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Course: EST 2 Biology
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EST 2 Biology

Cell Structure and Molecular Biology

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Ecology

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Genetics and Heredity

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Body Systems and Plants

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Classification and Evolution

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Text lesson

4.7 Plant biology (Part 4)

Testing leaves for starch: chlorophyll, light and carbon dioxide — EST 2 BIOLOGY notes: Describe the starch test and use it to show that chlorophyll, light and carbon dioxide are needed for photosynthesis.

Although plants make glucose, leaves cannot reliably be tested for glucose because it is quickly used, converted to other substances and transported or stored as starch. Starch is stored in the chloroplasts where photosynthesis takes place, so testing a leaf for starch shows which parts have been photosynthesising.

1

Kill the leaf by dipping it in boiling water for about 30 seconds (this also softens it).
2

Turn off any Bunsen burner, then heat the leaf in ethanol in a water bath to remove chlorophyll.

  • Ethanol is extremely flammable, so the safest way to heat it is in an electric water bath, not over an open flame.
3

Rinse the leaf in warm water to soften it.
4

Spread the leaf on a tile and add iodine solution.
5

Blue-black shows starch is present; orange-brown means it is absent.

A variegated leaf (green and white) shows that chlorophyll is needed: only the green parts turn blue-black. The same method with a leaf covered in foil shows light is needed, and a plant in a sealed bell jar with soda lime (which absorbs CO2) shows that carbon dioxide is needed.

Aquatic pondweed (for example Elodea) is usually used in experiments to investigate the rate of photosynthesis, because the oxygen it releases forms bubbles that can be counted or collected.

1

Place the pondweed in water in a beaker or tube and place a lamp at a measured distance.
2

Count the bubbles of oxygen released per minute, or collect the gas in a capillary tube or gas syringe.
3

Change one factor: light intensity (move the lamp), temperature (change the water temperature) or carbon dioxide (dissolve different amounts of sodium hydrogencarbonate in the water).
4

Keep all other variables constant for a fair test.

  • For example, a glass tank of water placed between the lamp and the beaker absorbs heat so the temperature does not change as well as the light intensity.

Alternative ways of measuring the gas include collecting it in an upturned measuring cylinder or a gas syringe, rather than counting bubbles.

Plants respire all the time, so plant cells take in oxygen and release carbon dioxide by aerobic respiration.

Respiration, photosynthesis and indicators
In daylight they also photosynthesise, taking in carbon dioxide and releasing oxygen.
At night there is no photosynthesis, but respiration continues, so plants take in oxygen and give out carbon dioxide.
In bright light photosynthesis is faster than respiration, so there is a net uptake of carbon dioxide and a net release of oxygen.

Hydrogencarbonate indicator detects changes in carbon dioxide concentration: it is orange-red at atmospheric levels, turns yellow when carbon dioxide is high and purple when it is low.

Respiration, photosynthesis and indicators
Several leaves from the same plant are placed in stoppered boiling tubes with the indicator, and the effect of light is investigated over a few hours.
A tube in the dark turns yellow (respiration adds CO2), whereas a tube in the light turns purple (photosynthesis removes CO2).

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light. Green plants make glucose from carbon dioxide and water, and oxygen is released as a waste product. The reaction requires energy, which is obtained by the pigment chlorophyll trapping light from the Sun.

Chlorophyll is a green pigment found in chloroplasts. It reflects green light (giving plants their colour) and absorbs light energy, transferring it into chemical energy for the synthesis of glucose. Photosynthesis will not occur without chlorophyll.

6CO2 + 6H2O → C6H12O6 + 6O2 (light energy, chlorophyll)

Worked example 1. Describe how to test a leaf for starch. [5]

Dip in boiling water, boil in ethanol in a water bath (no flame) to remove chlorophyll, rinse in water and add iodine; blue-black shows starch.

Worked example 2. A variegated leaf is tested for starch. Describe and explain the result. [3]

Only the green areas turn blue-black, showing chlorophyll is needed for photosynthesis.

Worked example 3. Why is ethanol heated in a water bath? [1]

Ethanol is extremely flammable, so no naked flame must be used.

Worked example 4. Why do we test for starch and not glucose? [2]

Glucose is quickly used or converted; starch is stored in chloroplasts, showing where photosynthesis occurred.

Term Meaning
Starch test Test using iodine; blue-black means starch is present.
Variegated leaf A leaf with green and white areas.
Destarch To remove starch from a plant by keeping it in the dark.
Photosynthesis Manufacture of carbohydrates from carbon dioxide and water using light energy.
Limiting factor A factor in short supply that restricts the rate of a process.
Stomata Pores in the leaf for gas exchange.
Chlorophyll The green pigment that absorbs light for photosynthesis.
1

Heating ethanol directly over a flame.
2

Saying a negative starch test proves there was no respiration.
3

Testing for glucose with iodine.
4

Forgetting that the leaf must be destarched before the experiment.
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4.7 Plant biology (Part 4)

EST 2 Biology / Body Systems and Plants

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