CIE A Level Biology (9700) 2019-2021

Revision Notes

1.1.5 Calculating Actual Size

Calculating Actual Size

  • When investigating the size of organisms and biological structures you will use a microscope of a specific magnification to produce an image
  • Photomicrographs are images obtained from a light microscope, these are used for specimens above 200 nm (a bacteria cell is about 1000 nm)
  • Electron micrographs are images obtained from electron microscopes, both scanning and transmission, these are used for specimens above 0.5 nm
    • Electron microscopes are useful for looking at organelles and biological molecules, eg. DNA can be seen replicating
  • To better understand the images we produce using microscopes we need to know the actual size of the specimen

Worked example: Calculating the actual size of a specimen

A scientist looks at a sample of red blood cells under a light microscope.

The eyepiece lens of the microscope has a magnification of x10 and an objective lens of x40 was used to view the blood cells. The scientist takes a photomicrograph of the blood cells, in which the average size of each cell is 3 mm.

What is the average size of the red blood cells in the sample? Give your answer in micrometres.

Known values:

  • Eyepiece lens magnification: x10
  • Objective lens magnification: x40
  • Image size: 3 mm

Step 1: Calculate the total magnification of the specimen

eyepiece lens magnification x objective lens magnification
= total magnification

x10   x   x40   =   x400

Step 2: Calculate the image size in the units asked for (micrometres)

1 mm = 1000 μm

3 mm = 3000 μm

Step 3: Calculate the actual size of the red blood cell

Calculating Actual Size equation

  • Therefore, the average size of a red blood cell in this sample is 7.5 micrometres

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Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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