AQA A Level Biology复习笔记2.2.2 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

   

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

   

TEM electron micrograph of an animal cell showing key features

TEM electron micrograph of a plant cell showing key features

• 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)