The Importance of Chlorophyll in Photosynthesis

The Importance of Chlorophyll in Photosynthesis

Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize food from carbon dioxide and water. Chlorophyll is a green pigment present in all green plants. It is responsible for the absorption of light which provides energy for photosynthesis to occur. A variegated leaf is a leaf that has two different colours (i.e. green and white or green and red). Only the parts of the leaves which are green contain chlorophyll.

Learning Objectives

To demonstrate the importance of chlorophyll in photosynthesis.

Materials

Variegated 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 that student cover their test tubes to avoid excess release of ethanol vapour. If a test tube catches re, instruct student to cover the tube with a non- ammable object to extinguish the ame.

Preparation Procedure

1.           Collect variegated leaves from the environment.Try to find leaves that do not have a waxy outer coat.

2.           Heat water to boiling using a heat source.

Activity Procedure

1.            Choose a small piece of one leaf (the piece should not be bigger than the lid of a soda bottle) and draw a picture to show the colour pattern. Label which parts of the plants are green and which parts are not.

2.           Submerge this leaf in boiling water for about 3 minutes.

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

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

5.           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.

Spread the decolourized leaf on a white tile and add iodine solution drop wise until the whole leaf is covered. Record your observations and draw a diagram showing the colour pattern of the leaf.

Figure 6.5: A diagram of a variegated leaf.

Results and Conclusions

The leaf is dipped in boiling water to kill the cells. The leaf is then submerged in boiling ethanol to extract the colour from the leaf. The ethanol will change to a green colour while the leaf should lose all of its colour, becoming white. When iodine solution is added, the leaf will turn a dark blue/black colour in all the places that the leaf was green. The non-green parts of the leaf should not turn dark, but should remain the colour of iodine solution. This indicates that chlorophyll is necessary for photosynthesis because only the parts of the leaf containing chlorophyll were able to photosynthesise and produce starch.

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. Varie-gated leaves can be found in areas that contain decorative plants, such as in front of houses and buildings. Before doing this experiment with students, test some variegated leaves from the local environment to ensure that they respond well to the experiment. Make sure that the leaf has been in sunlight for at least 6 hours prior to the experiment or there may not be enough starch to detect. This practical should NOT be done in the morning.

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