The microscope in cell studies

The light microscope remains an essential tool because it allows quick observation of living and stained cells through temporary slides, where thin specimens are mounted under a coverslip that is lowered carefully to avoid trapping air bubbles.

Biological drawings from slides

• Accurate, showing only what is genuinely visible (not what is expected)
• Unshaded, with no shading or colouring
• Properly and fully labelled
• Drawn using clean, continuous (single, unbroken) lines

Quantitative work depends on the magnification formula, with both the image size and the actual size expressed in the same unit:

magnification = image size ÷ actual size actual size = image size ÷ magnification

Real cell sizes are obtained by calibrating an eyepiece graticule against a stage micrometer, so that the arbitrary eyepiece graticule divisions can be converted into micrometres (μm) for each objective lens.

The key conceptual distinction in this topic is between magnification and resolution:

• Magnification is the number of times larger an image is than the actual size of the object; it simply enlarges and adds no further detail.
• Resolution is the ability to distinguish between two points that are close together as separate; it determines how much detail can actually be seen and is set by the wavelength of the radiation used.

Light microscopes are limited to a resolution of around 200 nm, whereas electron microscopes use beams of electrons of much shorter wavelength to achieve resolutions down to about 0.2 nm.

This understanding underpins the rest of Unit 1, because cell ultrastructure can only be discussed once students recognise why electron microscopy is necessary to reveal it.