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Chapter: Biology laboratory Practicals: Biology Activities

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The Importance of Light in Photosynthesis

Photosynthesis is the process through which green plants and some other organisms use sunlight to synthesize food from carbon dioxide and water. Light energy is required for photosynthesis to occur.

The Importance of Light in Photosynthesis

Photosynthesis is the process through which green plants and some other organisms use sunlight to synthesize food from carbon dioxide and water. Light energy is required for photosynthesis to occur.

Learning Objectives

To show that light is needed for photosynthesis.

Materials

Aluminium foil or black carbon paper, clips, a green leaf, ethanol*, iodine solution*, heat source, cooking pot, water, test tube*, white tile*, dropper*, and cotton wool.

Hazards and Safety

Ethanol is very ammable! Make sure you cover the test tubes with cotton wool to avoid excess release of ethanol vapour. If the test tube catches on re, cover the tube with a non- ammable object to extin-guish the ame.

Preparation Procedure

1.           Put a potted plant in a dark place for 24 hours to de-starch it.

2.           Use aluminium foil or carbon paper to cover a portion of the upper and lower epidermis of a leaf (see diagram).

3.           Allow the plant to sit in sunlight for at least 6 hours.

Activity Procedure

1.           Submerge the leaf in boiling water for about 3 minutes.

2.           Remove the leaf from the water and insert it into a test tube containing ethanol and plug the test tube with a piece of cotton wool. Note: the test tube should be less than half full of ethanol.

3.           Submerge the test tube in the boiling water and leave it to boil until the leaf loses all of its colour.

4.           Once the leaf has lost its colour, remove it from the ethanol solution and dip it brie y into the boiling water to remove the ethanol and soften it.

5.           Spread the decolourized leaf on a white tile and add iodine solution drop wise until the whole leaf is covered.

6.           Record your observations and draw a diagram showing the colour pat-tern of the leaf after the addition of iodine solution.

Results and Conclusion

The aluminium foil blocks sunlight from reaching the leaf, thus preventing photosynthesis from taking place. Any part of the leaf that was covered with foil will test negative for starch while the parts exposed to sun will test positive for starch. This proves that sunlight is necessary for photosynthesis.

Clean Up Procedure

Collect all the used materials, cleaning and storing items that will be used later. No special waste disposal is required.

Notes

It is important that the leaf does not contain a thick waxy coating. Do the procedure for testing a plant for starch on other leaves from this plant before setting the aluminium foil to ensure it works.

 

Because ethanol boils at a lower temperature that water, the ethanol solution can be boiled in a water bath. Ethanol is very ammable and it is possible that the top of test tube will catch re. If this happens, cover the top of the test tube with a non- ammable material such as glass or metal to cover the ame and deprive it of oxygen.

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