A-LevelBiologyPhotosynthesis9700/41 · May/June 2021 · Q112 Marks

Photosynthesis is a complex process involving the transfer of light energy into chemical energy. The rate of photosynthesis is affected by many environmental factors. A student carried out an experiment to investigate the relationship between the concentration of carbon dioxide available to an aquatic plant, Cabomba caroliniana, and its rate of photosynthesis. Fig. 1.1 shows the experimental set-up for this investigation. [Figure 1.1] Sodium hydrogencarbonate solution was used as a source of carbon dioxide. The concentration of carbon dioxide was varied using six different concentrations of sodium hydrogencarbonate solution. All C. caroliniana plants were kept in the dark before the light was switched on at the start of the experiment. Five replicates were carried out at each concentration. The rate of photosynthesis was obtained by calculating the percentage change in dissolved oxygen concentration in the solution over five minutes. Fig. 1.2 shows the results of the investigation. [Figure 1.2]

(a)(i)Describe the role of photosynthetic pigments.[2]
(a)(ii)Name the precise location in a chloroplast of photosynthetic pigments.[1]
(a)(iii)Name a practical technique to separate photosynthetic pigments.[1]
(b)(i)With reference to Fig. 1.2, explain the pattern of results obtained between 0.25 moldm⁻³ and 1.25moldm⁻³ of sodium hydrogencarbonate solution.[3]
(b)(ii)The percentage change in dissolved oxygen for C. caroliniana at 0.00 moldm⁻³ of sodium hydrogencarbonate solution is negative. Suggest reasons for this negative value.[2]
(b)(iii)To minimise temperature changes, the student decided to use an LED lamp as a light source. LED lamps release very little heat energy. Explain the importance of minimising temperature changes in this experiment.[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.

[1] absorb / harvest / capture / trap, light / photons ;
[1] (to) excite electrons ;
[1] (for, cyclic /non-cyclic) photophosphorylation / electron transport chain ;
[1] accessory pigment passes energy to, primary pigment / reaction centre ;
[1] (pigments) form a, light harvesting cluster / photosystem / PSI / PSII ;
[1] thylakoid membrane(s) / granum / grana / lamella(e);
[1] chromatography;
[1] (from 0.25) to 1.00 (mol dm-³), concentration / CO2, is limiting factor or at/>, 1.00 (mol dm-³), concentration / CO2, no longer limiting factor ;
[1] at />, 1.00 (mol dm-³), light intensity / temperature, is limiting factor ;
[1] CO2, needed / used, for Calvin cycle / for light independent stage or CO2, reacts with / fixed by, RuBP ;
[1] little/ no, photosynthesis ;
[1] (aerobic) respiration uses oxygen ;
[1] to control a variable / to ensure a fair test / so CO2 is the only variable;
[1] low, temperature / kinetic energy, decreases, rate / reaction / photosynthesis / O2 production / O2 increase ;
[1] low, temperature / kinetic energy, decreases, (named) enzyme-substrate, collisions / complexes ;
[1] low(er) temperature, is a limiting factor / may limit rate ;
[1] high, temperature / kinetic energy, decreases, rate / reaction / photosynthesis / O2 production / O2 increase ;
[1] high, temperature / kinetic energy, denatures (named) enzyme(s);
[1] high/increased, temperature, causes / increases, photorespiration ;

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