Photosynthesis
Monacia Williams, Contributor

Last week I left you with a challenge. Did you find the answer to it? Was your answer a variegated leaf? If so, then you were correct, because that is the type of leaf we would need to use to determine that chlorophyll is necessary for photosynthesis to take place.

Variegated leaves are those that have both white and green colourations. Two plants with this type of leaf are the Aurelia and the variegated Hibiscus. They are quite common in home gardens. Variegated leaves would be suitable for both the test and the control experiment. Let us test your knowledge now. What area of the leaf would represent the test experiment? What section would represent the control? Was your answer for the test, the white section, and for the control, the green section?

Use diagrams

Another tip that I have for you is this: for this experiment, the best way to present your observations is to use diagrams. You will need two, one of the leaf before the experiment is carried out and the other after the starch test is done, using annotated labelling to connect both diagrams.

In the previous lesson we defined the control experiment. Do you remember the definition? The controlled experiment is a second experiment which differs from the first in one feature only. Why is it necessary to use control experiments,? We do this for comparison to show the effect of one variable or to eliminate variables because biological materials or systems are complex, with many systems operating at the same time. This is why the green section of the leaf can become the control for the chlorophyll experiment, because it differs from the test by one feature only, which is the presence of chlorophyll. It is exposed to the same conditions to which the white section is, since they are both on the same leaf.

The other photosynthesis experiments are:

• To determine that carbon dioxide is necessary. For the test experiment, potassium hydroxide solution is used to remove carbon dioxide. The potassium hydroxide combines with the carbon dioxide in the air and removes it from the air so it is no longer available to the plant.
• To determine that oxygen is produced as a result of photosynthesis. For this experiment, the water plant Elodea is used and the gas is collected in a test tube over an inverted filter funnel. The gas is tested with a glowing splint to confirm that it is oxygen.

If you have been working along with me through the weeks, you would now have more than a working knowledge of photosynthesis and be ready to answer any question on this topic. I will now include a few that you can use to test your knowledge.

Questions:

1. What is the difference between autotrophic and heterotrophic nutrition?

2. Which one comes first and why?

3. State five ways in which leaves are adapted for photosynthesis.

4. When the leaf is decolourised by ethanol, it becomes very brittle. Why does this happen?

5. Why is it necessary to destarch the leaf before the experiment begins?

Answers:

• Autotrophic nutrition is feeding by converting inorganic materials into organic ones, while heterotrophic nutrition is using organic food made by other organisms.
• Autrophic nutrition comes before heterotrophic, because autotrophs make the food that is eaten by heterotrophs.
• Leaves:
  a. Leaves are spread around the stem at right angles to the sun's rays so that they can catch as much light as possible.
  b. Leaves are green from the chlorophyll that they contain. Chlorophyll captures the light energy that is needed for photosynthesis.
  c. Leaves contain xylem vessels which transport water to the palisade cells.
  d. Leaves are thin and flat so sunlight can reach the cells that carry out photosynthesis.
  e. Leaves have stomata, which allow gases to diffuse in and out of them. This means that carbon dioxide can enter and oxygen can leave.
• The leaf becomes very brittle because the ethanol removes water from the leaf as well as chlorophyll.
• The leaf must be destarched in order to remove any starch present prior to the start of the experiment.

I hope you enjoyed this series of lessons. I certainly did! See you next week.

Elated players from Bridgeport High School cheer after winning the Braeton United Sports Club invitational match against Waterford High School at the Ferdie Neita Park, in Braeton, St Catherine, on Saturday, December 27, 2008. Bridgeport won 5-3 on penalities after the teams were locked at 2-2 at the end of regulation time. - Anthony Minott/Freelance Photographer

Monacia Williams teaches at Glenmuir High School.