Microscopy, scale and biological organisation

What you’ll learn

• Distinguish magnification from resolution and compare microscopy methods.
• Calculate actual size using magnification, scale bars or a calibrated graticule.
• Relate cells to tissues, organs and organ systems.

Understand the idea

Magnification describes how much larger an image is than the object. Resolution is the ability to distinguish two close points as separate; increasing image size alone does not reveal detail below the resolution limit. Electron microscopes can resolve finer structures than light microscopes because their effective wavelength is much shorter. Transmission electron microscopy shows internal detail in thin sections, whereas scanning electron microscopy shows surface form. Conventional electron-microscope preparation does not preserve living specimens.

Staining increases contrast between structures that would otherwise be difficult to distinguish. A light-microscope eyepiece graticule has arbitrary divisions and must be calibrated against a stage micrometer at the objective magnification being used. A change of objective changes the real length represented by a graticule division. A scale bar is particularly useful because it enlarges along with the image when a figure is resized. Convert image length and actual length to the same units before dividing.

In a multicellular organism, specialised cells cooperate in tissues. Different tissues form an organ with a shared function, and interacting organs form an organ system. For example, cardiac muscle, connective tissue and vascular tissue contribute to the heart, which is part of the circulatory system. Core practical 5 develops observation and measurement of animal cells: biological drawings show clear outlines and labels based on visible evidence, rather than artistic shading or imagined organelles.

Key formulae

$$\text{magnification}=\frac{\text{image size}}{\text{actual size}}$$

$$1\ \mathrm{mm}=1000\ \mathrm{\mu m}$$

$$\text{actual size}=\text{graticule divisions}\times\text{length per division}$$

Worked example

At one objective setting, 40 graticule divisions match 100 micrometres on a stage micrometer. A cell spans 18 divisions. Calculate the cell’s length.

1. The calibration is 100/40 = 2.5 micrometres per division.
2. The cell spans 18 divisions at this same objective setting.
3. Its length is 18 × 2.5 = 45 micrometres.

Answer: $45\ \mathrm{\mu m}$.