The Microscope in Cell Studies (CIE A Level Biology)

Preparing a microscope slide

• Specimens can be viewed under a light microscope; this allows some details of cellular material to be observed
• Pre-prepared permanent slides can be viewed
  • Such slides are produced by cutting very thin layers of tissue which are stained and permanently mounted on a glass slide for repeated use
• Different methods will be used to view different types of specimen, e.g. temporary slide preparations can be produced in the school laboratory as described below

Preparing a slide using a liquid specimen

1. Add a few drops containing the liquid sample to a clean slide using a pipette
2. Lower a coverslip over the specimen and gently press down to remove air bubbles
   • Coverslips protect the microscope lens from liquids and help to prevent drying out 

Preparing a microscope slide using a solid specimen

1. Use scissors or a scalpel to cut a small sample of tissue, and peel away or cut a very thin layer of cells from the tissue sample
   • The preparation method always needs to ensure that samples are thin enough to allow light to pass through
2. Place the sample onto a slide
   • A drop of water may be added at this point
3. Apply iodine stain
4. Gently lower a coverslip over the specimen and press down to remove any air bubbles

Preparing a microscope slide using onion cells diagram

Tissue from an onion is as a solid specimen, and can be prepared here using iodine stain

Preparing a slide using human cells

1. Brush teeth thoroughly with normal toothbrush and toothpaste
   • This removes bacteria from teeth so they don't obscure the view of the cheek cells
2. Take a sterile cotton swab and gently scrape the inside cheek surface of the mouth for 5-10 seconds
3. Smear the cotton swab on the centre of the microscope slide for 2-3 seconds
4. Add a drop of methylene blue solution
   • Methylene blue stains negatively charged molecules in the cell, including DNA and RNA
   • This causes the nucleus and mitochondria to appear darker than their surroundings
5. Place a coverslip on top
   • Lay the coverslip down at one edge and then gently lower the other edge until it is flat
   • This reduces bubble formation under the coverslip
6. Absorb any excess solution by allowing a paper towel to touch one side of the coverslip

Preparing a microscope slide using cheek cells diagram

Cheek cells can be stained using methylene blue

Staining specimens

• The cytoplasm and other cell structures may be transparent or difficult to distinguish; stains allow them to be viewed clearly under a light microscope
• As with the type of preparation required, the type of stain used is dependent on the specimen being viewed

Common microscope stains & uses table

• To record the observations seen under a microscope, a labelled biological drawing is often made
• Biological drawings are line drawings 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 drawing must have a title
  • The magnification under which the observations shown by the drawing are made should be recorded if possible
    • A scale bar may be used
  • A sharp pencil should be used
  • Drawings should be on plain white paper
  • Lines should be clear, single lines without sketching
  • No shading should be used
  • The drawing should take up as much of the space on the page as possible
  • Well-defined structures should be drawn
  • Only visible structures should be drawn, and the drawing should look like the specimen
  • The drawing should be made with proper proportions
  • Structures should be clearly labelled with label lines that:
    • Do not cross
    • Do not have arrowheads
    • Connect directly to the part of the drawing being labelled
    • Are on one side of the drawing
    • Are 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)

Plant cell biological drawing

Bacterial cell biological drawings

Animal cell drawing