Motion

Abstract

It has been known for many years that the membranes of living cells can flow. Hydrodynamic flow of a biological membrane in the liquid crystal state can be expected, and its quantitative analysis is the province of fluid dynamics. In this chapter we will be mainly concerned with rather different kinds of motion that have been revealed more recently by spectroscopic and optical experiments on bilayers and natural membranes. We refer to lateral diffusion of lipids across bilayers, rotation and translation of proteins and other microscopic forms of motion that can be broadly classified as diffusional and involve the displacement or rotation of complete molecules with respect to a set of coordinates passing through the local center of mass. Motions of these kinds certainly occur in natural membranes and are implicated in several important biological phenomena such as capping, a process familiar in immunology involving the aggregation of membrane proteins. Here we concentrate on the basic molecular mechanisms responsible for motion rather than its biological consequences. Experiments in this field are usually summarized in the form of diffusion coefficients (see Chapter 12).