Learning focus
Develop accurate biological explanations, interpret plant diagrams and data, and connect practical evidence with theory.
Why starch is tested
Glucose formed during photosynthesis is often converted to starch. Detecting starch therefore provides indirect evidence that photosynthesis has occurred over the experimental period.
The test sequence
Boil the leaf in water to kill cells and stop reactions. Heat it in ethanol in a hot-water bath to remove chlorophyll. Rinse in warm water to soften the brittle leaf and spread it on a white tile. Add iodine solution. Blue-black indicates starch; yellow-brown indicates no detectable starch.
Safety
Ethanol is flammable and must not be heated directly over a flame. Use a water bath, wear eye protection and handle hot glassware with care.
Destarching
Keep a healthy plant in darkness for long enough that stored leaf starch is used in respiration. Then any starch detected after treatment is more likely to have been made during the experiment.
Controls
A valid experiment changes one condition and keeps others constant. A control shows that the procedure and plant are capable of producing a positive result. Without controls, a negative result may be caused by an unhealthy plant or flawed technique.
Practical or data skill
Perform the starch test on a normal green leaf. Record observations at each stage and explain the purpose of each step.
Examination tip
The ethanol removes chlorophyll; it does not remove starch.
Review questions and suggested answers
Question 1
Why is the leaf boiled in water first?
Suggested answer
To kill the cells and stop metabolic reactions.
Question 2
Why is ethanol heated in a water bath?
Suggested answer
Because ethanol is flammable.
Question 3
Why destarch the plant?
Suggested answer
To remove pre-existing starch so starch made during the experiment can be identified.
