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Testing leaves for starch

Testing leaves for starch is an experiment that is usually performed in school science laboratories, by pupils grade 7-9. It is a simple experiment that works well in the summer but can be disappointing in the winter months when leaves make little starch. Variations include testing leaves destarched by lack of light, lack of carbon dioxide, or lack of chlorophyll (by using a variegated leaf).

Table of contents
1 Introduction
2 Hypothesis
3 Equipment
4 Safety procedures
5 Method
6 See also

Introduction

Leaves come in different shapes, sizes, colours, thicknesses and different types. The plant where the leaf is usually obtained is a geranium.

Photosynthesis happens in the mesophyll cells of leaves. The mesophyll cells contain tiny bodies called chloroplasts, which contain chlorophyll, which is used to catch the light energy needed in photosynthesis. Glucose can be converted into starch and stored. Both starch and sucrose can be turned back into glucose and used in respiration. Most plants store starch. They can turn starch back into glucose when they need it for respiration.

Aim

The aim of the experiment is to see if a green leaf that had been left in the light for 48 hours would have the same amount of starch as a leaf that had no light for 48 hours.

Hypothesis

The prediction is that both the green leaf that has been in the light will have starch present.The one left in the dark will not have starch present. The leaf will turn brown/black if starch is present.

Equipment

Safety procedures

Method

  1. Set up the equipment.
  2. Light the bunsen and boil the water. When the water had boiled add the first geranium leaf (the one that had been the light for 48 hours).
  3. Wait one minute for the leaf to boil (this is to get rid of the waterproof layer and break the open cells and make it soft).
  4. Turn off the Bunsen burner (for safety reasons, we are going to use ethanol), and take out the leaf.
  5. Put the leaf in a boiling tube and cover with ethanol.
  6. Put the tube of ethanol plus leaf into the beaker of hot water. Ethanol boiles at 80° so it should come to boil even though the bunsen is off.
  7. Dip it back into the hot water so it can get the ethanol off.
  8. Spread the leaf out on a tile. Add about five drops of iodine on to the leaf and observe. After about two minutes the iodine had soaked in.
  9. Repeat using a leaf that had been in the dark for 48 hours.

See also