Osmosis is the diffusion of water across a membrane against the
gradient. this means that water molecules diffuse across the membrane
from a low ionic strenght to high ionic strenght containing solutions.
diffusion is where molecules travel across membranes along the gradient.
it means that molecules will travel across a membrane from a solution
of higher concentration to a solution of lower concentration.
diffusion also includes diffusion of water ie osmosis. therefore osmosis
is a part of diffusion. whenever molecules diffuse across a membrane,
water also diffuses across the membrane in the opposite direction that
the molecules diffuse across.