photosynthesis

Photosynthesis converts light energy into the chemical energy of sugars and other organic compounds. this process consists of a series of chemical reactions that require carbon dioxide (CO2) and water (H2O) and store chemical energy in the form of sugar. Light energy from light drives the reactions. Oxygen (O2) is a byproduct of photosynthesis and is released into the atmosphere.

During the lesson student will:

1. Explain the process of photosynthesis from prior knowledge.

2.  Perform the starch test on leaves and interpret the results  and then discuss their results and share their understanding of photosynthesis.

Starch testing

Iodine solution is used to test leaves for the presence of starch. The step for testing for photosynthesis is as follow:

1. heat a plant leaf in boiling water for 30 seconds (this kills the leaf, stopping any chemical reactions)
2. add the leaf to boiling ethanol in a water bath for a few minutes (the boiling ethanol dissolves the chlorophyll and removes the green colour from the leaf - it turns white so it is easy to see the change in colour)
3. wash with water to rehydrate and soften the leaf and spread onto a white tile
4. add iodine solution from a dropping pipette

After a few minutes, the parts of the leaf that contain starch turn the iodine from brown to blue/black.

The teacher will give students some questions to answer 

Ques 1. What is starch? (2 marks)

Ques 2. What is the chemical formula of starch? (1 mark)

Ques 3. What is the purpose of starch? (2 marks)

Ques 4. What is stroma? (1 mark)

Ques 5. How would you determine whether or not leaves contain starch? (1 mark)

Ques 6. Describe Photosynthesis. (3 mark)

Ques 7. Why is starch tested for and not glucose? (2 marks)

Ques 8. Explain why boiling the leaf in water is important. (3 marks)

Ques 9. Explain why boiling the leaf in ethanol is important. (2 marks)

Starch is an odourless, tasteless white material generated in the green leaves of plants as a reserve food supply from surplus glucose produced during photosynthesis.The end result of a starch test on a leaf would make it either brown or blue-black. If starch has been produced (that is if photosynthesis has occurred), the leaf would turn blue-black, otherwise, it would remain brown.

http:/www.phschool.com/science/biology_place/biocoach/photosythesis/overview…

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