The diagram alongside refers to an experiment in which the apparatus was set up with the light source 10 cm away from the plant. After 15 minutes the number of bubbles evolved per minute from the cut stem was recorded. The light source was moved to 20 cm away from the plant, left for 15 minutes and the number of bubbles evolved per minute was again recorded The experiment was repeated with the light source at distances of 40, 60, 80 and 100 cm away from the plant. Plot a graph for the results obtained and answer the following questions.
Figure

(i) From the graph it seems likely that the rate of bubbing per minute at 50 cm would have been
(a) 2.0
(b) 2.5
(c) 3.0
(d) 3.5
(e) 4.0

(ii) The gas produced by the plant during the experiment was
(a) air
(b) oxygen
(c) carbon dioxide
(d) nitrogen
(e) hydrogen

(iii) The gas collected comes due to the breakdown of
(a) glucose
(b) starch
(c) water
(d) air
(e) ATP

(iv) If ice cubes were added to the water, the rate of bubble formation would
(a) stay the same.
(b) increase because more water is added.
(c) decrease because the temperature drops.
(d) decrease because water freezes.
(e) cannot tell from the information given.

(v)
(a) increases because more respiration occurs.
(b) increases because more photosynthesis occurs.
(c) increases because the gas becomes less soluble.
(d) decreases because carbon dioxide acts as limiting factor.
(e) decreases because respiration decreases.

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Solution

(i) (d) 3.5
It can be extrapolated from the graph.

(ii) (b) Oxygen
Oxygen is produced as the by-product of photosynthesis.

(iii) (c) Water
Photolysis or breakdown of water produces oxygen.

(iv) (c) Decrease because the temperature drops
Photosynthesis depends on temperature. With the decrease in temperature, the rate of photosynthesis decreases; thus, liberation of oxygen also decreases.

(v) (b) Increases because more photosynthesis occurs
More photosynthesis occurs because of more carbon dioxide released from sodium bicarbonate; thus, plant ending up releasing more oxygen.

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