6c. Plant Systems: Experiments to Investigate Processes in Plants

Question 1

How do you ensure that starch is absent in leaves before the start of the experiment? Why does this remove starch? Why must starch by removed?

Answer 1

Destarch the plant by placing it in darkness for two days.

In darkness, photosynthesis stops and starch is converted into glucose.

So that if starch is formed in leaves at the end of the experiment, photosynthesis must have taken place.

Question 2

Test for Starch in a Leaf

Steps + Reasons

Answer 2

1. Place the leaf in boiling water for two minutes.

• To stop any metabolism and arrest the cell activity to stop any enzymatic reaction.
• Makes cell membrane more permeable as proteins in membrane denature.

2. Put boiled leaf in alcohol. Place the boiling tube in a beaker of hot water.

• Extract chlorophyll from leaf so that the green pigment will not affect the staining of the leaf and make the blue-black colouration more visible.

3. Gently remove the leaf from alcohol and put it into hot water.

• Leaf is brittle because alcohol removes water from the leaf.
• Hot water softens the leaf and makes it more permeable to iodine solution.
• Also to prevent the leaf from tearing.

4. Place leaf on white tile and add a few drops of iodine solution.

• Test for presence of starch.

Question 3

How to test whether sunlight is necessary for photosynthesis?

Answer 3

1. Destarch a potted plant by placing it in darkness for two days.
2. Remove one leaf and test it for starch to ensure that starch is absent in leaves before the start of the experiment, so that if starch is formed in leaves at the end of the experiment, photosynthesis must have taken place.
3. Use a leaf and sandwich a part of it between two pieces of black paper before exposing it to strong sunlight.
4. After a few hours, remove the leaf and test for starch.

Question 4

How to test whether chlorophyll is necessary for photosynthesis?

Answer 4

1. Destarch a potted plant with variegated leaves by placing it in darkness for two days.
2. Remove one leaf and test it for starch to ensure that starch is absent in leaves before the start of the experiment, so that if starch is formed in leaves at the end of the experiment, photosynthesis must have taken place.
3. Expose plant to strong sunlight.
4. After a few hours remove leaf and test for starch.

Question 5

How to test whether carbon dioxide is necessary for photosynthesis?

Answer 5

1. Destarch two potted plants by placing them in darkness for two days.
2. Remove one leaf from each and test it for starch to ensure that starch is absent in leaves before the start of the experiment, so that if starch is formed in leaves at the end of the experiment, photosynthesis must have taken place.
3. Enclose the pots in polythene bags to prevent carbon dioxide released by respiration of microorganisms in the soil from being released to the air.
4. Place one pot in a bell jar with soda lime and potasium hydroxide solution, which removes carbon dioxide from the bell jar
5. Set up a control with pebbles and water instead
6. Expose both plants to strong sunlight
7. After a few hours, remove a leaf from each and test for starch

Question 6

How to test how temperature, carbon dioxide concentration, and temperature affects rate of photosynthesis?

Answer 6

1. Assume that rate of oxygen produced, and rate of bubble production, is directly proportional to the rate of photosynthesis.
2. Place aquatic plant with the cut end facing upwards in dilute sodium bicarbonate solution in a boiling tube, place boiling tube in water bath, place lamp facing plant.
3. Depending on the independent variable, vary the temperature of water bath, concentration of sodium bicarbonate solution, or distance of lamp from plant, keeping the rest constant.
4. Allow some time for plant to adapt to the conditions before taking readings.
5. Count number of bubbles over a period of 5 minutes and take average.
6. Record results in a table then plot graph.

Question 7

How to study the path food takes through a plant?

Answer 7

1. Using aphids in studying translocation
2. Using radioisotopes like carbon-14 in studying translocation

Question 8

How to measure the rate of transpiration?

Answer 8

1. Assume that the mass/volume of water absorbed is equal to the mass/volume of water lost by transpiration.
2. Cut a shoot of plant while it is underwater and immerse in water for a few hours to allow it to adjut to the conditions.
3. Use bubble potometer or weight potometer (basically an electronic balance) which directly measures the rate of absorption of water of the plant.
4. Rate of transpiration is either distance travelled by air bubble per unit time or weight of water lost per unit time.