosystems had not evolved, and all photosynthetic organisms were restricted to the cyclic photophosphorylation used by the sulfur bacteria?
Answer: The evolution of the photosystems from cyclic photophosphorylation resulted in the formation of the oxygen gas that accumulated in the atmosphere. If the photosystems hadn't evolved, oxygen would not have accumulated in the atmosphere. Also, without the evolution of the photosystems, the Calvin cycle would not have evolved because without the photosystem I, NADPH would not be produced to feed hydrogens into the formation of carbohydrates.

4). In theory, a plant kept in total darkness could still manufacture glucose, if it were supplied with which molecules?
Answer: The molecules that are required to produce glucose are the molecules required for the Calvin cycle, which are:


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