Biology Practical: Photosynthesis

GCSE Biology Practical - Photosynthesis

Investigate the conditions necessary for photosynthesis: Carbon Dioxide, Light and Chlorophyll.

GCSE Biology practical - CO₂ is necessary for photosynthesis

1. Take a potted plant and place it in a completely dark place for about 48 hours to destarch its leaves. (During photosynthesis plants form starch in the leaves. Destarching a plant involves preventing photosynthesis, usually by keeping the plant in the dark for 24 to 48 hours until the starch is used by the plant for their food and energy).
2. Take a conical flask having a wide mouth and put some potassium hydroxide solution into it.
3. Insert one leaf into the conical flask through a split rubber cork and fit it tightly.
4. The cork will prevent carbon dioxide from entering the flask.
5. Potassium hydroxide in the flask absorbs the carbon dioxide gas present in the flask.
6. The potted plant with its one leaf inside the flask containing potassium hydroxide is kept in sunlight for few hours. During this period, the other leaves of the plant gets carbon dioxide from the air but the leaf which is inside the flask does not get any carbon dioxide. This is because all the carbon dioxide of the air present in the flask has been absorbed by potassium hydroxide solution.
7. Pluck the leaf from the plant and take it out from the flask.
8. Pluck another leaf from the plant which was exposed to atmospheric air.
9. Test both the leaves for presence of starch.
10. Put the two leaves in boiling water for few minutes.
11. Now boil the two leaves in alcohol over a water bath. In this way, we get a decolorized leaf.
12. Wash the decolorized leaf with water to remove any alcohol present
13. Pour iodine solution over the two leaves and observe any change in colour.
14. We will observe that the leaf from conical flask does not turn blue- black ( showing absence of starch), while the leaf exposed to atmospheric air turns blue-black on adding iodine solution (showing the presence of starch).

From this observation we conclude that in the absence of carbon dioxide no photosynthesis takes place and hence no starch is formed. Hence, carbon dioxide is necessary for photosynthesis.

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GCSE Biology practical - Light is necessary for photosynthesis

1. Take a potted plant having green leaves and place it in a completely dark place for about 48 hours to destarch its leaves. During photosynthesis plants form starch in the leaves. Destarching a plant involves preventing photosynthesis, usually by keeping the plant in the dark for 24 to 48 hours until the starch is used by the plant for their food and energy.
2. Take a thin strip of black paper and wrap it on one leaf on both the sides so that sunlight may not reach it from both the sides.
3. Cover the leaf partially so that the remaining part of the leaf remains exposed to sunlight.
4. Keep this potted plant with partially covered leaf in bright sunlight for a few hours.
5. After few hours pluck the partially covered leaf from the plant and remove the black paper.
6. Put the leaf in boiling water for a few minutes.
7. Now boil the leaf in alcohol over a water bath. In this way, we get a decolourised leaf due to removal of chlorophyll.
8. Wash the decolourised leaf with water to remove any alcohol present.
9. Put the leaf on a white paper and pour iodine solution over the leaf and observe any change in colour.
10. The middle part of leaf which was covered with black paper does not turn blue-black on adding iodine solution showing that no starch is present in the middle part of the leaf. This is because sunlight could not reach the covered part of the leaf due to which the leaf could not do photosynthesis to produce starch.
11. The uncovered part of leaf which was exposed to sunlight turns blue-black on adding iodine solution showing that starch is present in this part of leaf. This means that the part of leaf which was exposed to sunlight could do photosynthesis to produce starch.

We conclude that sunlight is necessary for photosynthesis to produce food like starch.

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GCSE Biology practical - Chlorophyll is necessary for photosynthesis

1. Place the potted plant with variegated leaf in the dark for about 2-3days. This step is essential in order to rid the plant of any starch that was formed in the leaves prior to the start of the experiment. At the end of this time period, remove the potted plant from the dark and place it in sunlight for several hours.
2. Pluck a leaf from the plant and place it into the test tube containing alcohol solution. Place the tube in the boiling water bath. This bleaching step is necessary for a better visualisation of the iodine-starch reaction on the leaf surface in the later part of this experiment.
3. Remove the leaf from the alcohol using a forcep. Briefly place the leaf for a few seconds in the hot water bath in order to soften the leaf.
4. Transfer the leaf to a petri dish containing iodine solution. After a couple of minutes, you’ll see that the parts of the leaf that originally contained the green spots test positive for iodine by becoming blue black in colour. However, the variegated spots on the leaf, which did not have chlorophyll, do not show any significant change in colour. This is a negative test for starch and thereby photosynthesis.

This experiment demonstrates that chlorophyll is an absolute essential for photosynthesis to take place in a plant.

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