Microscopy & Drawing Scientific Diagrams (AQA A Level Biology): Revision Note
Exam code: 7402
Use of an optical microscope
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
Microscopy method
School students often use pre-prepared microscope slides to view cells or tissues as follows:
Place the slide containing a tissue specimen on the microscope stage
Turn the objective lens so that the lowest power lens is in use
Use the coarse focus wheel to lower the lens as far down towards the stage as possible
Adjust the fine focus to raise the lens until the specimen is in focus
If the specimen is too small, turn the objective lens to the next-highest power and refocus
Repeat with the highest power objective lens if necessary

Using a graticule
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

Observing starch grains in plant cells
Starch is the storage polysaccharide of plants
Sugars formed during photosynthesis are stored as starch inside starch grains
These starch grains are commonly found in:
the stroma of chloroplasts
storage organs such as potato tubers
the seeds of cereals and legumes
Starch grains are large enough to be seen with a optical microscope but they first require staining in order to be seen easily
Iodine (I2) in potassium iodide (KI) solution can be used to stain starch grains
This can be done by adding a drop of potassium iodide onto the specimen (mounted on a slide) before placing a coverslip over the top
This makes the starch grains darker in colour, making them easier to see
Remember, iodine in potassium iodide solution has a light orange-brown colour but turns blue-black in the presence of starch
Examiner Tips and Tricks
Remember - only a very small amount of stain is required (usually just a drop or two will be enough)! Also, when adding a cover slip, remember to slowly lower the cover slip onto the specimen at an angle to avoid any air bubbles getting trapped (these could create ‘artefacts’ and obstruct your view of the specimen).
Scientific drawings
Biological drawings are line pictures that show specific features that have been observed when the specimen was viewed
Scientific drawings must follow standard drawing conventions:
Add a title and magnification
Use plain paper and a sharp HB pencil
Draw clear, single lines – no shading
Make it large, proportional, and well-defined
Use straight label lines (no arrows or crosses), on one side, drawn with a ruler
Plant cell biological drawing

Bacterial cell biological drawing

Examiner Tips and Tricks
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.
Unlock more revision notes. It’s free!
By signing up you agree to our Terms and Privacy Policy.
Already have an account? Log in
Did this page help you?