AQA AS Level Biology Paper 1, June 2022: Question 3

10 marks · Medium difficulty · Short Answer

Answer questions about cell structure, electron microscopes, bacterial features, and viral characteristics based on microscopy images and biological knowledge.

Practise this question

Question

Figure 2 shows a scanning electron microscope image of a bacterium with a whip-like structure labelled X. Questions 03.1 to 03.6 ask to identify structure X, compare SEM and TEM images, explain electron microscope resolution, describe how to calculate cell structure size from a photograph, name bacterial structures absent in plant cells, and name HIV particle features absent in bacteria.
Question text

03.1 Figure 2 is an image of a bacterium obtained using a scanning electron microscope.

Figure 2

Name the structure labelled X.

[1 mark]

03.2 Figure 2 is different from an image of this bacterium obtained using a transmission

electron microscope.

Describe and explain one difference between these images.

[2 marks]

Description

Explanation

03.3 The resolution of an image obtained using an electron microscope is higher than the

resolution of an image obtained using an optical microscope.

Explain why.

[1 mark]

03.4 A student determined the size of a cell structure from a photograph obtained using a

microscope.

He used a ruler and a calculator and gave the answer in μm

Describe how the student determined the size of the structure.

[2 marks]

03.5 Name two structures found in all bacteria that are not found in plant cells.

[2 marks]

03.6 Name two features of HIV particles that are not found in bacteria.

Do not include attachment protein in your answer.

[2 marks]

Mark scheme

Show the mark scheme The mark scheme provides answers for questions 03.1 through 03.6, accepting flagellum for X, differences between SEM (3D/surface) and TEM (2D/internal), shorter wavelength of electrons for higher resolution, measuring image length and dividing by magnification/using scale bars, bacterial features like circular DNA, murein cell wall, and 70S ribosomes, and HIV features like capsid, reverse transcriptase, RNA genome, and envelope.

Question Marking Guidance Mark Comments

03.1 Flagellum/flagella; 1

(AO1)

03.2 1. 3D with SEM, but 2D with TEM

OR

(Only) surface visible with SEM, but internal

structures visible with TEM;

2 2. Accept Resolution

2. (Because) electrons deflected/bounce off is lower (with SEM)

(2 x AO1)

(using SEM)

OR

Electrons transmitted/pass through (using – LOGY – – JUNE 2022

TEM);

6 03.3 Shorter wavelength between electrons Accept ‘of electrons’

OR ‘of electron beam’

OR for ‘between

electrons’

Longer wavelength in light (rays); 1

Reject Electron

(AO1) microscope has a

shorter wavelength

Reject Light

microscope has a

longer wavelength

03.4 Mark in pairs: 1 and 2 OR 3 and 4

1. Measure (length of structure) and divide by 1. Accept correct

magnification; equation making

reference to a

2. Correct conversion from measured length to

measurement

μm, either

× 10 000 from cm

(2 x AO1)

OR

× 1000 from mm;

3. Measure (length of structure) and divide by

(image) length of scale bar;

4. Multiply by actual length of scale bar;

03.5 1. Circular DNA (molecule in cytoplasm);

2. Murein cell wall

Accept mesosome

OR

2 max

Peptidoglycan cell wall

(2 x AO1)

OR

Glycoprotein cell wall;

– LOGY – – JUNE 2022

3. Small(er)/70S ribosomes (in cytoplasm);

03.6 1. Capsid; 1. Accept capsomere

2. Reverse transcriptase; 1. Ignore protein coat

3. RNA genome; 2 max 3. Accept ‘genetic

material’ OR

4. Envelope; (2 x AO1)

‘genes’ for

‘genome’

How to answer it

Microscopy and Cell Ultrastructure Study Guide

What this question tests

This question assesses your understanding of cell biology and microscopy techniques. You will be tested on your ability to identify cellular structures from electron micrographs, distinguish between Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), explain resolution in terms of beam wavelength, calculate actual sizes using magnification or scale bars, and compare the distinct ultrastructural features of prokaryotes, eukaryotic plant cells, and viruses (HIV).

Question 03.1

Naming Bacterial Structures

Name the structure labelled X. [1 mark]

✅ Correct Answer

Flagellum (Accept plural: flagella)

💡 Key Knowledge

Bacterial flagella are whip-like appendages used primarily for locomotion. In scanning electron micrographs, their slender, thread-like outer structure is clearly visible projecting from the cell wall.

Mark allocation: 1 mark for AO1 (Recall of biological terminology).
Question 03.2

SEM vs TEM Comparison

Describe and explain one difference between these images. [2 marks]

✅ Correct Answers (Choose one pair)

  • Pair 1: Image is 3D with SEM, but 2D with TEM; because electrons bounce off/are deflected (SEM) vs transmitted/pass through (TEM).
  • Pair 2: Only surface visible with SEM, but internal structures visible with TEM; because electrons bounce off the surface (SEM) vs pass through thin sections (TEM).

🧠 Exam Technique

This is a two-part requirement: a description of what is seen visually (3D vs 2D, or surface vs internal) linked directly to an explanation of how the electron microscope operates (electrons reflected vs transmitted). You must score both parts to get the marks.

Mark allocation: 2 marks total (2 x AO1). Both description and valid linked explanation required.
Question 03.3

Principles of Resolution

Explain why the resolution of an image obtained using an electron microscope is higher than the resolution of an image obtained using an optical microscope. [1 mark]

✅ Correct Answer

Electron microscopes use a beam of electrons which has a shorter wavelength than light (or optical microscopes use light which has a longer wavelength).

❌ Common Errors

Students often lose this mark by stating "electron microscopes have a shorter wavelength" instead of specifying that it is the electrons/beam of electrons that have the shorter wavelength. Precision with physics principles is strictly enforced by examiners.

Mark allocation: 1 mark for AO1.
Question 03.4

Calculations and Magnification

A student determined the size of a cell structure from a photograph obtained using a microscope. He used a ruler and a calculator and gave the answer in micrometres (μm). Describe how the student determined the size of the structure. [2 marks]

📐 Step-by-Step Method

  1. Measure: Measure the image length of the structure using a ruler (in cm or mm).
  2. Calculate or Convert: Either divide the measured length by the magnification, OR use a scale bar (measure image length of scale bar, divide by its actual length, then divide structure length by this scaling factor).
  3. Unit Conversion: Convert centimetres to micrometres by multiplying by 10 000 (or millimetres to micrometres by multiplying by 1 000 ).

🧠 Exam Technique

Marked in pairs (Mark 1 & 2, OR Mark 3 & 4 from the mark scheme). You must state both how you measured/calculated the size AND how you handled unit conversion to micrometres.

Mark allocation: 2 marks (2 x AO1).
Question 03.5

Prokaryotic vs Plant Cell Ultrastructure

Name two structures found in all bacteria that are not found in plant cells. [2 marks]

✅ Acceptable Answers (Any two)

  • Circular DNA (molecule free in cytoplasm)
  • Murein (or peptidoglycan / glycoprotein) cell wall
  • Small / 70S ribosomes (in cytoplasm)
  • Mesosome (allowed by mark scheme)

❌ Common Errors

Do NOT write "cell wall" on its own—plant cells also have cell walls (made of cellulose). You must specify murein or peptidoglycan. Similarly, stating "ribosomes" is insufficient because plants also have ribosomes (80S); you must specify 70S or small ribosomes.

Mark allocation: 2 marks max (2 x AO1). 1 mark per correct structure.
Question 03.6

Viral Ultrastructure (HIV vs Bacteria)

Name two features of HIV particles that are not found in bacteria. Do not include attachment protein in your answer. [2 marks]

✅ Acceptable Answers (Any two)

  • Capsid (or capsomere)
  • Reverse transcriptase
  • RNA genome (Accept RNA / genetic material / genes)
  • Envelope (viral membrane)

💡 Key Knowledge

Viruses are acellular and lack standard cellular machinery. HIV contains a protein capsid enclosing RNA and the enzyme reverse transcriptase, surrounded by a lipid envelope stolen from host cell membranes. None of these features exist in bacterial cells.

Mark allocation: 2 marks max (2 x AO1). 1 mark per correct feature.

Topics

Biology · 3.2 Cells

Question and mark scheme from the AQA AS Level Biology examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.