At a coastal field centre, students grew green algae in clear alginate beads. Each tube contained the same volume of water with hydrogen carbonate indicator...
Question 1 Report
At a coastal field centre, students grew green algae in clear alginate beads. Each tube contained the same volume of water with hydrogen carbonate indicator. The indicator is red at its starting carbon dioxide concentration, turns yellow when carbon dioxide rises and becomes purple when carbon dioxide falls. Fig. 1 shows the apparatus used beside a window at 20 degrees C. A lamp was switched on for 10 minutes. Tube A contained algae; tube B contained only the indicator solution. The student counted oxygen bubbles leaving tube A during the test.
Fig. 1 Apparatus for investigating gas exchange in algae.
Tube A produced 36 bubbles in 3 minutes and its indicator became purple. Tube B remained red. The algae cells were supplied with mineral ions before the investigation.
(a) Name the gas in the bubbles produced by tube A. [1]
(b) Describe the purpose of tube B in this investigation. [2]
(c) Use the bubble count to calculate the mean number of bubbles produced each minute. [2]
(d) Suggest two variables, other than light, that should be kept the same if another student compares different masses of algae. [2]
(e) Describe how the changes in the indicator provide evidence that photosynthesis occurred in tube A. [3]
Answer Details
- (a) The bubbles contain oxygen, a product of photosynthesis. [1]
- (b) Tube B is a control because it contains no algae. Its unchanged red colour shows that any colour change in tube A was caused by the algae, not by the water, tube or lamp. [2]
- (c) \[\frac{36\text{ bubbles}}{3\text{ min}}=12\text{ bubbles min}^{-1}\] Mean bubble production is 12 bubbles per minute. [2]
- (d) Keep, for example, temperature and the volume of indicator solution or water the same. Starting carbon dioxide concentration, lamp distance, exposure time and number or mass of algae beads are also valid controls. [2]
- (e) The algae removed carbon dioxide from the water. Carbon dioxide is a raw material for photosynthesis. The fall in carbon dioxide made the hydrogen carbonate indicator turn purple; the oxygen bubbles also provide evidence that photosynthesis occurred. [3]
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