determines how fast a diffuses.
determines the direction in which a diffuses.
if a has low concentration at some cell then
will be positive and a will diffuse toward this cell.
if a has high concentration at some cell then
will be negative and a will diffuse away from this cell.
Key point: a small amount of variation in the initial concentrations gives an
initially unstable system which may then be driven to a stable state in which
the chemical concentrations vary across the surface, producing a pattern.