Photosynthesis experiments – Cont’d

Five daily reminders that will serve you well: 1. I am amazing. 2. I can do anything. 3. Positivity is a choice. 4. I celebrate my individuality. 5. I am prepared to succeed.

WELL, HERE we are together once more; how have you been? I certainly hope that you are not allowing yourself to feel pressured by anything! Just check the reminders at the beginning of the lesson! Are you ready and anxious to begin another lesson? Even with exams looming over your heads, biology can still be fun? Last week, we laid the foundation for the design and interpretation of photosynthesis experiments. What did you learn? You learnt that certain things must be done to ensure that the results obtained from your experiments are reliable. Let us recap. You must:

i. Know that the two end products are starch and oxygen. ii. Know that the test reagent for starch is iodine. iii. Know that iodine reacts with starch to produce a blue-black colour.

iv. Use aquatic plants in order to test for the end product, oxygen.

v. The production of oxygen is indicated by the formation of bubbles.

vi. De-starching the plant/leaf is most often the first step in the experiments.

vii. Ensure that all the raw materials are in adequate supply, i.e., none is limiting.

viii. Realise that it is difficult to test that water is necessary for the process.

Our first experiment is carried out to show that chlorophyll is necessary for photosynthesis. The write-up for the experiment will be done in the same way that it should be written up in your lab book.

Date: July 30, 2018

Topic: Photosynthesis

Aim: To determine if chlorophyll is necessary for photosynthesis

Apparatus/Materials: Potted plant with variegated leaves, Bunsen burner, tripod, gauze, 400ml beaker, test tubes, test tube rack, test tube holder, forceps, white tile, tap water, iodine, dropping pipette.

METHOD

1. De-starch the plant by placing it in a dark cupboard for at least 24 hours.

2. Remove the plant and place it in sunlight for a few days.

3. Remove a leaf from the plant.

4. Make an exact drawing of the leaf to the left of a sheet of blank paper. Label the drawing.

5. Put the water in the beaker and place it over the lit Bunsen burner.

6. As soon as the water boils, use the forceps to place the leaf in the boiling water. Leave it there for 30 seconds.

7. Fill a test tube with enough alcohol to cover the leaf. Place the leaf in the alcohol. You may have to roll the leaf in order to do this.

8. Put out the flame and place the test tube with the alcohol and the leaf in the hot water.

9. As soon as the green colour leaves the leaf, remove the test tube from the water bath.

10. Carefully remove the leaf from the test tube. Care is needed because the leaf is now ‘dry’ and brittle, it will tear easily.

11. Place the leaf in the hot water to soften it.

12. Remove the leaf and place it on the tile and pour on the iodine. Leave it for about one minute.

13. Drain off the iodine into the sink. Make a drawing of the leaf to the right of the original drawing. Label the drawing.

14. Paste your drawing under your observations/results title. The drawings will form your observations.

NB: Remember that you will need to rewrite the above method using reported speech/past tense.

DISCUSSION

Photosynthesis is the process by which plants convert carbon dioxide and water, in the presence of sunlight, to produce glucose and oxygen in the chloroplasts of leaves. The glucose produced is converted to starch, which is held in granules in the cells of the leaf. The leaf is green because it contains chloroplasts, which contain the chlorophyll and chlorophyll gives leaves their green colour. Iodine turns blue-black in the presence of starch; therefore, it is used to indicate the presence of starch. Variegated leaves are leaves with some areas that are not green, hence these areas do not have chlorophyll. This makes this type of leaf suitable to be used to show that chlorophyll is necessary for photosynthesis.

The plant was de-starched in order to ensure that starch present in the leaf was translocated and, hence, if starch is found at the end of the experiment, it would have been produced during the experiment. The leaf was plunged into hot water to kill the cells and to break down the cell membranes to allow entry and exit of the different chemicals. Alcohol was used to remove the chlorophyll. Chlorophyll is soluble in alcohol. This caused the chlorophyll to be removed from the leaf so it became colourless. The alcohol also removed the water from the leaf, making it dry and brittle. Plunging the leaf into the water caused it to reabsorb the water that was lost, hence making it soft and pliable again.

The area that was originally green had a blue-black colour after being covered with iodine solution. This meant that starch was present in the area that had chlorophyll. The area that was originally white retained the brownish-yellow colour of the iodine. This meant that there was no starch present in these areas.

All of the leaf was subjected to the same conditions, the only difference being that one area was green and the other white.

CONCLUSION

The absence of starch in the non-green areas indicates that chlorophyll is necessary for photosynthesis.

NB: In this experiment, the green part of the leaf formed the control experiment, while the white part was the test.