The central points of this paper can now be summarized. We consider here, for simplicity only, vanishing particle concentration. In equilibrium sedimentation equation (6) applies. The density increment is a measurable quantity. It can either be introduced into equation (6) to calculate M2, or it can be analysed by equations (7) and (8) to provide additional information on specific volumes and solute interactions.

Light scattering is determined by the analogous equation (20). The refractive index increment is also experimentally accessible and its structure (not considered here) is similar to that of the density increment. Small angle X-ray scattering is determined by equation (31) and the electron density increment which appears in this equation cannot be directly determined by experiment. Yet it can be obtained in straightforward fashion from the mass density increment, by equation (34). Similarly, in the case of neutron scattering (equation (38)), the scattering length density increment is obtained from the mass density increment by equation (40), or it may now be directly evaluated by neutron interferometry.

The apparent and often spectacular movements of animals and insects, movements of the whole organism in relation to its surroundings arising from internally generated forces, have always been, by their very ubiquity, uppermost in our perception of motion in the living world. Movement in plants, generally of one organ in relation to the whole plant, whilst sometimes spectacular, have often in the past been seen as rather esoteric events, amusing perhaps, but of little importance in the general biological scheme of things. However, this is not so; plant movements are quite widespread in occurrence and all are most probably manifestations of a single physiological process, the change in volume of special motor cells. One particular movement, the opening and closing of stomata, which provides a basic control of photosynthesis, is of fundamental importance to the existence of the whole biosphere.

The spin label method developed by McConnell 15 years ago is now widely used in studies of the structure and dynamic properties of a variety of the biological systems such as proteins and protein complexes, lipids and membranes, nucleic acids, nucleoproteins, etc.

The ESR spectrum of the nitroxide radcal – the spin label – is very sensitive to its microenvironment and permits easy registration of even subtle alterations in it. If spin labels are attached to different sites of a macromolecule the information can be gained about conformational properties of all these local regions and, as a result, about the dynamic behaviour of the object as a whole.