Testing leaves for starch: the technique - Photosynthesis - Energy - Introductory - Levels - Practical Biology
11/10/11 10:16 PM
TESTING LEAVES FOR STARCH: THE TECHNIQUE
Introductory Level
Testing leaves for starch: the technique
Introduction
This procedure kills a leaf, disrupts the cell membranes and softens the cuticle and cell
walls. This makes it possible to extract the chlorophyll with hot ethanol and also allows
the iodine solution to penetrate the cells and react with any starch present.
Lesson Organisation
You can run this as a teacher demonstration, or with students carrying out the procedure
in pairs.
Apparatus And Chemicals
For the class – set up by technician/ teacher:
Ethanol (IDA) (note 1)
Kettles of boiling water (note 2)
OR Electric water baths set at 90 °C containing a boiling tube rack
Iodine in potassium iodide, solution in dropper bottles (note 3)
Beaker or jar (at least 250 cm3), labelled ‘Waste ethanol’ (note 4)
Leaves, different types, such as pelargonium (pot geranium) (note 5)
For each group of students:
Eye protection
Beaker for boiling water, 250 cm3
Forceps, 1
Boiling tube, 1 for each type of leaf used
Anti-bumping granules (optional)
Glass rod
Marker pen
Petri dish
White tile
Health & Safety
Read our health and safety guidelines
Ethanol (IDA), iodine solution and hot liquids require safety precautions to be taken.
Wear eye protection.
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Testing leaves for starch: the technique - Photosynthesis - Energy - Introductory - Levels - Practical Biology
11/10/11 10:16 PM
Technical Notes
1 Ethanol (IDA) – refer to CLEAPSS Hazcard 40A and student safety sheet 60 – is highly
flammable (flash point 13 °C) and harmful (because of the presence of methanol). The
risks in this procedure are reduced by using hot water from kettles or in water baths
rather than heating with a Bunsen burner flame. Some protocols recommend propanol
(Hazcard 84A) in place of ethanol, as it removes chlorophyll more effectively. However, it
has the additional risk of eye damage, its flashpoint is very similar to that of ethanol (IDA)
and it may be more expensive.
2 Kettles are a safer source of hot water than heating with a Bunsen burner because of
the presence of flammable ethanol (IDA) in this procedure. Students are familiar with the
hazards of using kettles. Consider how to limit the movement of students around the
laboratory with kettles or beakers of near-boiling water. Electrically-heated and
thermostatically-controlled hot water baths may be a safer alternative.
3 Iodine solution – refer to CLEAPSS Hazcard 54B and Recipe card 39. A 0.01M solution
is suitable for starch testing. Make this by 10-fold dilution of 0.1M solution. Once made,
the solution is a low hazard but may stain skin or clothing if spilled, and may irritate the
eyes.
4 Save the waste ethanol as a source of chlorophyll for future work. Make sure it cannot
be tipped over and is in a safe place so it is not a fire hazard.
5 If the teacher or technician snips the leaves from the plants to give to the students, the
plants are more likely to survive to be used again. Variegated Pelargonium (pot
geranium) are good subjects for this experiment as are Tradescantia and Impatiens (busy
lizzie).
6 Ensure that the plants have been well-illuminated for 24-48 hours. In winter, it might be
worth using a halogen lamp to ensure the illumination is adequate.
Ethical issues
There are no ethical issues associated with this procedure.
Procedure
SAFETY: Ensure the ethanol is kept away from naked flames. Students should wear eye
protection when working with ethanol or iodine solution. Take care with hot liquids. Be
aware that plant sap may irritate the skin.
Investigation
a Collect leaves from the plants to be tested.
b At your desk, pour some boiling water from a kettle into a large beaker.
c Using forceps, pick up one of your leaves and hold it in the hot water for about one
minute.
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Testing leaves for starch: the technique - Photosynthesis - Energy - Introductory - Levels - Practical Biology
11/10/11 10:16 PM
d Using forceps, remove the leaf from the boiling water and note how it has changed.
e Drop the leaf into a boiling tube and push it to the bottom with a glass rod. Add some
anti-bumping granules (optional). Label this tube with your initials if you will be placing it
in a hot water bath.
f Put on your eye protection.
g Add enough ethanol to cover the leaf, and stand the boiling tube in your beaker of hot
water, or in the hot water bath.
h Watch as the ethanol boils and the green colouring (chlorophyll) is removed from the
leaf. This will take a few minutes.
i Replace the hot water with freshly-boiled water from the kettle after 5 minutes if there is
still some green colour in the leaf.
j Using forceps, remove the leaf from the boiling tube and rinse the leaf in cold water.
k Put the leaf in a Petri dish on a white tile.
l Add iodine solution to the leaf from the dropper bottle. Make sure the leaf is completely
covered with iodine.
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Testing leaves for starch: the technique - Photosynthesis - Energy - Introductory - Levels - Practical Biology
11/10/11 10:16 PM
m Watch for a few minutes to see if a blue-black colour develops in any part of the leaf. A
blue-black colour with iodine solution indicates that starch is present.
n Wash your hands to remove any traces of plant sap, or the chemicals that you have
used.
Teaching Notes
The familiar word equation for photosynthesis is:
carbon dioxide + water ! oxygen + glucose
In the leaf, excess glucose is rapidly converted to starch, so we test leaves for starch to
show that photosynthesis has happened, rather than testing for glucose.
We often indicate that light and chlorophyll are required for the process by adding them to
the equation near the arrow.
You could demonstrate the effect of iodine solution on starch by adding a few drops of
solution to a starch powder or starch suspension in a boiling tube. You can then discuss
the fact that this colour change might be hard to see in a dark green leaf until the
chlorophyll is removed.
Depending on student’s prior knowledge, you could discuss the structure of plant cells
and explain that the starch is within the cells, the cells are surrounded by cell membranes
and tough cellulose cell walls and that some leaves also have a protective waxy cuticle.
The hot water treatment softens up the protective structures, and disrupts the cell
membranes to let the chlorophyll out and the iodine solution in.
Having established the technique, students can apply it in a further practical – Identifying
the conditions needed for photosynthesis.
Health & Safety checked, December 2008
Related experiments
Identifying the conditions needed for photosynthesis
This protocol applies the technique of testing leaves for starch to plants that have been
kept in controlled conditions in order to establish the conditions needed for
photosynthesis to occur.
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Testing leaves for starch: the technique - Photosynthesis - Energy - Introductory - Levels - Practical Biology
11/10/11 10:16 PM
Investigating factors affecting the rate of photosynthesis
This procedure allows you to quantify the effect of some of the factors that affect the rate
of photosynthesis by following the changing rate of photosynthesis in pond weed as
conditions such as temperature, carbon dioxide concentration and light intensity change.
Investigating photosynthesis using immobilised algae
This procedure uses a more sophisticated method to follow the process of
photosynthesis by directly measuring the changes in carbon dioxide levels caused by
photosynthetic activity.
©2011 The Nuffield Foundation & Society of
Biology
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