A cell is the smallest LIVING unit in a body. An atom is the smallest "nonliving" unit.
Water doesn't have a large Ka (acid dissociation constant), hence the equilibrium of
H2O ⇌ H+ + OH-
lies on the far left.
therefore a container of water wouldn't have many H+ and OH- ions floating about.
There WOULD be a very very very small concentration and H+ and OH- in it, since a value of Ka means that it does ionise in itself, but on the whole there would be MORE water molecules floating about. This is why water acts as a very poor buffer.
You need the acid AND the conjugate base (salt) for a buffer to work.
The answer is: generation of graded potential. Graded potentials are membrane potential of varying sizes in mV that are a result of opening up of ionotropic channels. When the graded potentials net value add up and reach the threshold potential, then an impulse is generated in the post synaptic neuron.