A-LevelBiologyPhotosynthesis9700/42 · May/June 2018 · Q78 Marks

Fig. 7.1 shows part of a section through the leaf of a C4 plant such as maize. The letters A, B and C show three types of cell found in the leaf. [Fig. 7.1]

(a)Complete Table 7.1 by using the letters A, B or C from Fig. 7.1 to show the location of several compounds associated with photosynthesis in C4 plants. You may use A, B and C once, more than once, or not at all. [Table 7.1][3]
(b)Explain why the cells in C form a tight ring around the cells in B.[2]
(c)Several enzymes have a role in the light independent stage of photosynthesis. Fig. 7.2 shows the activity of one of these enzymes at different temperatures for a C3 plant, Pisum sativum, and a C4 plant, Amaranthus hypochondriacus. [Fig. 7.2] Describe the differences between the two curves and suggest explanations for these differences.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[3] compound location PEP carboxylase C RuBP B rubisco B
[2] any 2 of: 1 stop, air / oxygen, getting to cells B / rubisco / RuBP / bundle sheath cells ; 2 so rubisco does not catalyse reaction of oxygen with RuBP ; 3 reduce / stop, photorespiration ;
[3] any 3 of: describe 1 (enzyme in) Amaranthus / C4, peaks, at higher temperature or (enzyme in) Amaranthus / C4, has higher optimum temperature or (enzyme in) Pisum / C3, has higher activity at low(er) temperatures ; ora 2 data quote ; e.g. C4 – 45 °C and C3 – 30 °C at 100% or C4 - 70% and C3 – 25% at 50 °C; explain 3 enzyme in, Pisum / C3, less thermostable / denatures at lower temperature; ora 4 AVP; e.g. Amaranthus adapted to a hot climate / ora e.g. enzyme in Amaranthus has more S-S bonds

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.