Photosynthesis and Plant Nutrition: Question 3
Syllabus 6.1
A student wants to find out how light intensity affects the rate of photosynthesis in a sprig of hornwort, a small aquatic plant. The student seals the hornwort inside a flask of water containing a fixed, generous concentration of sodium hydrogencarbonate solution (an extra dissolved source of carbon dioxide), together with a dissolved-oxygen probe connected to a datalogger that automatically records the percentage of dissolved oxygen in the water. The flask sits in a water bath held at a constant 25°C throughout, and is placed under an adjustable LED panel, set in turn to three different brightness settings. Each brightness setting is left running for ten minutes before the increase in dissolved oxygen is read from the datalogger:
| LED brightness setting | Increase in dissolved oxygen over 10 minutes / % |
|---|---|
| 25% | 2 |
| 50% | 5 |
| 100% | 9 |
(a) Identify the independent variable and the dependent variable in this investigation. [2]
(b) State two variables, other than temperature, that the student has kept constant to make this a fair test. [2]
(c) Describe the relationship shown by the results in the table between the LED brightness setting and the increase in dissolved oxygen. [2]
(d) Explain, in terms of the word equation for photosynthesis, why an increase in dissolved oxygen in the flask indicates that the rate of photosynthesis has increased. [2]
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Worked solution
Part (a): Identifying the variables
The student deliberately changes the LED brightness setting between the three trials, so this is the independent variable. The student then measures how much the dissolved oxygen increases over ten minutes, so this is the dependent variable.
Part (b): Keeping a fair test
To make sure that light intensity is the only thing affecting the results, the student needs to keep several other variables constant, for example: the concentration of sodium hydrogencarbonate solution (so the carbon dioxide supply is not itself a limiting factor that changes between trials), the size or mass of the hornwort sprig used each time, the volume of water in the flask, and the length of time each reading is taken over.
Part (c): Describing the pattern in the results
Reading along the table:
- at a 25% brightness setting, dissolved oxygen increases by only 2% over ten minutes
- at 50% brightness, the increase rises to 5%
- at 100% brightness, the increase rises further, to 9%
So as the LED brightness setting increases, the increase in dissolved oxygen also increases. Because the LED brightness setting controls the light intensity reaching the hornwort, this shows that a higher light intensity leads to a faster rate of photosynthesis.
Part (d): Linking dissolved oxygen to the rate of photosynthesis
The word equation for photosynthesis is:
carbon dioxide + water → glucose + oxygen
Oxygen is a product of this reaction. If the percentage of dissolved oxygen in the flask rises faster in the same ten-minute period, this means the hornwort is producing oxygen at a faster rate. Since photosynthesis is the process making this oxygen, a faster build-up of dissolved oxygen shows that photosynthesis is happening at a faster rate, faster than any oxygen being used up by the plant’s own respiration.
Final answers
- (a) Independent variable: LED brightness setting. Dependent variable: increase in dissolved oxygen over 10 minutes.
- (b) Any two of: hydrogencarbonate concentration, size of the hornwort sprig, volume of water, time period, distance from the LED panel.
- (c) The increase in dissolved oxygen rises (from 2% to 9%) as the LED brightness setting rises (from 25% to 100%).
- (d) Oxygen is a product of photosynthesis, so a faster build-up of dissolved oxygen shows photosynthesis is happening at a faster rate.