One of the continuing intellectual challenges in the field of coated vesicle research is extrapolation from morphological and experimental data, obtained from observations at single points in time, to a concept of vesicle behaviour as a continuum. We would like to know more of the dynamics of coated vesicles expressed in supramolecular and, hopefully, molecular terms. Although little of the data presently available is conclusive there is sufficient circumstantial evidence to enable us to attempt to set up hypotheses about how coated vesicles are formed, move and are discharged. Specifically consideration will be given to the following problems:

1. (1) What biochemical pathways and structures are essential to coated vesicle function? We can expect the results of investigations using stimulators and inhibitors of endocytosis to eliminate some of the mechanisms put forward to account for coated vesicle behaviour.

2. (2) How does the membrane invaginate into the configuration of a coated vesicle? The induction of curvature entails changes in the local molecular arrangement or composition of the membrane or associated structures probably brought about by movement of constituent molecules. We will define this as membrane motility.

3. (3) Once a coated vesicle has formed it travels to one of a number of specified destinations (see Chapter 2). This process might or might not require a structured motile system. We will call this dynamic process vesicle motility to distinguish it from the membrane motility just described. We will consider the evidence relevant to it.

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