Can Leaf Discs Make Starch in the Dark?

Can leaf discs make starch in the dark?

Students’ Sheet

Introduction

This worksheet shows how you can investigate photosynthesis and the synthesis of starch using leaf discs. Before starting any practical work, you need to know

·  that carbon dioxide is the source of carbon in the starch formed by photosynthesis

·  that carbon dioxide enters the leaf mainly through the stomata, usually on the lower surface of the leaf

·  that starch is a carbohydrate made from the polymerisation of glucose

·  how to destarch a leaf, what happens to the starch and why you need to do this

·  how to test a leaf for starch, using iodine solution to give a blue-black colour if starch is present

Look carefully at your results and see if you get ideas for more investigations that you could do. You may get some surprises from possible extensions of the investigation which is described below.

Instructions

Testing for starch

·  Heat discs in 10cm3 of water, in a water bath of boiling water, for 5minutes. Pour off water

·  Boil in 10cm3 of ethanol, in the same water bath, for 3minutes. Pour off the ethanol

·  Rinse in water. Place on a tile. Add iodine solution

Questions

Some questions for you to interpret when you get your results

1. Do the discs in A and B behave in the same way as a whole leaf? Explain the colours obtained with iodine solution.

2. Why do discs in C behave differently from those in B? Explain the colours obtained with iodine solution.

3. What might happen if the leaves were floated the right way up?

Now make a prediction . . .

What will happen to a white leaf disc from a variegated pelargonium . . .?

(a) on water in the light

(b) on glucose in the dark

Science & Plants for Schools: www.saps.org.uk

Resource title here: p. 2

This document may be photocopied for educational use in any institution taking part in the SAPS programme.

It may not be photocopied for any other purpose. Revised 2010.