Question

In this experiment, you will measure the rate of photosynthesis in isolated chloroplasts using the dye 2,6-dichloroindophenol (DPIP). The transfer of electrons during the light-dependent reaction results in the production of H+. This H+ ion can reduce DPIP, changing its color from blue to clear. The color can be quantitatively measured with a spectrophotometer. However, in this experiment, you will qualitatively analyze your data.

Table 2: Photosynthesis Data Chloroplast Solution Present? Time for Color Test Tube Initial Color Final Color Change 1 Yes Blue Blue Nil 2 Yes Blue Green 20min Yes Blue Blue Nil

1. Write a detailed conclusion for your experiment. How did light affect the time it took for the DPIP solution to change color? How did the presence/absence of chloroplasts affect color change in the DPIP solution? Do the results support your hypothesis? Explain.
2. Explain why the DPIP solution changed color.
3. What were the controls in this experiment?
4. What was the purpose of the 0.5 M sucrose solution in this experiment? What do you think would happen if water or a 3 M sucrose solution was used instead?
5. How does an absence of light affect photosynthesis?
6. How could you modify this experiment to test the effects of different wavelengths of light on the rate of photosynthesis?
7. Maple leaves turn from green to yellow/orange/red in the fall. How do you think this pigment change affects the rate of photosynthesis? Develop a hypothesis and describe the experiment you would perform to test that hypothesis.

Answer 1

1. Hypothesis: As the time changes the chloroplast solution changes the DPIP color from blue to colorless.

As the time changes, DPIP accepts electrons from chloroplast and getting reduced to change the color from blue to clear. In the fist and third tubes, the reaction occurred. Here DPIP replaces NADPH and getting reduced. In the second cuvette, the effect of darkness on the reduction of DPIP is that the reaction occurs much slower. This is because the electrons move slower in the absence of light, and photosynthesis reactions need light to perform the light reactions. The implication is that the amount of sugar produced in photosynthesis depends upon the duration of exposure to light. Thus the result supports the hypothesis.

2. DPIP accepts electrons from chloroplast and getting reduced to change the color from blue to clear. Reduced DPIP is colorless. Here, DPIP acts as an electron acceptor instead of NADPH in photosynthetic light reaction.

3. The control can be everything (chloroplast solution+ distilled water) except the dye, DPIP. This control is used to calibrate the colorimeter to zero.

The positive control contains all the solutions (0.1%DPIP solution + distilled water) except chlorophyll. Positive control is to nullify the interference from temperature, and other errors.

4. 0.5M sucrose solution is used in this experiment to suspend the spinach chloroplasts in a 0.5 M sucrose solution. Because it is isotonic to the spinach leaf cellular fluid. Thus we can keep the cells in working condition.

5. Absence of light does not initiate photosynthesis. If the photons are falling from the light on the chloroplast, the photosyntheis occur.