Biology 5090/61 — October/November 2019
Cambridge O-Level · Alternative to Practical · worked solutions for every part, with the mark scheme
Topics Experimental Contexts · Use of Techniques, Apparatus and Materials · Observations and Measurements · Analysis, Conclusions and Evaluation · Planning Experiments and Investigations · Microscopy and Biological Drawing
Aquatic plants live in water and exchange gases with the water around them.
Hydrogencarbonate indicator (bicarbonate indicator) can be used to detect changes in the amount of carbon dioxide dissolved in water. It changes colour as follows:
| high concentration of carbon dioxide | atmospheric concentration of carbon dioxide | low concentration of carbon dioxide |
|---|---|---|
| yellow | red | purple |
A student carried out an investigation into the gas exchange of an aquatic plant using hydrogencarbonate indicator.
He took some fresh hydrogencarbonate indicator and bubbled air through it until there was the same level of dissolved carbon dioxide in the indicator as in the atmosphere.
A piece of aquatic plant was placed in test-tube A. of the indicator was poured into the test-tube to cover the plant. The test-tube was then sealed with a bung.
Name a suitable piece of apparatus the student could use to measure of indicator.
______
A similar piece of the same aquatic plant was placed in test-tube B and covered with the same volume of indicator. This test-tube was sealed with a bung.
Aluminium foil was then wrapped around test-tube B to prevent any light from entering.
Both test-tubes were placed in a well-lit location and left for 45 minutes.
The arrangement is shown in the diagram below.
After 45 minutes the student removed the aluminium foil from test-tube B.
He observed the colour of the indicator in both test-tubes and recorded his results in the table below.
| test-tube A | test-tube B | |
|---|---|---|
| colour | purple | yellow |
Describe what has happened to the concentration of carbon dioxide in test-tubes A and B.
test-tube A ______
test-tube B ______
Suggest an explanation for the colour changes observed.
test-tube A ______
test-tube B ______
Explain why a bung was added to test-tubes A and B.
______
Some students investigated the effect of light intensity on the rate of photosynthesis. A piece of the stem of the aquatic plant was placed in a beaker with the cut end uppermost and covered with water.
In a dark room, a lamp was used to shine light on it.
Bubbles of gas were seen coming out of the cut end of the stem as the plant photosynthesised.
This apparatus is shown in the diagram below.
They left the plant for five minutes. After five minutes they counted the number of bubbles of gas given off in one minute.
They varied the light intensity by moving the lamp to different distances from the plant, and repeated the process.
Explain why the students placed the apparatus in a dark room.
______
State one factor that the students should control in this investigation. Explain how the students could control this factor.
factor = ______
explanation = ______
Explain how the students could ensure that the result for each light intensity was reliable.
______
Explain why the plant was left for five minutes at each light intensity before counting bubbles.
______
The results of the students' investigation are shown in the table below.
| light intensity / arbitrary units | rate of photosynthesis / bubbles per minute |
|---|---|
| 4 | 6 |
| 7 | 10 |
| 11 | 14 |
| 16 | 16 |
| 28 | 18 |
| 50 | 19 |
Construct a line graph of the data in the table on the grid below. Join your plotted points with a smooth curve.
Use your graph to find the rate of photosynthesis at a light intensity of 20 arbitrary units.
Show your working on your graph.
______ bubbles per minute
Describe the effect of increasing light intensity on the rate of photosynthesis in this investigation.
______
Suggest an explanation for the shape of the graph above a light intensity of 28 arbitrary units.
______
The students found it difficult to count bubbles accurately and thought that some bubbles were bigger than others.
Suggest a modification to the investigation that would allow the volume of gas produced to be recorded.
______
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