Factors that Affect the Light Reaction of Photosynthesis: Student Sheet

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Purpose

The purpose of this experiment is to investigate if light and intact chloroplasts are necessary for the light reaction to occur.

Background Information

Photosynthesis is a most wonderful process in which plant with chloroplasts have the ability to make sugar from carbon dioxide. Glucose has more energy than carbon dioxide. This is because the glucose molecule has hydrogen.

6CO₂ + 6H₂0 ------> C₆H₁₂0₆ + 120₂

To make glucose chemical energy is needed. This chemical energy comes in the form of ATP and NADPH.

In order for this reaction to occur, the plant cell must regenerate the ATP (put the phosphate back on ADP) and NADPH (put hydrogen on NADP).

This also requires energy but this energy light and chlorophyll.

This is called the light reaction. The purpose of the light reaction is to make the chemical energy needed to be able to make glucose. It occurs in the thylakoid of the chloroplast. The question then arises, is it necessary for intact chloroplast for this reaction to occur or will chlorophyll by itself be enough? Secondly, is it really necessary for light for this reaction to occur?

To investigate this reaction, a dye DPIP is used. DPIP is blue but will turn clear if it is mixed with chloroplasts and the light reaction is occurring.

Materials

• 4 test tubes
• test tube rack
• phosphate buffer
• distilled water
• 5 cc syringe
• dropper
• aluminum foil

Procedure

1. Obtain the above materials.
2. Label the test tubes 1-4.
3. Put the above compound into each of the test tubes. Compare the colors.
4. For test tube, wrap it in foil and make a foil cap, so that no light can get in.
5. In test tube #2 and #3, add three drop of chloroplasts. Be sure to put # 3 back in the aluminum foil. Observe the color and compare it to #1.
6. In test tube #4 add three drop of CHLOROPHYLL not chloroplasts. Observe the color and compare it to #1.
7. Every five minutes compare the color of tests tube to one another and to test tube #1 for fifteen minutes.

Questions

1. Why is ATP and NADPH needed for photosynthesis?

2. Is light needed for photosynthesis? How do you know?

3. Are chloroplast needed or is chlorophyll by itself enough? How do you know?