AQA A-Level Biology Paper 1, June 2025: Question 6

10 marks · Medium difficulty · Practical Techniques & Data Analysis

Explain haemocyanin oxygen dissociation at different temperatures, describe colorimetric calibration curve production, calculate crab running distance from a graph, and evaluate a line of best fit.

Practise this question

Question

Question text

06.1 Crab blood contains the respiratory pigment haemocyanin.

Figure 3 shows the dissociation curve for oxyhaemocyanin in a species of crab at two

temperatures.

Figure 3

Crabs use gills for gas exchange.

Use Figure 3 to explain how the increase in temperature affects the ability of

haemocyanin to load or unload oxygen at the gills and in tissues.

Refer to high pO2 and low pO2 in your answer.

[2 marks]

06.2 Figure 4 shows the colour of haemocyanin with increasing concentrations of oxygen.

Figure 4

A calibration curve can be used to determine the percentage oxygen saturation of

*14haemocyanin in a sample of crab blood.*

You are given samples of crab blood in which the percentage oxygen saturation of

haemocyanin is known.

Use all the information provided to:

• describe how you could produce a calibration curve for the percentage oxygen

saturation of haemocyanin in crab blood

• identify one control variable used in the procedure to obtain data for the calibration

curve.

[3 marks]

Description

One control variable

06.3 Ghost crabs live on sandy beaches.

Ghost crabs can run for a long time before they get tired and have to stop.

A scientist calculated the mean time ghost crabs could run for at different speeds

before they stopped.

Figure 5 summarises the scientist’s results.

Figure 5

A ghost crab running at 0.14 m s–1 covered a distance of 504 m in 1 hour.

Use Figure 5 to calculate the extra distance this crab could cover if it continued

running at 0.14 m s–1 before it stopped running.

Show your working.

[2 marks]

Extra distance17 m

06.4 Figure 6 shows the data points from individual crabs that were used to plot the curve

in Figure 5.

Figure 6

Figure 6 not reproduced here due to third-party copyright restrictions.

The figure is adapted from Figure 1 in

https://journals.co.za/doi/pdf/10.10520/AJA00445096_586

The scientist selected the crabs at random from one sandy beach. Before the

investigation, the crabs were equally rested and fed identical diets.

Use information in Figure 6 to suggest why the line of best fit in Figure 5 is not a

good indicator of the time ghost crabs can run for.

[3 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. At low pO2/in tissues, Hc unloads more O2 at Accept pO2 figures

high(er) temperature/15oC below 7 for ‘low pO ’

Accept pO2 figures

OR above 7 for ‘high pO2’

1 Reject references at

At low pO2/in tissues, Hc (O2) affinity decreased low pO2/tissues to Hc

at high(er) temperature/15oC; being loaded

1. Accept reference at

2. At high pO2/in gills, temperature (change) has no 2 low pO2/tissues to

06.1 effect on (O2) loading (2 x unloading less

AO2) readily/has high(er)

affinity at low(er)

OR temperature/ 4oC

1. Accept ‘more

At high pO2/in gills, temperature (change) has no readily’/quicker’ for

effect on (changing) Hc (O2) affinity; ‘more oxygen’

2 Reject references at

high pO2/gills to Hc

being unloaded

1. Use colorimeter/colorimetry; 1. Reject calorimeter

2. Plot absorbance/transmission against 2. Reject if

(percentage oxygen) saturation (and draw absorbance/

line/curve of best fit); 3 transmission is plotted

06.2 (3 x on x-axis

(Control variable) AO1)

2. Accept oxygen

3. Volume (of samples) concentration for

– A-LEVEL BIOLOGY – 7402/1 –

‘saturation’

3. Ignore ‘amount’

Temperature

OR 3. Accept (same)

wave length/colour (of

Filter (in colorimeter); light) for ‘filter’

Correct answer of 403/403.2 (m) = 2 marks;;

1.8(h)/108(mins) (correct reading from the Figure)

= 1mark

0.8(h)/48(mins)/2880(s) (correct time until tired)

= 1 mark 2

06.3 (2 x

OR AO2)

907/907.2(m) (correct total distance covered)

= 1 mark

Incorrect reading from graph x 0.14 – 504

= 1 mark;

1. Only 3/limited number of speeds (investigated);

2. Accept correct

2. (Crab) number/sample varies at different speeds;

3 max figures for ‘different

06.4 (3 x speeds’

3. Difference in range/spread of (stopping) times at

AO3)

different running speed;

4. Small sample size/few crabs (investigated);

How to answer it

Haemocyanin Dissociation, Colorimetry, & Crab Locomotion

📋 What this question tests

This question assesses biological principles and practical competencies across four core areas:

Oxygen-binding pigments: Interpreting oxygen dissociation curves and explaining how environmental temperature changes loading at gas-exchange surfaces vs unloading in respiring tissues.
Quantitative practical skills (RPA): Outlining the production of a calibration curve using colorimetry and determining appropriate controlled variables.
Kinematics & unit conversions: Extracting graphical data accurately and calculating distance while managing mixed units (hours vs seconds).
Scientific methodology & data evaluation: Critiquing sample sizes, variation, and the validity of lines of best fit.

Part 06.1

Temperature & Oxygen Dissociation Curves

Explaining loading at gills and unloading in tissues

✅ Mark Scheme Model Answer

At low pO₂ / in tissues: Haemocyanin unloads more O₂ at higher temperature (15 °C)
OR haemocyanin affinity for O₂ decreases at higher temperature [1 mark].
At high pO₂ / at gills: Temperature change has no effect on oxygen loading / oxygen affinity [1 mark].

Total: 2 marks (AO2)

💡 Key Knowledge

The curves show oxyhaemocyanin dissociation at 4 °C (solid line) and 15 °C (dashed line):

High pO₂ (>7 kPa, e.g. at gills): Both curves converge at virtually 100% saturation. Haemocyanin is fully saturated regardless of temperature.
Low pO₂ (<7 kPa, e.g. in respiring tissues): The 15 °C curve shifts to the right (lower % saturation at any given pO₂), meaning haemocyanin releases/unloads oxygen more readily when warmer.

🧠 Exam Technique

Always address both scenarios stated in the prompt:

High pO₂ / gills: explicitly link this to loading.
Low pO₂ / tissues: explicitly link this to unloading.

Quote values from the graph if possible (e.g. at 2 kPa, saturation drops from ~88% to ~45% as temperature rises from 4 °C to 15 °C).

❌ Common Misconceptions & Errors

Incorrect location: Stating that haemocyanin "loads oxygen at tissues" or "unloads at gills" (automatic mark loss).
Assuming temperature always reduces loading: Failing to notice that at high pO₂ (gills), saturation remains at ~100% at both temperatures.
Vague language: Writing "temperature affects affinity" without stating direction (decreases affinity) or where it happens.

Part 06.2

Colorimetry & Calibration Curves

Quantifying haemocyanin saturation using absorbance

✅ Mark Scheme Model Answer

Description:

Measure absorbance (or transmission) of known samples using a colorimeter [1 mark].
Plot absorbance / transmission (y-axis) against percentage oxygen saturation (x-axis) and draw a line or curve of best fit [1 mark].

Control Variable (any 1 from): [1 mark]

Volume of crab blood/sample.
Temperature.
Filter / wavelength / colour of light used in colorimeter.

Total: 3 marks (AO1)

🧠 Exam Technique & Axes Rules

X-axis: Known independent variable (percentage oxygen saturation).
Y-axis: Dependent variable measured by the instrument (absorbance or percentage transmission).
Do not reverse the axes: The mark scheme explicitly states: Reject if absorbance is plotted on x-axis .

❌ Critical Errors to Avoid

Spelling: Writing calorimeter (heat measuring) instead of colorimeter (light absorbance). Calorimeter receives 0 marks.
Vague control variables: Writing "amount of blood" — the examiner guidance specifically states Ignore 'amount' . Always specify volume.
Forgetting best fit: Always mention drawing a line or curve of best fit when asked how to construct a calibration curve.

💡 Why Colorimetry Works Here

Figure 4 shows that deoxygenated haemocyanin is colourless, while oxygenated haemocyanin is dark blue. As oxygen saturation increases, the intensity of blue increases, causing higher absorbance of complementary light (e.g. red/orange filter) and lower light transmission.

Part 06.3

Kinematics & Graph Interpretation

Calculating extra running distance before fatigue

📐 Step-by-Step Calculation

Question Context: Running speed = 0.14 m s⁻¹ . Crab has already run for 1 hour and covered 504 m . Calculate the extra distance it can cover before stopping.

Step 1: Read the maximum running time from Figure 5

Locate 0.14 m s⁻¹ on the x-axis and read across to the y-axis:
→ Total running time = 1.8 hours (or 108 minutes) [1 mark for reading or working].

Step 2: Determine remaining (extra) running time

The crab has already run for 1.0 hour:
→ Extra time = 1.8 h − 1.0 h = 0.8 hours

Step 3: Convert extra time to seconds (SI units)

Speed is in metres per second (m s⁻¹), so time must be in seconds:
→ Extra time in seconds = 0.8 × 3600 s = 2880 s

Step 4: Calculate extra distance

Distance = Speed × Time:
→ Extra distance = 0.14 m s⁻¹ × 2880 s = 403.2 m (or 403 m) [1 mark].

Alternative Method (Total Distance − Distance Covered):
Total distance = 0.14 m s⁻¹ × (1.8 × 3600 s) = 0.14 × 6480 s = 907.2 m.
Extra distance = 907.2 m − 504 m = 403.2 m.

❌ Common Calculation Traps

Unit Mismatch: Multiplying speed ( m s⁻¹ ) by time in hours ( 0.8 h ) gives 0.112 m. You must convert hours to seconds (× 3600)!
Misreading the prompt: Calculating the total distance (907.2 m) rather than the extra distance (subtracting 504 m).
Scale reading error: Each major grid line on the y-axis represents 0.5 h; reading 1.8 h incorrectly as 1.7 h or 1.9 h forfeits full marks.

✅ Final Answer

Extra distance = 403.2 m (or 403 m)

2 marks awarded for 403.2 or 403

Part 06.4

Evaluating Data & Lines of Best Fit

Why the line of best fit in Figure 5 is not a good indicator

✅ Mark Scheme Points (Choose any 3)

Limited speeds: Only 3 running speeds investigated (or very few speeds tested) [1 mark].
Unequal sample sizes: The number/sample of crabs varies at different speeds tested [1 mark].
Large variation: Wide difference in range / large spread of stopping times at different speeds [1 mark].
Small sample size: Few crabs / small overall sample size investigated [1 mark].

Total: 3 marks max (AO3)

🧠 Exam Technique for "Evaluate the Line of Best Fit"

Whenever raw data points are compared against a summary line of best fit, scrutinise:

Spread/Error: Are the points tightly clustered around the line, or widely scattered? (Spread indicates low reliability).
Density of x-values: Did they test enough intervals along the x-axis to justify a continuous trend? (Only 3 speeds means interpolations between points are unverified assumptions).
Sample size consistency: Were the same number of organisms tested at each condition?

❌ Examiner Pitfalls

Repeating controlled factors: The question states crabs were "equally rested and fed identical diets" from the "same sandy beach". Suggesting that diet, rest, or beach type caused differences gains no credit.
Vague comments: Writing "it isn't accurate" or "it is anomalous" without referring to spread, range, or sample size.

💡 Top Student Tip

Notice that in Figure 5, the line of best fit has two distinct straight-line segments joined by a sudden angle at 0.17 m s⁻¹. Drawing an abrupt change with only 3 speeds investigated is scientifically flawed because there are no data points between 0.14, 0.17, and 0.24 m s⁻¹ to establish the actual shape of the curve.

Topics

Biology · Practical skills · 3.3 Organisms exchange substances with their environment · Experimental design · Data analysis · Uncertainty and evaluation

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.