AQA GCSE Biology

Revision Notes

1.1.5 Microscopy

A Brief History of the Microscope

  • 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

Electron Microscopes

  • 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 Calculations

  • Magnification is calculated using the following equation:
Magnification = Drawing size ÷ Actual size
  • A better way to remember the equation is using an equation triangle:

Magnification Equation, IGCSE & GCSE Biology revision notes

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 Magnification Equation, IGCSE & GCSE Biology revision notes

Worked example using the equation triangle for magnification

 

Exam Tip

It is easy to make silly mistakes with magnification calculations. To ensure you do not lose marks in the exam:

  • Always look at the units that have been given in the question – if you are asked to measure something, most often you will be expected to measure it in millimetres NOT in centimetres – double-check the question to see!
  • Learn the equation triangle for magnification and always write it down when you are doing a calculation – examiners like to see this!

Converting Units

  • 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 extended magnification question, IGCSE & GCSE Biology revision notes

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

Exam Tip

If you are given a question with 2 different units in it, make sure you make a conversion so that both measurements have the same unit before doing your calculation. Also, watch out for the units you are given in the answer-prompt space.

Remember the following to help you convert between mm and µm:

 

Converting Units AQA, IGCSE & GCSE Biology revision notes

Author: Jenna

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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