A-LevelBiologyCell membranes and transport9700/21 · May/June 2021 · Q37 Marks
Visking tubing can be used to investigate the properties of cell membranes. A student carried out an experiment to investigate osmosis using Visking tubing. An outline of the investigation is shown in Fig. 3.1. [Figure 3.1]. Six pieces of Visking tubing were filled with 10cm³ of different concentrations of sucrose solution: 0.0, 0.4, 0.8, 1.2, 1.6 and 2.0moldm⁻³. The height of the meniscus of each solution in the Visking tubing was measured. The pieces of Visking tubing were put into test-tubes containing 15cm³ of 0.9moldm⁻³ sucrose solution. After 20 minutes, the pieces of Visking tubing were removed from the test-tubes and the height of the meniscus in each was measured. The results are shown in Table 3.1. Table 3.1: concentration of sucrose solution inside Visking tubing /moldm⁻³ and difference in height of meniscus after 20 minutes/mm. Data: 0.0 / -12, 0.4 / -4, 0.8 / -2, 1.2 / +1, 1.6 / +6, 2.0 / +11.
- (a)The Visking tubing used by the student was not permeable to sucrose. Explain the results shown in Table 3.1.[3]
- (b)When red blood cells are placed in water they are destroyed by bursting. The student also investigated how red blood cells are affected by immersion in solutions of sodium chloride of different concentration. Blood samples of the same volume were added to solutions of sodium chloride in separate test-tubes. After 10 minutes, the student took 0.1cm³ of the blood samples from the test-tubes and estimated the percentage of red blood cells that had burst in each blood sample. Fig. 3.2 shows the student's results. [Figure 3.2]. Describe and explain the effects on red blood cells of immersion in different concentrations of sodium chloride as shown in Fig. 3.2.[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 2021). The mark or mark type is shown in brackets.
- [3] any three from
- 1 differences in height show that
- concentrations of sucrose, 0, 0.4 and 0.8 (mol dm⁻³) or < / less than, 0.8/0.9, water moves out of Visking tubing;
- A one of 0, 0.4 or 0.8 R sucrose moving
- 2 concentrations of sucrose 1.2, 1.6, 2.0 (mol dm⁻³) or ≥ / more than, 0.9/1.2, water moves into Visking tubing;
- A one of 1.2, 1.6 or 2.0 R sucrose moving
- 3 ref. to net water movement;
- if water enters Visking tubing – A ora for water leaving
- 4 external solution has higher water potential (than contents of Visking tubing) ;
- A high water potential to low water potential
- 5 water moves, down, water potential/ Ψ, gradient ;
- [4] mark whole question to a max of four marks
- descriptions
- 1 at concentrations, less than / <, 0.04 mol dm⁻³ all cells, burst / AW;
- 2 at concentrations between 0.04 and 0.14 mol dm⁻³ decreasing percentage of cells burst / AW ; A use of percentages
- 3 at concentrations, greater than / >, 0.14 mol dm⁻³ no cells, burst / AW;
- explanations to max 3
- 4 in low concentrations / ≤ 0.04, of sodium chloride water moves into cells down water potential gradient / from high Ψ to low Ψ ;
- 5 cells increase in, volume / size / internal pressure;
- 6 either
- cell membranes are not strong enough to withstand increase in volume / pressure
- or
- red blood cells burst because they have no cell wall;
- 7 between 0.04 and 0.14 (mol dm⁻³) water potential gradient into cells, decreases / becomes less steep / AW ;
- 8 above 0.14 (mol dm⁻³) / in high concentrations, water potential inside cells is the same or higher than the sodium chloride solution ;
- 9 at high concentration / ≥ 0.14, water leaves cells / cells shrink / cells shrivel / cells show crenation ;
Write your answer and get it marked
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.
Sign up free →