A-LevelBiologyPhotosynthesis9700/42 · Feb/Mar 2019 · Q113 Marks

The effect of light intensity on the rate of photosynthesis can be investigated using a cut shoot of a pond plant. The apparatus used in the investigation is shown in [Figure 1.1]. The light intensity can be changed by placing the lamp at different distances from the pond plant.

(a)(i)Apparatus X, shown in [Figure 1.1], is a thin glass container filled with water. Explain the function of apparatus X.[2]
(a)(ii)Before completing the assembly of the apparatus shown in [Figure 1.1], sodium hydrogencarbonate is added to the water surrounding the pond plant in the test-tube. Explain why sodium hydrogencarbonate is added to the water surrounding the pond plant in the test-tube.[1]
(a)(iii)Name the gas collected in the test-tube.[1]
(b)(i)The investigation was carried out with the lamp at distances of 10, 20, 30, 40 and 50 cm from the pond plant. For each of these distances, the air bubble in the capillary tube was initially positioned at 0cm on the scale and, after 5 minutes, the distance moved by the air bubble was measured. The rate of movement of the air bubble was then calculated. The results are shown in [Figure 1.2]. With reference to [Figure 1.2], describe the relationship between the rate of photosynthesis and light intensity.[2]
(b)(ii)Further investigations showed that at distances of less than 10cm, the rate of movement of the air bubble was the same as at 10cm. Explain why there was no change in the rate of movement of the air bubble at distances less than 10cm.[2]
(c)Some of the light energy absorbed by the pond plant is used in cyclic photophosphorylation. Outline the process of cyclic photophosphorylation.[5]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

[2] any two from: 1 idea of stopping heat (from lamp) reaching plant; 2 (so) temperature does not change; 3 (because) temperature affects, the rate of photosynthesis / enzymes (involved in photosynthesis) ;
[1] to provide carbon dioxide;
[1] oxygen;
[2] as light intensity increases the rate of photosynthesis increases / ora; data quote (two values of rate of movement of air bubble plus two values of distance of lamp from pond, plus units);
[2] light intensity is no longer a limiting factor ; temperature / carbon dioxide (concentration), could be the limiting factor ;
[5] any five from: 1 ref. to (only) photosystem 1 / P700 ; 2 light energy absorbed by, chlorophyll a / primary pigment / reaction centre ; 3 electrons, excited / move to higher energy level ; 4 (electron) emitted by, chlorophyll a / primary pigment / reaction centre; 5 (to) electron, carrier / acceptor ; 6 passes along, electron transport chain / ETC; 7 chemiosmosis / description; 8 (leading to) ATP synthesis ; 9 electron returns to, photosystem 1 / P700/ chlorophyll a / primary pigment / reaction centre ;

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