Light intensity, carbon dioxide, temperature and chlorophyll as limiting factors, reading rate graphs, the inverse square law, and the required practical with pondweed.
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1.What is a limiting factor?
A factor that is in short supply and is therefore holding back the rate of a process, in this case photosynthesis.
2.Name the four limiting factors of photosynthesis.
Light intensity, carbon dioxide concentration, temperature, and the amount of chlorophyll.
3.Describe the effect of increasing light intensity.
The rate increases in proportion at first, then levels off when something else becomes the limiting factor.
4.Describe the effect of increasing carbon dioxide concentration.
The rate increases at first, then levels off once carbon dioxide is no longer the factor in shortest supply.
5.Describe the effect of increasing temperature.
The rate increases up to an optimum, then falls sharply as the enzymes controlling photosynthesis denature.
6.Why does the rate fall at high temperature rather than levelling off?
The enzymes that control photosynthesis are denatured, so they can no longer catalyse the reactions.
7.What can reduce the amount of chlorophyll in a plant?
Disease such as tobacco mosaic virus, a lack of magnesium ions, or damage to the leaves from the environment.
8.On a rate graph, what does a flat line tell you?
That the factor being changed is no longer limiting. Something else is now holding the rate back.
9.If a graph levels off as light increases, name two possible limiting factors.
Carbon dioxide concentration or temperature.
10.Why do greenhouse growers add extra carbon dioxide?
To remove carbon dioxide as the limiting factor, so plants photosynthesise faster and grow more quickly.
11.Why do greenhouse growers use artificial lighting?
To extend the hours of light and increase the intensity, so photosynthesis continues for longer.
12.What must a grower balance when using heaters and lights?
The cost of running them against the extra money earned from a larger or faster crop.
13.What is the inverse square law in this context?
Light intensity is inversely proportional to the square of the distance from the source, so doubling the distance quarters the intensity.
14.Write the inverse square law as a proportion.
light intensity is proportional to 1 divided by distance squared.
15.A lamp is moved from 10 cm to 20 cm from a plant. What happens to the light intensity?
The distance doubles, so the intensity falls to a quarter of what it was.
16.In the required practical on photosynthesis, what organism is used?
Pondweed, usually Cabomba or Elodea.
17.What is the independent variable in that practical?
The distance of the lamp from the pondweed, which changes the light intensity.
18.What is the dependent variable?
The number of oxygen bubbles produced in a set time, or the volume of oxygen collected.
19.Name three control variables for the pondweed practical.
The temperature of the water, the concentration of carbon dioxide in the water, and the same piece of pondweed throughout.
20.Why is sodium hydrogen carbonate added to the water?
It provides a constant supply of carbon dioxide, so that carbon dioxide does not become the limiting factor.
21.How is the temperature kept constant in the practical?
By placing a glass tank of water between the lamp and the pondweed to absorb the heat from the lamp.
22.Why is a heat shield needed at all?
The lamp gives out heat as well as light. Without a shield, moving it closer would change both temperature and light intensity, so you could not tell which caused the change.
23.Why is counting bubbles not a very accurate measure?
Bubbles vary in size, and they are easy to miscount when they come quickly. Collecting and measuring the volume of gas is better.
24.Why should the pondweed be left for a few minutes after moving the lamp?
To let the rate of photosynthesis adjust and settle at the new light intensity before you start counting.
25.How would you make the results of this practical more reliable?
Repeat each distance three times and calculate a mean, ignoring any anomalous results.
26.Why is pondweed used rather than a land plant?
The oxygen it produces forms visible bubbles in the water, which can be counted or collected easily.
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