AQA GCSE Combined Science: Trilogy Biology Paper 1 (Foundation), 2023: Question 7
6 marks · Standard Demand difficulty · Extended Answer
Describe a method using a light microscope and a ruler to estimate the mean length of onion cells on a slide.
Practise this questionQuestion
Question text
07 Figure 10 shows onion cells viewed using a light microscope.
Figure 10
Figure 11 shows the apparatus given to a student.
Figure 11
Describe how the student could use the apparatus to estimate the mean length of
onion cells on the slide.
[6 marks]
Mark scheme
Show the mark scheme
Question 7
AO / 25
Question Answers Mark
Spec. Ref.
Level 3: The method would lead to the production of a valid 5‒6 AO2
outcome. All key steps are identified and logically sequenced.
AO1
Level 2: The method would not necessarily lead to a valid 3‒4
outcome. Most steps are identified, but the plan is not fully logically
sequenced.
Level 1: The method would not lead to a valid outcome. Some 1‒2 AO1
relevant steps are identified, but links are not made clear.
No relevant content 0
Indicative content 4.1.1.2
Method 1 4.1.1.5
• place ruler on (microscope) stage RPA1
• focus on (scale on) ruler
• measure diameter / width of field of view
• in millimetres
• replace ruler with slide
• focus on cells
• use same magnification
• estimate / count number of cells that fit (lengthwise) across field
of view
• (to calculate mean length of onion cells):
divide diameter / width of field of view by (estimated) number of
cells
or
divide diameter / width of field of view by 3 cells
Method 2
• focus on cells
• note magnification
• take a photograph of the image
• calculate a mean:
o measure length of several cells (3 or more) cells on
the photograph
o add all image lengths together and divide total by
number of cells (to calculate mean cell image length)
OR
o measure the length of one cell
o calculate the real size
o repeat for other cells (3 or more)
o calculate the mean real size
• (mean) size of real cell = (mean cell) image size / magnification
General points
• focus under low power first
• using focusing knob
• then focus at ×100 magnification
• reference to how total magnification is calculated
• equation:
(mean) size of real cell = (mean cell) image size / magnification
For Level 2 attempt at a method to measure cells plus an indication
of how to calculate mean length
For Level 3 suitable method and calculation of mean length.
Total Question 7 6
How to answer it
Estimating Cell Size using Microscopy
What this question tests
This question assesses your ability to plan a scientific method (Required Practical 1). It tests your knowledge of microscopy, measuring field of view, and using magnification to calculate the real size of biological structures. You must demonstrate how to produce a mean (average) to improve accuracy.
The Method: Measuring Onion Cells
Describe how the student could use the apparatus to estimate the mean length of onion cells on the slide.
💡 Key Knowledge
- Field of View: The circular area you see when looking through the microscope.
- Magnification Formula: Real Size = Image Size / Magnification.
- Units: 1 cm = 10 mm = 10,000 µm.
- Mean: Total length of cells divided by the number of cells.
🧠 Exam Technique
This is a Level of Response question. To get 5-6 marks:
- Your steps must be in a logical order.
- You must explain how to get the mean.
- Mention specific equipment (e.g., "stage", "focusing knob").
✅ Model Answer (Method 1: Field of View)
- Place the transparent ruler on the microscope stage.
- Focus the microscope and measure the diameter of the field of view in millimetres (mm).
- Replace the ruler with the prepared slide of onion cells.
- Use the same magnification (×100) to view the cells.
- Count the number of cells that fit across the diameter of the field of view.
- Calculate the mean length by dividing the diameter by the number of cells.
📐 Calculation Steps
If you chose to use the magnification formula instead of the field of view method:
- Measure the length of 3 or more cells on a photograph of the image using the ruler.
- Calculate the mean image length (Total length / number of cells).
- Use the formula: Real Size = Mean Image Size / 100 (the magnification shown in Figure 10).
- Ensure units are consistent (e.g., convert mm to µm).
❌ Common Errors
- Mixing Magnifications: Measuring the ruler at ×40 but the cells at ×100 without adjusting the math.
- Vague Measuring: Saying "measure the cell" without explaining that you need to measure the image or the field of view first.
- Forgetting the "Mean": Only measuring one cell. The question specifically asks for the mean length.
📢 Examiner Commentary
Top-level students clearly stated that the ruler must be placed on the stage. Many students lost marks by not explaining how to use the ruler to find the width of the circle (field of view) before looking at the cells. Always mention focusing under low power first!
• Level 3 (5-6 marks): Logical sequence that leads to a valid outcome. Includes measuring field of view or using magnification formula correctly.
• Level 2 (3-4 marks): Most steps identified but logic is slightly flawed or missing the "mean" calculation.
• Level 1 (1-2 marks): Isolated facts (e.g., "look through the lens") without a clear plan.
Topics
Biology · B1: Cell Biology
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Foundation), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.