Use of alcohol during photosynthesis experiments

In the following experiment, alcohol was used to extract the chlorophyll

found in a leaf.

The aim was to be able to test for the presence of starch in a leaf.

The following method was used:

1. Dip a freshly cut leaf in beaker of boiling water for 10 seconds

(in order to kill it and soften it).

2. Place leaf in test tube of ethanol.

3. Place test tube in beaker of hot water for 10 minutes (in order

to remove the chlorophyll from the leaf).

4. Wash leaf in cold water

5. Dip leaf in a beaker of hot water (to soften the leaf again)

6. Place in a petri dish and cover with iodine solution.

Types of leaves used

In this experiment 3 types of leaves were used:

1. A totally green leaf.

2. A variegated leaf, to check that it is truly the chlorophyll that allows photosynthesis to take place.

3. A green leaf, but with a thicker waxy cuticle, to check if alcohol allows total extraction of chlorophyll.

Leaves dipped in boiling water

The 3 leaves were dipped in boiling water for about 10 seconds to kill and soften them

Killed leaves placed in ethanol

Each leaf was placed in a separate boiling tube filled 2/3 with ethanol.

Each boiling tube was then placed in a hot water bath for about 10 minutes.

The ethanol turned green, showing that chlorophyll was extracted from the leaf.

Leaves removed from alcohol and laid flat on tile

The leaves were removed from the alcohol, washed with cold water, and were then re-dipped in the hot water to soften them again.

Then they were laid flat onto a tile.

As can be observed the leaves with a thin cuticle, had most of their chlorophyll extracted. The leaf with a thick cuticle remained green, showing that it was difficult for the alcohol to extract the pigment.

Leaves tested with Iodine

The leaves were then completely covered with Iodine solution, to test for the presence of starch.

Presence of starch

A change in the colour of Iodine, from brown to blue-black, indicates that starch is present.

In the case of the variegated leaf, a blue-black colour was only observed in the areas that were previously green.

This shows that chlorophyll is necessary for photosynthesis.

This experiment was performed by a group of Form 4 Biology students.

St.Theresa Girls’ School,

Mriehel