AQA A-Level Biology Paper 1, June 2024: Question 3

10 marks · Medium difficulty · Short Answer

Describe the mechanism of breathing in, explain the need for thin tissue slices when using an optical microscope, identify a labeled tube from lung tissue, calculate mean alveolar diameter in micrometres given image magnification and measurements, and calculate percentage error from ruler measurements.

Practise this question

Question

A multi-part exam question starting with 03.1 asking to describe how we breathe in (3 marks). 03.2 asks to explain why very thin slices of alveolar tissue are used with an optical microscope (2 marks). 03.3 features Figure 2 showing lung tissue with a pointer to tube A and alveolar epithelium, asking to identify tube A (1 mark), and includes Table 2 giving eight measurements of alveolus diameter in mm. 03.4 asks to calculate the mean diameter in micrometres using magnification x40 and Table 2 (2 marks). 03.5 asks for the uncertainty of a ruler with 1 mm graduations and the percentage error for a 4 mm measurement (2 marks).
Question text

03.1 Describe how we breathe in.

[3 marks]

03.2 A scientist prepared alveolar tissue to view using an optical microscope. The scientist

cut very thin slices of the alveolar tissue.

Explain why the scientist used very thin slices of alveolar tissue with the

optical microscope.

[2 marks]

03.3 Figure 2 is an image of the lung tissue observed using an optical microscope.

Figure 2

Identify the tube labelled A.

[1 mark]

A

The scientist used a ruler to measure the diameter of some of the alveoli.

Table 2 shows the scientist’s results.

Table 2

Alveolus diameter / mm

Alveolus diameter / mm 4 2 5 1 2 3 5 2

03.4 The magnification of the image in Figure 2 is × 40

Use this information and Table 2 to calculate the mean diameter, in μm, of the alveoli.

Show your working.

[2 marks]

10 Answer μm

03.5 Give the uncertainty associated with taking a measurement using a ruler with

1 mm graduations.

Calculate the percentage error for a measurement using the ruler of 4 mm

[2 marks]

Uncertainty ± mm

Percentage error

Mark scheme

Show the mark scheme Mark scheme detailing accepted answers for questions 03.1 through 03.5. For 03.1, points include diaphragm contraction, external intercostal muscle contraction, and volume/pressure changes. For 03.2, mentions single/few cell layers for light transparency. For 03.3, identifies bronchiole(s). For 03.4, shows calculation steps arriving at 75 micrometres. For 03.5, gives uncertainty as 1 mm and percentage error as 25 percent.

Question Marking Guidance Mark Comments

1. Diaphragm (muscle) contracts pulling diaphragm 1. Accept flattening

down; OR moves down for

‘pulling diaphragm

2. External intercostal muscles contract

3 down’

pulling/moving ribs upwards/outwards;

03.1 (3 x 2. Accept ribs lifted for

3. (Causes) volume increase and pressure AO1)

‘pulling ribs upwards’

decrease in thoracic cavity;

3. Accept thorax OR

lung(s) for thoracic

cavity

1. (Create a) single/few layer(s) of cells/tissue; 2 1. Accept to avoid

03.2 overlapping cells

2. (So) light can pass through; (2 x

AO1)

Bronchiole(s); Accept broncheole(s)

OR bronchiol(les) OR

broncheol(les) OR

bronkiol(les) OR

1 bronkeol(les) OR

03.3 brochiol(les) OR

(1 x

brocheol(les)

AO1)

Reject bronchiolus

OR broncheolus OR

OR branchiol(les) OR

brancheol(les) OR

bronchus OR bronchi

Correct answer for 2 marks, 75 (μm);;

Accept for 1 mark, evidence of

Image

Real/actual =

Mag(nification)

03.4 (correct rearranged magnitude word equation)

(2 x

OR AO2)

÷ 40 (correct denominator from equation)

OR

– A-LEVEL BIOLOGY – –

3000 (μm) (correct mean diameter)

OR 7

× 1000 (correct conversion of mm to μm)

OR

0.075 (correct calculation with incorrect units (mm))

OR

Answer shows correct number but incorrect

number of decimal places eg 0.75 / 7.5 / 750

1. (Uncertainty ±) 1 (mm); 2. Accept, if incorrect

uncertainty

2. (Percentage error) 25 (%);

÷ 4 and multiplied by

OR – for example

2 50% (if uncertainty is

03.5 2)

(2 x

AO2) OR

12.5% (if uncertainty

is 0.5)

OR

2.5% (if uncertainty is

0.1)

How to answer it

A-Level Biology Study Guide: Gas Exchange, Microscopy & Calculations

What this question tests

This question assesses your understanding of the mechanics of breathing (ventilation), preparation techniques for optical microscopy, identification of lung tissue structures, application of the magnification formula with unit conversions, and the calculation of experimental uncertainty and percentage errors.

Question 03.1: Mechanics of Breathing

Describe how we breathe in. [3 marks]

✅ Correct Answer / Mark Scheme

  • 1. Diaphragm muscles contract, causing it to flatten / move down.
  • 2. External intercostal muscles contract, pulling/moving ribs upwards and outwards.
  • 3. This causes volume in the thoracic cavity to increase and pressure to decrease below atmospheric pressure.

💡 Key Knowledge

  • Pressure Gradients: Air always moves down a pressure gradient from high (atmosphere) to low (lungs).
  • Antagonistic Muscles: Diaphragm and intercostal muscles work to alter thoracic volume.

🧠 Exam Technique

  • Ensure you sequence the events logically: Muscle contraction → skeletal movement → volume/pressure changes.
  • Always specify external intercostal muscles (internal are used in forced expiration).

❌ Common Errors

  • Saying "lungs expand" rather than stating the thorax volume increases.
  • Confusing ribs moving "up and out" with "down and in".
  • Failing to link volume increase directly to a decrease in pressure.

Question 03.2: Microscopy Preparation

Explain why the scientist used very thin slices of alveolar tissue with the optical microscope. [2 marks]

✅ Correct Answer / Mark Scheme

  • 1. To create a single or few layers of cells/tissue.
  • 2. So that light can pass through the specimen.

💡 Key Knowledge

  • Optical microscopes rely on visible light passing through a specimen to form an image. If tissue is too thick, light is blocked or scattered, rendering the image unclear.

🧠 Exam Technique

  • Link your structural point (thin slice) directly to the optical requirement (light transmission) to secure both marks.

❌ Common Errors

  • Vague answers like "so you can see it clearly" without mentioning light or cell overlapping.

Question 03.3: Tissue Identification

Identify the tube labelled A in Figure 2. [1 mark]

✅ Correct Answer / Mark Scheme

  • Bronchiole(s) (Accept phonetic spellings like bronchioles or bronchiol).

❌ Common Errors

  • Writing "bronchus" or "bronchi" (incorrect scale and location).
  • Writing "alveolus" (label A clearly points to the conducting airway tube, not the air sacs).

Question 03.4: Magnification Calculation

Use the information and Table 2 to calculate the mean diameter, in um, of the alveoli. Show your working. [2 marks]

📐 Step-by-Step Calculation

  • Step 1: Calculate the mean image diameter from Table 2.
    Readings: 4, 2, 5, 1, 2, 3, 5, 2 mm
    Sum = 24 mm
    Mean = 24 / 8 = 3 mm
  • Step 2: Convert mm to um.
    3 mm × 1000 = 3000 um
  • Step 3: Apply the magnification formula ( Actual = Image / Magnification )..
    Actual Size = 3000 / 40 = 75 um

✅ Final Answer

Answer: 75 um

Mark breakdown: 2 marks for 75; 1 mark for correct evidence of formula rearrangement or calculating mean image size (3000 um) or correct conversion steps.

❌ Common Calculation Traps

  • Forgetting to convert millimetres (mm) to micrometres (um), resulting in an answer of 0.075 mm.
  • Incorrectly counting the number of values in Table 2 when finding the mean.

Question 03.5: Uncertainty and Percentage Error

Give the uncertainty associated with taking a measurement using a ruler with 1 mm graduations. Calculate the percentage error for a measurement using the ruler of 4 mm. [2 marks]

📐 Step-by-Step Calculation

  • Step 1: State the absolute uncertainty.
    With 1 mm graduations, a measurement can be uncertain by half a division on each end, or typically stated as ± half the smallest scale interval, but for digital/analogue scales with clear graduations, the rule is often ± 1 mm or ± 0.5 mm depending on convention. Here, the mark scheme accepts ± 1 (mm) (accounting for potential errors at both ends of a ruler measurement).
  • Step 2: Calculate percentage error.
    Formula: (Uncertainty / Measured Value) × 100
    = (1 / 4) × 100 = 25%

✅ Final Answer

Uncertainty = ± 1 mm
Percentage error = 25%

Mark breakdown: 1 mark for uncertainty (± 1), 1 mark for percentage error (25%). ECF (error carried forward) applies if an alternative valid uncertainty logic was used.

❌ Common Calculation Traps

  • Forgetting to multiply by 100 when calculating percentage error.
  • Confusing absolute uncertainty with percentage error.

Topics

Biology · Practical skills · 3.3 Organisms exchange substances with their environment · 3.2 Cells · Uncertainty and evaluation · Data analysis

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.