A-LevelBiologyPhotosynthesis9700/42 · May/June 2021 · Q511 Marks

(a) Photosynthesis is a complex process involving a light dependent stage and a light independent stage. A student carried out an experiment to investigate the effect of light intensity and light wavelength on the rate of photosynthesis. • An aquatic plant, Elodea canadensis, was put into a beaker containing sodium hydrogencarbonate solution as a source of carbon dioxide. • To minimise changes in temperature, an LED lamp was used as a source of light. • The lamp was switched on and the number of bubbles released by the aquatic plant in 1 minute was counted. • The lamp was placed at seven different distances from the beaker to change light intensity. • Five replicates were carried out at each lamp distance. • All other variables were controlled. (b) The student calculated the light intensity for each distance (d) using 1/d² . Table 5.1 shows the calculated light intensities for each distance. distance between plant and lamp/m | light intensity/1/d² ---|--- 0.025 | 1600 0.050 | 400 0.100 | 0.150 | 44 0.200 | 25 0.250 | 16 0.300 | 11 (c) At each distance from the lamp, the experiment was repeated using a red filter in front of the lamp to give a different wavelength of light. The experiment was repeated using a blue filter and then using a green filter. Each filter transmitted the same light intensity. The student calculated the mean rate of bubble production as a measure of the rate of photosynthesis. Fig. 5.1 shows a graph of the results. [Figure 5.1]

(a)(i)Name the products of the light dependent stage that are needed in the light independent stage.[2]
(a)(ii)Describe the role of chlorophyll b in photosynthesis.[2]
(b)Complete Table 5.1 by calculating the light intensity for distance 0.100m.[1]
(c)(i)With reference to Fig. 5.1: • state the range over which light intensity is the limiting factor • explain why light intensity above this range is not limiting the rate of photosynthesis.[3]
(c)(ii)At a light intensity of 1600, explain why different colour filters result in different rates of photosynthesis.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[2] mark first two answers ATP; reduced NADP; A NADPH
[2] any two from: 1 an accessory pigment; 2 absorb, light (energy) / photons; A harvest I trap / capture 3 pass on energy to, primary pigment / chlorophyll a / reaction centre ; 4 AVP; e.g. idea of extending the range of light wavelengths absorbed / absorbs wavelengths not absorbed by primary pigment
[1] 100;
[3] 1 0 – 400 ; A up to 400 I units any two from: 2 temperature / concentration of carbon dioxide, becomes the limiting factor; 3 (at a lower than optimum temperature) less kinetic energy so fewer collisions between rubisco and CO2 or a lower temp will limit the rate of (named) enzyme-controlled reactions ; 4 (with a lower than optimum CO2 concentration) less fixation of CO2 / AW ; 5 all, enzymes / processes, in photosynthesis, already at, highest rate / optimum or maximum amount of light is being absorbed (by the pigments);
[3] any three from: 1 different wavelengths of light absorbed by different pigments ; 2 red light is absorbed the most ; 3 green light is, absorbed less / reflected; 4 shorter the wavelength the greater the energy / red light has more energy (than blue light); ora 5 the more, light / energy, absorbed the more, light dependent reaction / photophosphorylation, occurs ; ora

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