AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2024: Question 5

✅ Correct answer

Type of microscope: electron microscope

Reason: it has high resolution / high magnification.

💡 Key knowledge

• Electron microscopes can produce much more detailed images than light microscopes.
• Bacteria are very small, so a microscope with higher resolution is needed to see fine detail clearly.
• The image shown is highly detailed and three-dimensional in appearance, which strongly suggests electron microscopy.

🧠 Exam technique

• This is a 1-mark question, but the mark scheme expects both the type and a valid reason.
• Keep the reason short and precise: high resolution or high magnification .
• Do not waffle about colour or brightness unless the question asks.

❌ Common errors

• Saying just microscope is too vague.
• Writing light microscope loses the mark.
• Giving a weak reason like because it looks clear is not precise enough.

✅ Correct answers

Any three of these score:

1. Prokaryotic cells do not have a nucleus.
2. Their DNA is free in the cytoplasm.
3. They do not have mitochondria.
4. They have a single loop of DNA / single circular strand of DNA.
5. They may contain plasmids.
6. They are smaller than eukaryotic cells.

💡 Key knowledge

• Bacteria are prokaryotes.
• Prokaryotic cells are simpler than eukaryotic cells.
• They do not contain membrane-bound organelles like a nucleus or mitochondria.
• Their genetic material is not enclosed in a nucleus.

🧠 Exam technique

• You need three separate differences, not the same idea repeated in different words.
• A very safe set of three is:
  • no nucleus
  • no mitochondria
  • has plasmids / single loop of DNA / smaller
• If you write the eukaryotic version instead, that can still be accepted if clearly stated, for example: eukaryotic cells have a nucleus, but prokaryotic cells do not .

❌ Common errors

• Repeating the same point twice, for example no nucleus and DNA free in cytoplasm. These are closely linked, so it is safer to choose other differences too.
• Saying just different DNA is too vague.
• Giving features not in the mark scheme without enough detail.
• Confusing cell wall differences with plant cells rather than eukaryotic cells in general.

📐 Calculations

1. Measure the image of the cell
   Cell image length = 36 mm
   The mark scheme allows about 35–37 mm.
2. Convert the image size into the same unit as the real size
   36 mm = 36 × 1000 = 36 000 μm
3. Find the real size from the scale
   The cell spans from 20 to 50 = 30 small divisions
   Each small division = 1.5 μm
   Real size = 30 × 1.5 = 45 μm
4. Use the magnification formula
   magnification = image size ÷ real size
5. Substitute the values
   Magnification = 36 000 ÷ 45 = 800

Final answer: magnification = ×800

✅ Correct answer

The magnification is 800.

Important: the mark scheme says do not accept if a unit is given. Magnification is a ratio, so write it as 800 or ×800.

💡 Key knowledge

• Formula: magnification = image size ÷ real size
• Both sizes must be in the same unit before dividing.
• 1 mm = 1000 μm
• The real size comes from the scale, not from measuring the printed picture directly.

🧠 Exam technique

• Show every stage. This question awards method marks, so working matters.
• Write down the formula first to secure a mark if you get stuck.
• Be careful to count the scale correctly: from 20 to 50 is 30 divisions, not 3.
• Top answers clearly show:
  • measurement
  • unit conversion
  • real size calculation
  • substitution into the formula
  • final answer

❌ Common errors

• Forgetting to convert mm to μm.
• Using 36 ÷ 45 instead of 36 000 ÷ 45.
• Misreading the scale and using the wrong real size.
• Writing a unit after magnification, for example 800 μm, which is wrong.
• Measuring the wrong part of the cell, such as including extra space around it.

05.1 Model answer

Electron microscope, because it has high resolution / high magnification.

05.2 Model answer

1. Prokaryotic cells do not have a nucleus.
2. They do not have mitochondria.
3. They have plasmids.

05.3 Model answer

Measured image size = 36 mm = 36 000 μm.
Real size = 30 × 1.5 = 45 μm.
Magnification = 36 000 ÷ 45 = 800.
Magnification = ×800