The Microscope in Cell Studies (Cambridge (CIE) A Level Biology): Revision Note

Microscope slide preparation

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 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

Drawing cells

• 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)