A-LevelBiologyTransport in plants9700/42 · May/June 2025 · Q612 Marks

Guard cells are located in the epidermis of the leaves of most plants. When environmental conditions change, this causes changes in guard cells that control the opening and closing of stomata.

(a)Describe the structure of guard cells.[4]
(b)(i)Stomata have daily rhythms of opening and closing. Fig. 6.1 shows the percentage of stomata open at different times of the day, over a period of three days, in the thale cress plant, Arabidopsis thaliana. [Figure 6.1] With reference to Fig. 6.1, describe the rhythm of stomatal opening and closing.[2]
(b)(ii)Suggest other environmental factors, apart from the time of day, that can contribute to stomatal closure.[2]
(c)Table 6.1 shows some of the events occurring during the closure of a stoma. The events are not listed in the correct order. [Table 6.1] Complete Table 6.2 to show the correct order of the events shown in Table 6.1. Three of the events have been completed for you. [Table 6.2][4]

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 2025). The mark or mark type is shown in brackets.

[4] (a) any four from: 1 variable thickness of cell wall described; 2 no plasmodesmata (between guard cell and other epidermal cells); 3 many, chloroplasts / mitochondria ; 4 chloroplasts have few grana ; 5 mitochondria have many cristae ; 6 cell surface membrane, often folded / contains many transport proteins; 7 AVP; e.g. cellulose microfibrils arranged in bands / shape description has several small vacuoles nucleus is relatively larger than in mesophyll cells
[2] (b)(i) 1 (more) stomata open at (mid)day and (more) stomata closed, at (mid)night / in darkness ora; 2 data quote – two values of time of day and percentage of stomata open ;
[2] (b)(ii) any two from: 1 water stress / drought ; 2 high temperature ; 3 low humidity / dry (atmosphere); 4 high carbon dioxide concentration in leaf (air spaces); 5 low light intensity; 6 high wind speed ;
[4] (c) correct order: 1 D, 2 E, 3 G, 4 B, 5 F, 6 J, 7 I, 8 A, 9 H, 10 C. E G B between D and F ; EGB in correct order throughout table; JIA H between F and C; JIA H in correct order throughout table ;

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.