∞ generated and posted on 2016.12.16 ∞

Movement of water across membranes, down water's concentration gradient.

Osmosis can be viewed either in terms of water movement relative to itself or instead water movement relative to other things, with both perspectives correct as the more other things (solutes) that are present dissolved in water then the less water must present, resulting in water movement down its concentration gradient towards regions of higher solute concentration/lower water concentration.

Osmosis thus is the movement from regions of high water concentration to regions of low water concentration. Since it is dissolved substances (solutes) that serve to lower water's concentration within a solution, osmosis generally involves movement from regions of low solute concentration (and therefore high water concentration) to regions of high solute concentration (and therefore low water concentration).

Note the dependence of osmosis on membranes being selectively permeable, that is, permeable to water but not to whatever solute that is creating water's concentration gradient that in turn is driving osmosis. That is, these solutes, as we are considering them, cannot themselves cross the membrane, or lipid bilayer.

Sufficient water movement across lipid bilayers can result in what is known as osmotic lysis.

Figure legend: Red blood cells (RBCs), because they lack cell walls, are prone to osmotic lysis given suspension in a hypotonic, that is, low-solute solution. Because solute concentrations inside of these cells is high in comparison with outside, the water osmotically 'rushes' into the cells, expanding the cytoplasm to the point of bursting. Here the small, black circles found both inside and outside of the RBCs are solute particles, i.e., ions or molecules.

The following video provides nice experiments using dialysis tubing indicating the impact of osmosis on cell volume:

The following video provides a short visualization of osmosis though doesn't stress the importance of how collision with the membrane is limiting to movement across it:

It alternatively is possible to move water against its concentration gradient through an input of energy, which experimentally can be provided by adjusting the pressure found on one side of a membrane so that it is higher than that found on the other side. The result can be water being pushed up its concentration gradient from the high solute side of a membrane and towards a new dynamic equilibrium. The pressure applied to the high-solute side of the membrane that just balances osmosis, such that there is no net movement of water across a membrane, provides a measure of a solution's osmotic pressure.

Figure legend: Osmotic pressure is typically measured using a U-tube apparatus. Here the higher solute concentration is found to the right of a dashed line, which represents a semipermeable membrane. Water flows osmotically therefore from the left to the right and as a consequence the water in the column to the right rises. This continues until the water has sufficiently risen that the pressure exerted by the force of gravity exactly counters the force exerted osmotically by water across the membrane. That pressure that exactly balances the force of gravity is known as osmotic pressure which in turn is a measure of the force exerted by water molecules as they attempt to move down their concentration gradient.

The following video discusses the above figure:

The following video is really nicely done, save for some math issues, and uses chicken eggs with dissolved shell as shrinking and expanding cell:

The following video nicely shows contraction of cytoplasm of onion cells while in a hypertonic solution:

See also the concepts of isotonic solution, hypertonic solution, and hypotonic solution.