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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