Bulk flow is the mass movement of one or
more substances in response to pressure, gravity, or some
other external force, like the flow of blood in the circulatory
system. Osmosis is the passive movement of water across a
differentially permeable membrane in response to solute
concentration gradients, pressure gradients, or both.
Osmotic movements are affected by the relative concentrations
of solutes (called tonicity) in the fluids inside and
outside the cell.
Three conditions can occur:
A hypotonic fluid has a lower concentration of solutes than
does the fluid in the cell; cells immersed in it may swell. In a
hypOtonic solution, a cell will swell until it looks like an O, and
eventually burst.
A hypertonic fluid has a greater concentration of solutes than
does the fluid in the cell; cells in it may shrivel. In a hypERtonic
solution, a cell will shrivel.
An isotonic fluid has the same concentration of solutes as
the fluid in the cell; immersion in it causes no net movement of
water. In an isotonic solution, a cell will remain the same size.
Any volume of fluid exerts hydrostatic pressure against a cell
membrane. Osmotic pressure is the amount of force that
prevents any further increase in the volume of solution inside a
cell.
