www.BioTopics.co.uk
Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.


../nextblue.gif ../contentsblue.gif

Experiments to show the factors required for photosynthesis (1) - Chlorophyll

Zebrina is a variegated plant similar to Tradescantia. This means that it has both green and white areas. Other variegated plants would probably work instead.

Procedure

1 Take a leaf of Zebrina.

2 Draw it accurately, noting the colours of the various areas.

zeb1

3 Drop it directly into boiling water in electric water bath. CARE!

Alternatively: use a beaker of water heated over a bunsen burner.

Leave it for 2 - 15 minutes.

4 Carefully extract leaf and place it into a wide test-tube. Shake it down to the bottom of the tube.

5 Just cover leaf with I.M.S. (ethanol).

6 Transfer tube containing leaf to other water bath, set at 80 C.

Alternatively: use the beaker of water just heated BUT with the BUNSEN BURNER OFF.

CARE! Ethanol vapour is flammable.

Leave it for 15 - 30 minutes.

7 Tip away liquid.

8 Rinse leaf in (preferably hot) water.

9 Note appearance of leaf.

zeb2
10 Spread out leaf in plastic dish, e.g. Petri dish.

11 Add about 5 drops of iodine solution. Wait 10 minutes.

12 Note the colours of the various parts now.

13 Draw again, and answer the questions.

zeb3

What are the green areas of the leaf composed of?

> cells containing chlorophyll

What are the white areas of the leaf composed of?

> cells containing NO chlorophyll

What was the purpose of boiling the leaf in water?

> to kill the cells and burst their chloroplasts

What was the purpose of the industrial methylated spirits?

> (as an organic solvent) to dissolve the chlorophyll

Why was pure ethanol not used?

> too expensive - Customs and Excise tax potable alcohol

Why was the second waterbath set at 80 C?

> ethanol's boiling point is 78C,it would boil away too quickly

What did the I.M.S. look like after the leaf had been in it for some time?

> green

What did the leaf look like after it had been in the I.M.S. for some time?

> not so green - +/- colourless

What does the iodine solution react with?

> starch

If the reaction is positive, what is the resulting colour?

> blue-black

If the reaction is negative, what is the resulting colour?

> orange

Look at the equation for photosynthesis again. What is the reason for using this test?

> glucose is turned into starch, which is easily tested for,

and being insoluble, it stays where it is!

What is the connection between the 2 colours on the leaf now and at the beginning?

original colour final colour
green blue-black
white orange

Write a sentence explaining what you can conclude about factors required for photosynthesis from this experiment.

> chlorophyll is necessary to produce starch





Return to the previous unit? Return to the contents page? Another look? Back to the BioTopics index page? Next Unit?



www.BioTopics.co.ukHome Contents Contact via form Contact via email Howlers Books WWWlinks Terms of use Privacy