Biophysical chemists study protein structure and the functional structure of cell membranes. For example, enzyme action can be explained in terms of the shape of a pocket in the protein molecule that matches the shape of the substrate molecule or its modification due to binding of a metal ion. Similarly, biophysical chemists study the structure and function of the biomembranes using model supramolecular structures. These include liposomes or phospholipid vesicles of different compositions and sizes.