Required practical: microscopy of plant and animal cells

What you need to know

• Prepare a wet mount using water, a thin flat specimen, a suitable stain where appropriate, and a coverslip lowered at an angle.
• Start with the lowest-power objective and use the coarse adjustment carefully before sharpening the image with the fine adjustment.
• After switching to higher power, refocus with the fine adjustment rather than the coarse adjustment.
• Record observations as a clear labelled scientific drawing with a magnification scale, and apply magnification = image size ÷ real size using appropriate units.

The big picture

This required practical uses a light microscope to observe, draw and label plant and animal cells, with a magnification scale included. A wet mount is prepared carefully, stained where appropriate and covered with a coverslip lowered at an angle to reduce trapped air bubbles. Start with the lowest-power objective, focus using the coarse then fine adjustment, and use only fine focusing after switching to higher power. Cell or structure sizes can be measured or estimated using an eyepiece graticule, and magnification is calculated from image size divided by real size using appropriate units such as mm and µm.

Prepare the wet mount

Follow the preparation in order so the specimen is ready for clear observation.

1. 1

   Water on the slide

   Place a drop of water on the slide before adding the specimen.

2. 2

   Specimen lies flat

   Add a thin layer of the specimen and lay it flat so it can be viewed through the light microscope.

3. 3

   Add the stain

   Use iodine solution for plant cells or methylene blue for animal cells because staining makes structures easier to see.

4. 4

   Coverslip goes down

   Lower the coverslip onto the specimen at an angle to reduce the chance of trapping air bubbles.

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Focus the microscope safely

The direction of coarse focusing matters: close from the side first, then move away while looking through the eyepiece.

1. 1

   Start on low power

   Begin with the lowest-power objective lens positioned over the specimen.

2. 2

   Move close from the side

   With the low-power objective selected, look from the side and use the coarse adjustment to move the objective close to the slide without touching it.

3. 3

   Focus moving away

   Now look through the eyepiece and use the coarse adjustment to move the objective slowly away from the slide until the specimen comes into focus.

4. 4

   Sharpen the image

   Use the fine adjustment to sharpen the focused image.

5. 5

   Switch to higher power

   Change to a higher-power objective when you want to see more detail.

6. 6

   Fine focus only

   Refocus with the fine focusing knob rather than the coarse adjustment because the shorter working distance increases the risk of the objective contacting the slide.

Magnification

magnification = size of image ÷ size of real object

Compare how large the image appears with the real object to calculate how many times it has been magnified.

Use matching units such as mm or µm before dividing; magnification is a ratio and has no unit.

Practical toolkit

Know what each item or term contributes to the microscopy practical.

Memory hook

Low power: coarse then fine. High power: fine only.

Check yourself

Why should you use the fine adjustment rather than the coarse adjustment after switching to a higher-power objective?