AQA AS Level Biology Paper 1, November 2021: Question 8

11 marks · Medium difficulty · Practical Techniques & Data Analysis

Analyze an osmosis investigation using unfertilised chicken eggs, evaluating table design, temperature effects, water potential conclusions, and methods to determine water potential.

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Question

Question text

08 An unfertilised chicken egg is a single cell surrounded by a shell.

A student investigated osmosis in chicken eggs. She dissolved the shells of

two eggs without damaging the cell contained inside the shells. She then:

• measured the mass of each egg without its shell

• covered one egg with vinegar and covered the other egg with a sugar solution

• kept both eggs covered at 30 °C for 24 hours.

After 24 hours, she measured the mass of each egg.

The student designed Table 2 and added her results to this table.

Table 2

Ratio of final

Initial mass of Final mass of Name of solution

mass to initial

egg / g egg / g covering egg

mass

66 85 Vinegar 1.29:1

60 43 Sugar 0.7:1

08.1 Suggest one improvement to the design of Table 2 and one improvement to the way

she presented the data contained in Table 2.

[2 marks]

Improvement to design of table

Improvement to presentation of data

08.2 Suggest and explain an advantage of carrying out this investigation at 30 °C rather

than at 20 °C.

[2 marks]

*0208.*3 The student concluded from the information in Table 2 that the water potential of the

solution inside the egg is higher than the water potential of the vinegar.

Is the student’s conclusion correct? Justify your answer.

[3 marks]

08.4 The student wanted to determine the water potential of chicken eggs. She:

• produced a dilution series of sugar solution

• followed the procedure described on page 20.

She calculated the final mass to initial mass ratio of the egg covered in each sugar

solution.

How would you advise the student to use her calculated ratios to determine the water

potential of the eggs?

In your answer state the independent variable in the student’s investigation.

[4 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

08.1 1. Name of solution/independent variable in first Ignore headings lack

column; detail

2. Same number of decimal places in final/column Ignore units required

on right

Ignore percentage

OR change required

1.3 not 1.29 2. Accept same

degree of precision

0.72 not 0.7;

08.2 1. (Warmer water/water at 30 C) has more kinetic

energy;

2. More/quicker osmosis

Large(r) difference in mass (in time available);

08.3 (No) 3. Accept ψ/WP for

water potential

1. Egg mass increased;

2. Water moves in by osmosis; 3. Accept less

3. (So) egg water potential lower (than vinegar negative for

water potential) 3 "higher"

OR OR

Vinegar water potential higher (than egg water "more negative" for

potential); – LOGY –lower – JUNE 2021

08.4 Independent variable

1. Concentration of (sugar) solution; 1. Accept water

potential/dilution

Determining water potential for

"concentration" 15

2. Plot calibration curve

2. Ignore unqualified

OR "plot graph"

Graph of ratio against concentration (of sugar);

3. Interpolate from ratio of 1; 3. Accept

description of

4. Change concentration into water potential; interpolation

4. Accept for

example,

descriptions

using a

table/graph to

find water

potential

TOTAL 11

How to answer it

Investigation of Osmosis Using Chicken Eggs

What this question tests

This question assesses your practical design skills, understanding of osmosis and water potential principles, data processing, and graphical analysis (calibration curves) using biological tissue.

Question 08.1

Table Design and Data Presentation Improvements

✅ Correct Answers

Table Design: Put the name of the solution (independent variable) in the first column.
Data Presentation: Use the same number of decimal places for the ratio column (e.g., write 1.30:1 instead of 1.29:1 , or 0.72:1 instead of 0.7:1 ).

💡 Key Knowledge

Standard data presentation conventions require independent variables on the left side of a data table.
Calculated ratios or raw figures in a data column must maintain a consistent degree of precision (decimal places or significant figures).

❌ Common Errors

Vague table improvement suggestions like "add units to headers" when headers already contain units ( / g ).
Stating that percentage changes should be calculated when the table specifically deals with ratios.

Mark allocation: 2 marks total (1 mark for table design improvement, 1 mark for data presentation improvement).

Question 08.2

Effect of Temperature on Osmosis Rate

✅ Correct Answers

Warmer water/solution at 30 °C has higher kinetic energy.
Results in faster/more osmosis or a larger difference in mass within the available time.

🧠 Exam Technique

Always link temperature changes to kinetic energy and particle movement speed first before describing the rate of transport processes.

❌ Common Errors

Stating that enzymes denature at 30 °C (30 °C is well below denaturation temperatures for biological membranes).
Vague references to "molecules moving faster" without specifying water molecules or kinetic energy.

Mark allocation: 2 marks total (1 mark for kinetic energy mention, 1 mark for faster osmosis or larger mass change).

Question 08.3

Evaluating Water Potential Conclusions

✅ Correct Answers

Conclusion: No, the student's conclusion is incorrect.
Reasoning 1: The egg mass increased, showing water moved in by osmosis.
Reasoning 2: Therefore, the egg's water potential must be lower (more negative) than the vinegar's water potential, OR the vinegar's water potential is higher.

💡 Key Knowledge

Water always moves down a water potential gradient: from a higher (less negative) water potential to a lower (more negative) water potential.
Accept standard abbreviations such as ψ or WP for water potential.

❌ Common Errors

Confusing "higher" and "lower" water potential terms, especially given that water potentials are negative values.
Failing to state "No" at the beginning of the answer and diving straight into the explanation.

Mark allocation: 3 marks total (1 mark for stating "No", 1 mark for noting water moved in due to increased mass, 1 mark for correct relative water potentials).

Question 08.4

Determining Water Potential Using a Calibration Curve

✅ Correct Answers

Independent variable: Concentration of sugar solution (or water potential/dilution).
Plot a calibration curve (graph of ratio of final to initial mass against sugar concentration).
Interpolate from the ratio value of 1.0 (where mass does not change, meaning net osmosis is zero).
Convert that sugar concentration value into its corresponding water potential value using reference data or lookup tables.

📐 Step-by-Step Practical Procedure

Step 1: Identify the independent variable (concentration of sugar solution).
Step 2: Plot the calculated mass ratios on the y-axis against the sugar concentrations on the x-axis to create a calibration curve.
Step 3: Find the point on the curve where the ratio equals 1.0 (indicating no overall net movement of water).
Step 4: Read down the corresponding sugar concentration from the x-axis and convert it to water potential units.

❌ Common Errors

Forgetting to state the independent variable clearly as requested by the prompt.
Saying "extrapolate" instead of "interpolate" when finding the value where the ratio is 1.

Mark allocation: 4 marks total (1 mark for identifying independent variable, 1 mark for plotting calibration curve/graph, 1 mark for interpolating ratio of 1, 1 mark for converting concentration to water potential).

Topics

Biology · Practical skills · 3.2 Cells · Data analysis · Experimental design

Question and mark scheme from the AQA AS Level Biology examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.