AQA GCSE Biology Biology Paper 1 (Foundation), June 2022: Question 4

✅ Correct Matches

• A (Focus adjustment knob) → To adjust the focus of the microscope
• B (Mirror / light source) → To direct light into the viewer's eye
• C (Objective lens) → To magnify the image of a specimen

🧠 Exam Technique

The rubric specifically states: "do not accept more than one line from a box on the left". If you draw two lines coming from part A, neither can score. Use a ruler to keep your lines clean and unambiguous.

✅ Correct Answers

• 04.2: Why add iodine solution? → To stain the cells (1 mark)
• 04.3: Why use a thin layer? → To allow light to pass through the cells (1 mark)

💡 Key Knowledge

• Stains: Plant cells (like onion cells) are largely transparent. Iodine binds to starch and cell structures (such as cell walls and nuclei), making them clearly visible under a light microscope.
• Light transmission: Light microscopes rely on light passing through the specimen. If the layer is too thick (many cell layers deep), light is blocked and you will only see a dark shadow.

❌ Common Errors

Confusing "to dry the cells" or "to separate the cells" with staining. Also, confusing light passage with "allowing oxygen/water to pass"—remember, the cells are being viewed on a glass slide, not kept alive for respiration!

✅ Correct Matches

• Hazard → The knife is sharp (1 mark)
• Plan to minimise risk → Cut the onion on a chopping board (1 mark)

💡 Key Knowledge: Risk Assessment Definitions

• Hazard: Something with the potential to cause harm (e.g., a sharp blade that can cause cuts).
• Risk: The chance of harm actually happening.
• Control measure (plan to minimise risk): An action taken to reduce the likelihood or severity of that harm (e.g., cutting downwards onto a chopping board, keeping fingers away from the blade).

❌ Common Errors

Linking Hazard to "Call a first aider". Calling a first aider is an emergency response after an injury occurs; it does not prevent or minimise the risk.

❌ Common Calculation Traps

• Unit Trap: Dividing 49 mm by 400 directly without converting to µm first gives 0.1225 µm (off by a factor of 1000).
• Measuring Trap: Starting the ruler at 1 cm instead of 0 cm, or measuring in centimetres instead of millimetres.

🧠 Exam Technique: Error Carried Forward (ecf)

Even if you slightly mismeasure line A–B (e.g. you write 46 mm), you can still gain the remaining 3 marks if you convert your value correctly (×1 000), substitute it into the formula, and evaluate it accurately!

✅ Correct Options (Tick 2)

• ☑ The cells would look larger (1 mark)
• ☑ The cells would show more internal structures (1 mark)

💡 Key Knowledge

Electron microscopes differ from light microscopes in two fundamental ways:

• Higher Magnification: Specimen can be enlarged up to ×2 000 000 (making cells appear far larger).
• Higher Resolution: Shorter wavelength of electrons allows sub-cellular structures (e.g., ribosomes, mitochondria internal membranes) to be clearly distinguished rather than blurred.

❌ Common Misconceptions

• "Cells would be coloured": False. Electron microscope images are naturally greyscale (black and white). Any colour added is artificial false-colouring.
• "Cells would have no nuclei": False. Nuclei remain present.

✅ Correct Options (Tick 2)

• ☑ Complete the cell walls (1 mark)
• ☑ Include the magnification (1 mark)

💡 Biological Drawing Golden Rules

• Use a sharp pencil with continuous, unbroken single lines (no gaps in membranes or cell walls).
• Never shade or colour in cells (the student incorrectly shaded the nuclei).
• Draw proportions accurately and ensure overlapping or surrounding boundaries close fully.
• Always state the magnification used (e.g. ×400) or include a scale bar.

❌ Why the other options are wrong

• "Add colour": Shading or colouring is penalized in biological line drawings.
• "Use a ruler to draw cells": Plant cells are living structures with rounded corners and curved walls; drawing them with a straight ruler is scientifically inaccurate!