In an investigation to find the water potential of onion tissue, some students used epidermal tissue from the storage leaves of a red onion. The vacuoles of the cells in this tissue contain a red pigment. The students researched one method to use in this investigation. The first step in this method is to find the concentration of sucrose solution at which 50% of the cells in the epidermal tissue are plasmolysed and 50% are not plasmolysed. The students decided to use sucrose solutions of different molar concentrations in their investigation and record the effect on the number of cells plasmolysed in the onion epidermis. The students then investigated the water potential of two other tissues that have epidermal cells with coloured pigment in their vacuoles. For each of the three tissues investigated, the students determined the concentration of sucrose solution in which 50% of the cells were plasmolysed. All three investigations were carried out at 20 °C. Table 1.1 shows these results. Table 1.1 type of tissue concentration of sucrose solution in which 50% of the cells were plasmolysed/moldm⁻³ water potential of the tissue /MPa leaf epidermis of red onion 0.35 −0.85 leaf stalk epidermis of rhubarb 0.50 −1.22 petal epidermis of antirrhinum 0.44 −1.07

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

1. [2] independent variable: concentration / molarity, of sucrose solution ;
2. dependent variable: number of, plasmolysed / non-plasmolysed, cells ;
3. [1] (weigh / AW) 85.5 g sucrose and make up to 250 cm³ with, (distilled / deionised), water;
4. [2] range of at least 5 concentrations from 0 to 1.0 mol dm⁻³ sucrose at (equidistant) intervals ;
5. shows proportions of (distilled / deionised) water and 1.0 mol dm⁻³ sucrose to make up a volume of 20 cm³ of at least two different concentrations ;
6. [1] idea that pigment makes it much easier to, see cells / observe changes in vacuole size / judge plasmolysis ;
7. [8] any eight from:
8. 1 ref. to method of making epidermal strips (from onion leaves); e.g. tweezers / fingers / scalpel / needle
9. 2 ref. to cutting epidermal strips to a standard size / ref. to standardising sample size of epidermal cells ;
10. 3 ref. to at least one, strip / sample of epidermal cells, into different concentration solutions (in container or on a slide);
11. 4 ref. to making sure, strips / epidermal cells, are completely immersed in the solutions ;
12. 5 ref. to leaving for same period of time ;
13. 6 ref. to mounting on a slide in (appropriate) sucrose solution (with coverslip);
14. 7 ref. to using a (light) microscope to count cells ;
15. 8 ref. to counting, large number, of cells / ref. to making two counts from same strip / ref. to counting cells in two fields of view;
16. 9 ref. to counting / recording number of plasmolysed cells and number of non-plasmolysed / turgid cells AW ;
17. 10 ref. to minimum of two replicates or counting three different strips from the same solution and taking a mean;
18. 11 ref. to calculation of percentage by multiplying proportion of plasmolysed cells by 100;
19. 12 ref. to low / medium risk ;
20. [2] plot a graph of concentration of sucrose solution against percentage plasmolysis;
21. draw a line at 50% plasmolysis and read off the concentration ;
22. OR
23. find concentration above and below / closest to those that give a percentage plasmolysis of 50% ;
24. make solutions in this range and repeat experiment to find the concentration giving 50% plasmolysis;
25. [2] 1 red onion (epidermis cells) (-0.85) and antirrhinum (epidermis cells) (−1.07) cells will both lose water and because the water potential / MPa is, less negative / higher/ greater, than the solution or the solution water potential, is more negative / lower / less, than the cells ;
26. 2 rhubarb (epidermis cells) (−1.22) would gain water and because the water potential / MPa is, more negative / lower/lesser than the solution or the solution water potential is, less negative / higher / greater than the cells ;