AQA GCSE Biology Biology Paper 1 (Higher), June 2022: Question 3
14 marks · Standard Demand difficulty · Short Answer
Assess risks, justify preparation steps, calculate the real length of an onion cell using magnification, and evaluate scientific drawings and electron microscopy.
Practise this questionQuestion
Question text
03 A student prepared some onion cells.
The student viewed the onion cells using a light microscope.
This is the method used.
1. Cut an onion into pieces using a sharp knife.
2. Peel off a thin layer of onion epidermis from one piece of onion.
3. Place the onion epidermis onto a microscope slide in a single flat layer.
4. Add three drops of iodine solution.
5. Slowly lower a cover slip at an angle onto the onion epidermis.
6. Place the slide on the stage of the microscope.
03.1 Table 4 shows a risk assessment for this experiment.
Complete Table 4.
[2 marks]
Table 4
Hazard Risk Plan to minimise risk
Iodine solution May cause allergic reaction or
is an irritant skin rash
Sharp knife
03.2 Give a reason for each of the following steps in the method.
[3 marks]
A thin layer of onion epidermis is used.
Iodine solution is added to the onion epidermis.
The cover slip is lowered onto the onion epidermis at an angle.
Figure 3 shows what the student saw under the microscope at a magnification
of ×400.
Figure 3
03.3 The length of cell Z in Figure 3 is 4.8 cm.
Calculate the real length of cell Z.
Give your answer in micrometres (μm).
*15* [5 marks]
Real length of cell Z = μm
Figure 4 shows the student’s drawing of Figure 3.
*16* Figure 4
03.4 Give two ways the student could improve the drawing in Figure 4.
[2 marks]
03.5 Onion cells can be seen using an electron microscope.
Give two ways onion cells would look different when seen using an electron
microscope.
[2 marks]
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1
AO3
Hazard Risk Plan to minimise risk
4.1.1.5
RPA1
wash skin immediately
(after contact)
or
wear gloves
or 1
Iodine clean up spills
May cause allergic
solution is
reaction or skin rash
an irritant
allow method to prevent
spills e.g. use a dropper
bottle
ignore do not spill
cut away from the body
may cut you / or 1
someone / skin cut on a chopping board
or
Sharp keep fingers away from
knife blade (when cutting)
allow description of how to
carry knife safely
ignore use a blunt knife
1 mark for each correct row
AO /
Spec. Ref.
03.2 thin layer AO2
(to) help see individual cells allow so light can penetrate 1 4.1.1.5
RPA1
iodine solution
(to) stain / see the parts of the allow visible named sub-cellular 1
cell structures e.g. nucleus,
cytoplasm, cell wall,
starch grains
ignore chloroplast
14 ignore (to) stain the cell
at an angle
(to) prevent / reduce air bubbles 1
AO /
Spec. Ref.
03.3 recall of equation
magnification = allow 1 AO1
size of image magnification =
size of real object length of image
length of real object
ignore use of equation triangle
rearrangement of equation allow 1 AO2
size of real object = length of real object =
size of image length of image
magnification magnification
allow recall and rearrangement
of equation implied at any stage
substitution allow substitution of incorrectly AO2
4.8 converted value
allow answer using incorrectly 1
0.012 (cm) converted value AO2
conversion allow conversion to μm at any AO2
120 (μm) stage
4.1.1.5
RPA1
AO / 15
Spec. Ref.
ignore make it neater
03.4 any two from: 2 AO3
• include magnification / scale 4.1.1.2
• use continuous lines RPA1
or ensure no gaps in lines
• do not draw overlapping cells
• draw (wider) cell walls
• do not shade allow do not colour
• draw all the cells present
• draw correct cell shapes
• do not have gaps between
cells
• draw nuclei in correct location
• label cell part(s) allow label named cell part(s)
AO /
Spec. Ref.
03.5 (would) look more magnified / ignore reference to zoom 1 AO1
bigger 4.1.1.5
(cell would) have more detail 1
or
(would) be at a higher resolution
or
(could) see more sub-cellular allow correct examples of sub-
structures cellular structures such as
or ribosomes, mitochondria, cell
sub-cellular structures seen in membrane
detail ignore chloroplast
allow (could) be in 3D
allow would be in black and
white
Total Question 3 14
How to answer it
Required Practical 1: Preparing & Viewing Onion Cells
This question assesses core practical knowledge from Required Practical 1 (Microscopy): identifying laboratory hazards and controls, explaining standard slide preparation steps, performing 5-mark magnification calculations with unit conversions (cm to µm), rules for scientific drawings, and comparing light vs. electron microscopes.
Microscopy Risk Assessment
Completing the risk and precaution table for slide preparation
| Hazard | Risk | Plan to minimise risk |
|---|---|---|
| Iodine solution is an irritant | May cause allergic reaction or skin rash | Wear gloves / wash skin immediately / clean up spills promptly / use dropper bottle |
| Sharp knife | May cut skin / cut fingers / cut yourself | Cut away from body / cut on a chopping board / keep fingers clear of blade |
✅ Correct Answers
- Iodine plan (1 mark): Wear protective gloves, wash skin immediately after contact, or use a dropper bottle to prevent spills.
- Sharp knife risk & plan (1 mark): Risk is cutting fingers/skin; plan is cutting on a chopping board, cutting away from yourself, or carrying safely pointing down.
❌ Common Errors
- Vague precautions like "do not spill" or "be careful" gain 0 marks.
- Writing "use a blunt knife" is not accepted—scientific methods require sharp tools used properly.
Scientific Reasons for Preparation Steps
Explaining why each procedural step is carried out
✅ Mark Scheme Answers
- Thin layer: To see individual cells clearly / to allow light to pass (penetrate) through the specimen [1 mark].
- Iodine solution: To stain and make sub-cellular structures (e.g. nucleus, cell wall) visible [1 mark].
- Cover slip at an angle: To prevent or reduce trapping air bubbles [1 mark].
💡 Key Scientific Knowledge
- A light microscope is an optical microscope: light must pass through the sample into the objective lens. Thick samples appear completely black.
- Iodine binds to starch and cellular material, darkening organelles so they contrast with the background.
❌ Examiner Trap
- Never say iodine stains "chloroplasts" in onion cells—onion bulbs grow underground and have no chloroplasts!
- Do not just write "to stain the cell"—explain that it allows cell parts (like the nucleus) to be seen.
Magnification Calculation
Calculating real length in micrometres (µm) from image size
🔧 Step-by-Step Calculation (Full 5 Marks)
- Recall formula [1 mark]:
magnification = image size / real size - Rearrange formula for real size [1 mark]:
real size = image size / magnification - Substitute values [1 mark]:
real size = 4.8 cm / 400 = 0.012 cm (or in mm: 48 mm / 400 = 0.12 mm ) - Evaluate numerical value [1 mark]:
0.012 cm or 0.12 mm - Convert to micrometres (µm) [1 mark]:
Since 1 cm = 10 mm and 1 mm = 1000 µm (1 cm = 10 000 µm):
0.012 × 10 000 = 120 µm (or 0.12 mm × 1000 = 120 µm )
🧠 Exam Technique: Unit Ladder
- cm → mm: Multiply by 10
- mm → µm: Multiply by 1000
- cm → µm: Multiply by 10 000
- Tip: Convert image length to µm before dividing:
4.8 cm = 48 000 µm
48 000 / 400 = 120 µm .
❌ Calculation Mistakes
- Forgetting to convert units and leaving the answer as 0.012 or 0.12 .
- Multiplying image size by magnification instead of dividing.
- Formula triangles written without an actual equation do not earn the recall mark!
Improving Biological Drawings
Applying rules of biological scientific drawing
✅ Any Two Valid Improvements (1 mark each)
- Do not shade or sketch (leave organelles clear).
- Use single, clear, continuous lines (no gaps, overlapping lines, or sketchy lines).
- Include a magnification or scale bar.
- Label cell parts (e.g. nucleus, cell wall, cytoplasm) using straight, uncrossed label lines.
- Draw cell walls with double lines to show thickness.
- Draw cells touching without empty gaps between cell walls.
- Draw nuclei in accurate positions/proportions (not randomly placed or striped).
❌ What Examiners Reject
- "Make it neater" — too vague; examiners award zero marks for this.
- "Colour it in" — biological drawings must never be coloured or shaded.
Light vs. Electron Microscopes
How onion cells appear under an electron microscope
✅ Two Distinct Differences (1 mark each)
- Higher magnification: The image would look bigger / more magnified [1 mark].
- Higher resolution: Greater detail / sub-cellular structures (such as mitochondria, ribosomes, or nuclear membrane) would be visible [1 mark].
- Also accepted: Image would appear in black-and-white or in 3D (scanning electron microscope).
🧠 Key Definitions to Remember
- Magnification: How many times larger the image is compared to the actual object.
- Resolution: The ability to distinguish between two separate points close together (fine detail).
❌ Common Mistakes
- Do not refer to "zoom"—use the scientific term magnification.
- Do not claim you would see "chloroplasts" (onion epidermal cells do not have them!).
Topics
Biology · Required Practicals · B1: Cell Biology · Required Practicals
Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 1 (Higher), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.