  Inside every living cell is the cytoplasm: a fluid mixture of thousands of
different macromolecules, predominantly proteins. This mixture is where most of
the biochemistry occurs that enables living cells to function, and it is
perhaps the most complex liquid on earth. Here we take an inventory of what is
actually in this mixture. Recent genome-sequencing work has given us for the
first time at least some information on all of these thousands of components.
Having done so we consider two physical phenomena in the cytoplasm: diffusion
and possible phase separation. Diffusion is slower in the highly crowded
cytoplasm than in dilute solution. Reasonable estimates of this slowdown can be
obtained and their consequences explored, for example, monomer-dimer equilibria
are established approximately twenty times slower than in a dilute solution.
Phase separation in all except exceptional cells appears not to be a problem,
despite the high density and so strong protein-protein interactions present. We
suggest that this may be partially a byproduct of the evolution of other
properties, and partially a result of the huge number of components present.
