Microscopes and measuring cellsIB MYP Biology: Revision notes

A light microscope uses lenses and light to make small objects look bigger. The main parts are:

• Eyepiece lens: you look through it. It usually magnifies ×10.
• Objective lenses: several lenses of different power (for example ×4, ×10, ×40) on a rotating turret.
• Stage: the platform that holds the slide.
• Light source: lights up the specimen from below.
• Coarse and fine focus knobs: move the stage or lenses to make the image clear.

To use it: place the slide on the stage, select the lowest-power objective lens, and use the coarse focus to find the specimen. Then switch to a higher-power lens and use the fine focus to make the image sharp.

Total magnification = eyepiece magnification × objective magnification. For example, ×10 × ×40 = ×400.

Magnification tells you how many times bigger the image is than the real object.

magnification = image size ÷ actual size

The formula can be rearranged:

• image size = magnification × actual size
• actual size = image size ÷ magnification

Worked example: a cell is 0.02 mm long. In a drawing it is 40 mm long. Magnification = 40 ÷ 0.02 = ×2000.

Both sizes must be in the same unit before you divide.

Cells are tiny, so we use small units:

• 1 millimetre (mm) = 1000 micrometres (µm)
• 1 micrometre (µm) = 1000 nanometres (nm)

To go to a smaller unit, multiply by 1000. To go to a larger unit, divide by 1000.

Examples: 0.03 mm × 1000 = 30 µm. 5 µm × 1000 = 5000 nm. 200 µm ÷ 1000 = 0.2 mm.

A typical animal cell is about 10–100 µm across, and bacteria are about 1–5 µm long.

A good scientific drawing follows these rules:

• Use a sharp pencil and draw clear, single, continuous lines.
• Draw large, filling at least half the space, with no shading or colouring.
• Draw only what you can see, with the correct proportions.
• Label with ruled lines that touch the part and do not cross. Write the labels horizontally.
• Give a title and state the magnification or scale.

To find the scale of a drawing, measure the drawing, then use magnification = image size ÷ actual size.

An electron microscope uses a beam of electrons instead of light. It has a much greater magnification and a much greater resolution than a light microscope.

• Resolution is the ability to show two points close together as separate points. Higher resolution means more detail.
• A light microscope can show cells and large organelles such as the nucleus.
• An electron microscope can show very small structures, such as ribosomes, plasmids and the detail inside mitochondria, and even viruses.

Electron microscopes are far more expensive than light microscopes and need trained users, so light microscopes are used for everyday work.