Microscopy & Drawing Scientific Diagrams (AQA A Level Biology): Revision Note

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:

  1. Place the slide containing a tissue specimen on the microscope stage

  2. Turn the objective lens so that the lowest power lens is in use

  3. Use the coarse focus wheel to lower the lens as far down towards the stage as possible

  4. Adjust the fine focus to raise the lens until the specimen is in focus

  5. If the specimen is too small, turn the objective lens to the next-highest power and refocus

  6. 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

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