Shung Tak Catholic English College

W.Y.Chan

Experiments on photosynthesis

Destarching Destarching

Starch is a detectable product of photosynthesis.

Presence of starch photosynthesis has occurred.

Procedure :Procedure :

1. Put in dark cupboard for 2 days or overnight.

2. In darkness, starch is converted back to sugar which is removed out of the leaves through phloem. 3. One of the leaves is detached and tested for starch.

Test for starch in a green leaf

1. Choose a leaf that has been in light for several hours.

2. Boil the leaf in water

Make the leaf soft, break down cuticle and kill the leaf. More permeable to alcohol and iodine solution. Burst the starch grains.

3. Immerse the leaf in boiling alcohol for a few minutes

To decolourize the leaf i.e. to remove chlorophyll.

4. Wash the leaf in warm water

To remove the alcohol and make the leaf become soft.

5. Spread the leaf on a white tile and add iodine solution to it

Turn to blue-black Starch is present.

Turn to light brown Starch is absent.

Experiment to show that light is necessary for photosynthesis

Destarch a potted plant by placing it in a dark cupboard for 2 days

To remove starch from the leaf

Attach 2 pieces of black paper with circular hole on the two sides of a leaf

Leave the plant in sunlight for about 6 hours.

Test the leaf for starch

Result :

The exposed parts of the leaf turns blue-black.

The shaded part is stain pale brown

Light is necessary for photosynthesis

Experiment to show that chlorophyll is necessary for photosynthesis

A Coleus plant with variegated leaves

Destarch the plant by putting it in darkness for 2 days

To remove starch from the leaves

Leave the plant in sunlight for about six hours

Detach a leaf and map out the green parts of the leaf on a piece of paper.

Carry out the test for starch

Only the green parts of the leaf turn blue-black

The white parts of the leaf turn pale-brown.

Chlorophyll is necessary for photosynthesis

Experiment to show that carbon dioxide is necessary for photosynthesis

Destarch the plant by putting it in darkness for 2 days

Place leaf A in a conical flask containing potassium hydroxide.

Place leaf B in a flask filled with air / water

Leave the plant in sunlight for about six hours

Result :

Leaf A turns to pale brown.

Leaf B turns to blue-black.

Carbon dioxide is necessary for photosynthesis.