How to Show Light Functions in Photosynthesis
Plants use sunlight, water and carbon dioxide to make their own food in the process known as photosynthesis. You can demonstrate the necessity of light with dyes that change color in response to the certain photosynthetic reactions. During photosynthesis, light excites electrons contained in the chloroplasts within plant cells. These electrons latch on to an oxidized form of a molecule called NADP+. You can replace NADP+ with a blue-colored, electron-accepting dye called DPIP. When you mix plant chloroplasts and DPIP and expose them to light, the dye will become clear, thus demonstrating the role of light in photosynthesis.
Things You'll Need
- 3 beakers, 500 mL
- Water, 770 mL
- 2,6-Dichlorophenol-Indophenol (DPIP), 0.02 g
- Balance
- Spinach leaves
- Blender
- Ice
- Strainer
- Pipetter
- Dropper
- Phosphate buffer
- 3 medium test tubes
- Aluminum foil
- Test tube rack
- Sunlight or lamp with 100-watt light bulb
Instructions
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Prepare the Solutions
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1
Prepare the DPIP solution. Use your balance to measure 0.02 g of DPIP powder. Mix the DPIP with 270 mL of water in your first beaker until the powder is completely dissolved.
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2
Prepare the sucrose solution. Use your balance to measure 85.5 g of table sugar and dissolve it in 500 mL of water in your second beaker. Place this beaker in ice to chill it.
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3
Prepare the chloroplast solution. Pour enough of the cold sucrose solution into your blender to cover the blades and add enough fresh spinach leaves to form a layer roughly 2 cm deep. Blend the spinach using three or four grinding pulses, each lasting nine seconds. Pour the resulting solution through a strainer into your third beaker and put it on ice to keep it cold.
Prepare the Demonstration
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4
Place 2 mL of phosphate buffer, 6 mL of water and 2 mL of DPIP in test tube 1 using your pipetter. Then add 3 drops of water. This is your control solution where no photosynthesis will occur.
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5
Place 2 mL of phosphate buffer, 6 mL of water and 2 mL of DPIP in test tube 2 using your pipetter. Then add 3 drops of cold chloroplast solution to the test tube. This is an experimental solution where photosynthesis can occur given the correct conditions.
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6
Wrap test tube 3 in aluminum foil. Use your pipetter to place 2 mL of phosphate buffer, 6 mL of water and 2 mL of DPIP in the test tube. Add 3 drops of cold chloroplast solution. This too is an experimental solution where photosynthesis can occur given the correct conditions.
Demonstrate the Role of Light
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7
Place all three test tubes in a test tube rack and place the test tube rack in direct sunlight or under a 100-watt light bulb.
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8
Wait for 20 minutes. During this time, discuss with your students what they expect to see as photosynthesis does or does not occur in each test tube.
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9
Observe test tube 1 with your students. No photosynthesis should have occurred in test tube 1 because it contains no chloroplasts; therefore the DPIP should remain a deep blue.
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10
Observe test tube 2 with your students. Photosynthesis should have occurred in test tube 2 because it contains chloroplasts and is exposed to light; therefore the DPIP should be noticeably clearer than it is in test tube 1.
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11
Observe test tube 3 with your students. Very little photosynthesis should have occurred in test tube 3 because it is wrapped in aluminum foil; therefore the DPIP should remain a deep blue, though it may have cleared slightly because of residual energy within the chloroplasts.
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1
Tips & Warnings
Before making your chloroplast solution, let the spinach leaves sit overnight under a bright light.
When handling chemicals, you and your students should always wear gloves, protective eye wear and protective clothing.
