A-LevelBiologyPhotosynthesis9700/42 · Oct/Nov 2020 · Q112 Marks

Fig. 1.1 shows a transmission electron micrograph of a chloroplast. [Figure 1.1]

(a)On Fig. 1.1, use label lines and letters to label: A - the storage site of the carbohydrate product of photosynthesis B - the site of the light independent stage.[2]
(b)(i)Name the structure labelled C in Fig. 1.1. C[1]
(b)(ii)Explain how the structure of C is linked to its function.[4]
(c)(i)The anatomy of C4 plants is adapted to allow the rate of photosynthesis to remain high at high temperatures. C3 plants do not have these adaptations and an additional reaction occurs at high temperatures that reduces the rate of photosynthesis. Explain why the reaction that takes place at high temperatures in C3 plants reduces the rate of photosynthesis.[2]
(c)(ii)C4 plants have higher rates of photosynthesis than C3 plants when the ratio of atmospheric oxygen to atmospheric carbon dioxide is high. Fig. 1.2 shows the atmospheric carbon dioxide concentration in the last 50 million years. [Figure 1.2] There is evidence that C4 plants first appeared 30 million years ago. With reference to Fig. 1.2, suggest why C4 plants first appeared 30 million years ago.[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[2] A - label line pointing to starch grain;
[2] B - label line pointing to stroma;
[1] granum / grana ; A stack of thylakoids I thylakoid
[4] any four from:
[4] 1 (stack of) thylakoids ;
[4] 2 (membranes / thylakoids / C) form large / increase, surface area ;
[4] 3 for, (named) pigments / photosystems / light-harvesting clusters ;
[4] 4 for absorption of light energy ;
[4] 5 so, large number of / many, enzymes / ETC / ATP synthase / stalked particles ;
[4] 6 for, light dependent stage / photophosphorylation;
[2] 1 oxygen, combines / reacts, with, rubisco / RuBP ;
[2] 2 less / no, carbon dioxide, combines / reacts, with, rubisco / RuBP ; A less or no carbon fixation
[2] 3 ref. to photorespiration;
[2] ignore refs to denaturation
[3] any three from:
[3] 1 photosynthesis (by C3 plants) is occurring ;
[3] 2 decrease in (atmospheric) carbon dioxide or increase in (atmospheric) oxygen or increase in / high, oxygen to carbon dioxide ratio ;
[3] 3 rubisco favours reaction with oxygen / AW ;
[3] 4 C4 plants have a selective advantage / description;
[3] 5 oxygen acts as a selection pressure ;
[3] 6 AVP; e.g. ref. to mutation in C3 plants

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