AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2023: Question 2

6 marks · Standard Demand difficulty · Extended Answer

Describe how to use a microscope, ruler and onion slide to estimate the mean length of onion cells.

Practise this question

Question

The question shows Figure 2, a light microscope image of onion cells inside a circular field of view at ×100 magnification, with several rectangular plant cells visible and one arrow labeled “Field of view.” Figure 3 shows the apparatus provided: a microscope, a 15 cm transparent ruler, and a prepared slide of onion cells. The printed question asks the student to describe how the apparatus could be used to estimate the mean length of onion cells on the slide for 6 marks.
Question text

02 Figure 2 shows onion cells viewed using a light microscope.

Figure 2

Figure 3 shows the apparatus given to a student.

Figure 3

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 Mark scheme for AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2023: Question 2

Question 2

AO /

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

12 • 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 2 6

How to answer it

Estimating onion cell length with a microscope

What this question tests

You need to describe a clear method for measuring cell size using a microscope, including field of view, magnification, millimetres, and how to calculate a mean length. Full marks are for a logical sequence that would produce a valid result.

Question title: Estimating the mean length of onion cells

Overall skill: planning a valid practical method

💡 Key knowledge

  • Use the microscope to view the onion cells at the same magnification.
  • Measure the diameter of the field of view using a ruler.
  • Count how many cells fit across the field of view lengthways.
  • Calculate mean cell length: field of view diameter ÷ number of cells.
  • Alternative valid method: measure several cell images, find a mean, then divide by magnification.

🧠 Exam technique

  • Write the method in a clear order.
  • Include at least one way to get a numerical estimate.
  • Use scientific terms: field of view, magnification, mean.
  • For 5–6 marks, make sure the steps link together to form a valid method.

❌ Common errors

  • Saying “measure the cells” without explaining how.
  • Forgetting to keep the same magnification.
  • Using cm instead of mm for the field of view measurement.
  • Not explaining how the mean length is calculated.
  • Measuring only one cell without any repeat/mean.

Part (a): Describe how to estimate the mean length of onion cells

This is a 6-mark method question

✅ Correct answer: model full-mark method

Place the transparent ruler on the microscope stage and focus on the ruler at the same magnification as the cells. Measure the diameter of the field of view in millimetres. Replace the ruler with the onion slide and focus on the cells. Estimate how many cells fit lengthways across the field of view. Calculate the mean cell length by dividing the field of view diameter by the number of cells.

Alternative valid approach: focus on the cells, take a photograph, measure the length of 3 or more cells on the image, find the mean image length, and divide by the magnification to get the real mean length.

📐 Calculations / working method

  1. Measure the field of view diameter in mm using a ruler.
  2. Count or estimate how many onion cells fit across the diameter.
  3. Use: mean cell length = field of view diameter ÷ number of cells
  4. If using a photo: measure 3 or more cells, add the lengths, then divide by the number of cells.
  5. If using magnification: real size = image size ÷ magnification

🧠 What got top marks

  • The best answers were logical and complete.
  • They included a method to measure the field of view first, then used it to estimate cell size.
  • They clearly explained how to calculate a mean, not just a single measurement.
  • They used the same magnification throughout, which makes the result valid.

💡 Key knowledge to remember

  • Magnification tells you how much bigger the image is than the real object.
  • Field of view is the circular area visible through the microscope.
  • Measuring several cells and finding a mean improves reliability.
  • Use mm for ruler measurements, then be consistent with units.

❌ Common calculation traps

  • Forgetting to divide by the number of cells.
  • Using the image length as the real length without dividing by magnification.
  • Not stating the unit, or mixing mm and cm.
  • Measuring only part of a cell instead of the full length.
  • Not making it clear that the ruler and slide should be viewed at the same magnification.
How marks were awarded: Level 3 responses identified all key steps in a sensible order: measure field of view, replace with slide, count cells across, and calculate mean length. Level 2 answers usually had the right idea but missed a step or did not fully link the method. Level 1 answers mentioned some relevant equipment or measurements but did not produce a clear valid method.

Part (a): A clear step-by-step answer you could write in the exam

  1. Put the transparent ruler on the microscope stage.
  2. Focus on the ruler and measure the diameter of the field of view in mm.
  3. Remove the ruler and place the prepared onion slide on the stage.
  4. Focus on the cells using the same magnification.
  5. Count how many cells fit lengthways across the field of view.
  6. Calculate the mean cell length by dividing the field of view diameter by the number of cells.

🧠 Extra exam tip

If you want to be extra safe in a 6-mark question, add a sentence about measuring 3 or more cells and repeating the calculation to improve reliability. That shows you understand how to get a better estimate.

Topics

Biology · Required Practicals · B1: Cell Biology · Biology Required Practicals

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.