A-LevelBiologyTransport in plants9700/41 · Oct/Nov 2024 · Q211 Marks

The leaves of many plants have stomata that show a regular daily rhythm of opening and closing over a period of 24 hours.

(a)Explain why it is important for plants to open and close their stomata in a daily rhythm.[3]
(b)Fig. 2.1 shows the results of an experiment to monitor this rhythm over three days and nights for the plant Arabidopsis thaliana. The percentage of open stomata is shown. Each day consisted of 14 hours of light (white bar) and each night consisted of 10 hours of darkness (black bar). Fig. 2.1 shows that the percentage of open stomata increases in the first seven hours of the experiment. Describe the sequence of changes that occurs in the guard cells that leads to the stomata opening.[6]
(c)The experiment was repeated with A. thaliana plants that were left in darkness from 14 to 96 hours. The results are shown in Fig. 2.2. With reference to Fig. 2.1, explain what Fig. 2.2 shows about the role of genes and the role of the environment in controlling the rhythm of stomatal opening and closing in A. thaliana.[2]

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 (Oct/Nov 2024). The mark or mark type is shown in brackets.

[3] any three from:
1 open in, day / light;
2 to take in carbon dioxide for, photosynthesis / light-independent stage / Calvin cycle ;
3 close in, night / dark;
4 to decrease water loss by transpiration;
[6] any six from:
1 hydrogen ions / protons / H ions / H⁺, leave (cell) using, energy / ATP ;
2 low H+ (in cell) / more negative charge;
3 K⁺/ potassium ion, channel (proteins) open OR K⁺, move into / enter, cell (by facilitated diffusion);
4 Cl⁻ ions, move into / enter (cell);
5 water potential of cell decreases ;
6 water moves in (to cell) by osmosis ;
7 (cells) become turgid / swell / increase in volume / expand ;
8 (guard cell) inner wall thicker / outer wall thinner, so cells, bend / curve ;
[2] 1 (environment) light needed, to keep cycle normal / to achieve same peak % of stomata opening ;
2 genes have a role as (regular), rhythm / pattern / cycle, continues, in darkness / without light;

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