AQA AS Biology

Revision Notes

2.2.2 Microscopy & Drawing Scientific Diagrams

Practical Skill: Microscopy & Drawing Scientific Diagrams

  • Many biological structures are too small to be seen by the naked eye
  • Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied
  • For example, the movement of chromosomes during mitosis can be observed using a microscope
  • When using an optical microscope always start with the low power objective lens:
    • It is easier to find what you are looking for in the field of view
    • This helps to prevent damage to the lens or coverslip incase the stage has been raised too high
  •  A graticule must be used to take measurements of cells:
    • A graticule is a small disc that has an engraved scale. It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
    • As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
    • By using the two scales together the number of micrometers each graticule unit is worth can be worked out
    • After this is known the graticule can be used as a ruler in the field of view

Graticule Calibration, downloadable AS & A Level Biology revision notes

The stage micrometer scale is used to find out how many micrometers each graticule unit represents.

  • Electron microscopes can produce highly detailed images of animal and plant cells
  • The key cellular structures within animal and plant cells are visible within the electron micrographs below
    • The presence of a vacuole in a micrograph is a good indicator of the cell type

Animal Cell micrograph 2, downloadable AS & A Level Biology revision notes

TEM electron micrograph of an animal cell showing key features

Plant Cell Micrograph 2, downloadable AS & A Level Biology revision notes

TEM electron micrograph of a plant cell showing key features

Drawing Cells

  • To record the observations seen under the microscope (or from photomicrographs taken) a labelled biological drawing is often made
  • Biological drawings are line pictures which show specific features that have been observed when the specimen was viewed
  • There are a number of rules/conventions that are followed when making a biological drawing
  • The conventions are:
    • The drawing must have a title
    • The magnification under which the observations shown by the drawing are made must be recorded
    • A sharp HB pencil should be used (and a good eraser!)
    • Drawings should be on plain white paper
    • Lines should be clear, single lines (no thick shading)
    • No shading
    • The drawing should take up as much of the space on the page as possible
    • Well-defined structures should be drawn
    • The drawing should be made with proper proportions
    • Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled
    • Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler
  • Drawings of cells are typically made when visualizing cells at a higher magnification power, whereas plan drawings are typically made of tissues viewed under lower magnifications (individual cells are never drawn in a plan diagram)

Exam Tip

When producing a biological drawing, it is vital that you only ever draw what you see and not what you think you see.

To accurately reflect the size and proportions of structures you see under the microscope, you should get used to using the eyepiece graticule.

You should be able to describe and interpret photomicrographs, electron micrographs and drawings of typical animal cells.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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