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Plants  Photosynthesis    Making

Experiment to show that a plant needs light for photosynthesis

Photosynthesis is the name of the process by which the plant manufactures sugars.

The plant takes CARBON DIOXIDE from the air and WATER from the soil and combines them together to form SUGAR.

OXYGEN is released as a waste product.

The energy for the process comes from the SUN which is absorbed with the help of CHLOROPHYLL (green).

Photosynthesis takes place in the CHLOROPLASTS which are found mostly in the leaves.

Word Equation for photosynthesis:

Carbon dioxide  + water   ------->      sugar   +    oxygen
Energy from the sun+ chlorophyll

 Gases are exchanged through tiny holes (called stomata) in the underside of the leaf. The stomata close up during the night or during hot weather to prevent the plant loosing too much water.

The sugar is used three ways:

i. Converted into STARCH and transported away to other parts of the plant.

ii. Converted into CELLULOSE for the cell walls.

iii. Used by the plant for its own respiration to provide energy for other processes.


Demonstrating photosynthesis


Photosynthesis can be demonstrated by placing an inverted test-tube over a piece of oxygenating pond weed in a beaker filled with water.

The whole apparatus is placed in bright light.

After a few days a colourless gas collects in the test tube.

The gas is shown to be oxygen by testing it with a glowing splint which relights.


Testing for the presence of starch in a leaf

This experiment shows that a leaf carries out photosynthesis in the presence of light.

The plant to be used is left in the dark for 24 hours before the experiment started. This removes all the starch from the leaves.

Reason: The leaves cannot carry out photosynthesis which makes starch, but they are still carrying out respiration, using up starch.

It is then taken out of the dark and left in the light again for another 24 hours, but this time part of the leaf was covered up using aluminium foil (Diagram1)

Diagram 1

A leaf covered by aluminium foil.
There is a hole in the middle of the foil
Diagram 4

The same leaf after being boiled in alcohol (see below) and then tested with iodine solution

It is likely that the parts of the leaf left in the dark (or parts without chlorophyll) will have no starch as they cannot carry out photosynthesis.

The leaf is now treated as follows:

Diagram 2

Diagram of apparatus before heating

Diagram 3

Diagram of experiment after heating

1.The leaf is dipped in boiling water for 2-3 seconds. This kills the cells and softens the leaf.

2. The leaf is boiled in alcohol for about 5 minutes
This dissolves out any chlorophyll.

After boiling in alcohol the leaf should have lost its green colour.
Compare diagrams 2 and 3 which shows the apparatus before and after boiling 

3. The leaf is now dipped back into boiling water. This is to soften it again.

4. The leaf is laid out on a white tile 1-2 drops of iodine solution are added.

Wherever there is starch present the leaf will turn a blue-black colour. It will remain a yellow colour where there is no starch. (see diagram 4)

Result and Conclusion:
The leaf turned a blue-black colour under the hole in the aluminium foil. This showed that starch was made in the presence of light but not where the leaf remained in the dark.

We conclude from this that light is needed for photosynthesis


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