The Microscope in Cell Studies (Cambridge International A-Level Biology)

Microscope Slide Preparation

Spec point — 1.1.1 · make temporary preparations of cellular material suitable for viewing with a light microscope

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

1. Place the sample onto a slide

• A drop of water may be added at this point

1. Apply iodine stain
2. Gently lower a coverslip over the specimen and press down to remove any air bubbles

Tissue from an onion is 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

1. Take a sterile cotton swab and gently scrape the inside cheek surface of the mouth for 5-10 seconds
2. Smear the cotton swab on the centre of the microscope slide for 2-3 seconds
3. 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

1. 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

1. Absorb any excess solution by allowing a paper towel to touch one side of the coverslip

Cheek cells can be stained using methylene blue

Drawing Cells

Spec point — 1.1.2 · draw cells from microscope slides and photomicrographs

• 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 drawing

Bacterial cell biological drawing

Animal cell biological drawing

Exam Hint When producing a biological drawing, it is vital that you only ever draw what you see and not what you think you should see!