The inside of a cell is crowded with molecules, and these are in continuous vigorous motion that is driven by thermal energy. In prokaryotic cells the cytoplasm is the only compartment; in most eukaryotic cells it is still the largest single compartment. The cytoplasm (also called the cytosol) is so crowded with small and large molecules that it is significantly more viscous than a typical aqueous solution encountered in laboratory experiments. As molecules in random motion collide, they diffuse throughout the cell; large molecules diffuse more slowly than small ones. It is the diffusion and collisions between molecules that enable biochemical reactions to occur.