Electron Microscopy (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Electron microscopes

Why are electron microscopes better?

• An electron microscope has much higher magnification and resolving power than a light microscope

  • The magnification of an electron microscope is up to a million times (x 1 000 000) or more

  • The magnification of a light microscope is only about a thousand times (x 1000)

• Electron microscopes can therefore be used to study cells in much finer detail, enabling biologists to see and understand many more subcellular structures such as the mitochondrion

• They have also helped biologists develop a better understanding of structures such as the nucleus and cell membrane

Magnification calculations

Magnification equation

• Magnification is calculated using the following equation:

Magnification = Drawing size ÷ Actual size

Magnification equation triangle

• An equation triangle can help with rearranging simple equations

An equation triangle for calculating magnification

• Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle – whatever you are trying to find, place your finger over it and whatever is left is what you do, so:

  • Magnification = image size / actual size

  • Actual size = image size / magnification

  • Image size = magnification x actual size

• Remember magnification does not have any units and is just written as ‘X 10’ or ‘X 5000’

Worked example

• An image of an animal cell is 30 mm in size and it has been magnified by a factor of X 3000. What is the actual size of the cell?

• To find the actual size of the cell:

Worked example using the equation triangle for magnification

Converting Units

Converting units in Biology

• Cells are very small and require a microscope to be seen

• Scientists measure the size of cells in micrometers (µm)

• You may be given a question in your Biology exam where the measurements for a magnification calculation have different units

  • The following units may be used:

    • centimetre (cm): 1cm = 10mm

    • milimetre (mm): 1mm = 1000 µm

    • micrometre (µm): 1µm = 1000 nm OR 0.001 mm

    • nanometre (nm)

• You need to be able to convert between different units of measurement, particularly mm and µm

• You need to ensure that you convert them both into the same unit before proceeding with the calculation (usually to calculate the magnification)

• For example:

Example of an extended magnification question

Step 1: convert into mm

1mm = 1000µm

2000 / 1000 = 2 mm

Step 2: Substitute the numbers into the formula

• The actual thickness of the leaf is 2 mm

• The thickness of the leaf in the image is 50 mm

Magnification = image size / actual size

Magnification = 50 / 2 = 25

So the magnification is X 25

How to Use Standard Form

• When biologists talk about the size of cells and the structures within them, they are dealing with very small numbers. Very small (or very big) numbers are represented using standard form – this helps to avoid confusion

• Let’s say we want to represent the length of a Vibrio cholerae cell which is 1.5µm in mm

• First, we need to convert the measurement in µm into mm (see image in Scale & the Size of Cells)

• 5 µm = 0.0015 mm

• To write this in standard form:

Practise converting numbers into standard form – you may be asked to do this in the exam!

Scale & the Size of Cells

• You need to show an understanding of the size and scale of cells (and the subcellular structures within them)

You need to be aware that many subcellular structures in eukaryotic cells are the same size as or bigger than prokaryotic cells!

• Differences in size can be described as differences in order of magnitude, essentially the difference in size calculated by a factor of 10

Size of cells table

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