GENERAL IDEA

In this chapter we discuss the techniques which are available to the experimental scientist who wishes to visualize or detect single biological molecules primarily using visible light, both in the test tube and in the living cell.

Introduction

How can we ‘see’ something like a single biological molecule, which is of the order of a thousand million times smaller than a typical object in the macroscopic world that we visualize with our naked eyes? The region of the human eye responsible for detecting light, the retina, consists of two types of cells called rods and cones (rods differ by being 100 times more sensitive than cones but they respond more slowly, have less spatial resolution and do not discriminate colour), both of which can convert detected photons of light into electrical signals, conveyed via ion channels and nerve fibres into the brain. The resolving power of the human eye, the visual acuity, is a measure of the smallest angular separation that the eye can resolve, which for humans has a theoretical limit equivalent to ~0.01°, about 20 milliradians, determined by the limit of optical diffraction set by the wavelength of the incident light and the diameter of the aperture in front of the ‘imaging device’ (here set by the pupil of the eye), as shown in Figure 3.1A.