A-LevelBiologyPhotosynthesis9700/41 · Oct/Nov 2020 · Q113 Marks

Fig. 1.1 is a transverse section through a leaf from the maize plant, Zea mays. Maize is a C4 plant. [Figure 1.1]

(a)On Fig. 1.1, use label lines and letters to show: A – a cell in the epidermal layer B – a cell that contains PEP carboxylase.[2]
(b)(i)Identify the cell type labelled C in Fig. 1.1. C[1]
(b)(ii)Explain how the leaf anatomy shown in Fig. 1.1 adapts the C4 plant to maintain a high rate of photosynthesis at high temperatures.[4]
(c)(i)Fig. 1.2 shows the results of an experiment comparing the rate of carbon dioxide uptake in a C3 plant (Chenopodium album) and a C4 plant (Amaranthus retroflexus) in high and low carbon dioxide (CO2) conditions. The rate of CO2 uptake is used to measure the rate of photosynthesis. [Figure 1.2] Using Fig. 1.2, compare the rates of photosynthesis in high CO2 conditions in the C3 and C4 plants.[4]
(c)(ii)Using Fig. 1.2, compare the rates of photosynthesis in low and high CO2 conditions in the C4 plant between 30°C and 35°C and suggest an explanation for this difference.[2]

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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) A - line to epidermal cell ; B - line to mesophyll cell;
[1] (b)(i) bundle sheath (cell);
[4] (b)(ii) any four from: 1 decrease / stop, photorespiration ; 2 mesophyll cells, surround / protect / form ring round, bundle sheath; 3 air/oxygen, cannot reach, bundle sheath (cells); 4 oxygen cannot, reach / bind to, RuBP / rubisco (in BSC) ; 5 high/much, carbon dioxide (in bundle sheath cells); 6 AVP;
[4] (c)(i) any four from: 1 as temperature increases rate increases, up to 25 °C / at low temperatures, for both; 2 as temperature increases rate decreases, above 35 °C / at high temperatures, for both; 3 C3 rate higher, at low temperatures / < 22 °C ; 4 C3 rate lower, at high temperatures / > 22 °C ; 5 C3, peak rate / maximum rate / optimum T °C, at 25 °C and C4, peak rate / maximum rate / optimum T °C, at 30–35 °C ; 6 C3 has lower, maximum rate / peak; 7 C3, maximum rate / peak, is at lower temperature or C3 has a lower optimum temperature ; 8 comparative data quote ;
[2] (c)(ii) comparison rate, higher/faster, high CO2 conditions ; (at 30–35 °C) rate is maximum for both; explanation (max 1) carbon dioxide, limits rate / is limiting factor, in low CO2 conditions ; AVP;

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