INTRODUCTION

Biochemical analysis is frequently accompanied by microscopic examination of tissue, cell or organelle preparations. Such examinations are used in many different applications; for example, to evaluate the integrity of samples during an experiment, to map the fine details of the spatial distribution of macromolecules within cells, or to directly measure biochemical events within living tissues.

There are two fundamentally different types of microscope: the light microscope and the electron microscope (Fig. 4.1). Light microscopes use a series of glass lenses to focus light in order to form an image whereas electron microscopes use electromagnetic lenses to focus a beam of electrons. Light microscopes are able to magnify to a maximum of approximately 1500 times whereas electron microscopes are capable of magnifying to a maximum of approximately 200 000 times.

Magnification is not, however, the best measure of a microscope. Rather, resolution, the ability to distinguish between two closely spaced points in a specimen, is a much more reliable estimate of a microscope's utility. Light microscopes have a resolution limit of about 0.5 micrometres (μm) for routine analysis. In contrast, electron microscopes have a resolution of up to 1 nanometre (nm). Both living and dead specimens are viewed with a light microscope, and often in real colour, whereas only dead ones are viewed with an electron microscope, and never in real colour. Recent advancements have improved upon the 0.2μm resolution limit of the light microscope for some special applications (Section 4.8).