Microscopy (AQA GCSE Biology)

• Microscopy techniques have developed over time, increasing our understanding of cell subcellular structure
• The first light microscopes were developed in the 17th Century
• Scientists such as Anton van Leeuwenhoek and Robert Hooke are responsible for using microscopes to develop our first understanding of cells
• Light microscopes use light and lenses to form a magnified image of a specimen
• Over the centuries, the design of the light microscope has evolved, increasing magnification and resolution to enhance the detail of what can be visualised
• With a light microscope it is possible to see images of cells and large subcellular structures (like nuclei and vacuoles), although stains are often required to highlight certain parts of cells
• The first electron microscopes were developed in the first half of the 20th Century
  • Electron microscopes use beams of electrons, rather than light, to visualise specimens
  • The wavelength of an electron beam is much smaller than that of visible light, which gives electron microscopes a much higher resolution and magnification

• An electron microscope has much higher magnification and resolving power than a light microscope
• They 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 the structure of the nucleus and cell membrane

• Magnification is calculated using the following equation:

Magnification = Drawing size ÷ Actual size

• A better way to remember the equation is using an equation triangle:

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

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

• Remember that 1mm = 1000µm
• 2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm
• Magnification = image size / actual size = 50 / 2 = 25
• So the magnification is x 25