BIOL 2011L Lecture Notes - Lecture 3: Osmotic Pressure, Colligative Properties, Lipid Bilayer

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23 Nov 2017
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OSMOSIS
Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable
membrane into a region of higher solute concentration, in the direction that tends to
equalize the solute concentrations on the two sides. It may also be used to describe a
physical process in which any solvent moves across a semipermeable membrane
(permeable to the solvent, but not the solute) separating two solutions of different
concentrations. Osmosis can be made to do work.
Osmotic pressure is defined as the external pressure required to be applied so that there is
no net movement of solvent across the membrane. Osmotic pressure is a colligative
property, meaning that the osmotic pressure depends on the molar concentration of the
solute but not on its identity.
Osmosis is a vital process in biological systems, as biological membranes are
semipermeable. In general, these membranes are impermeable to large and polar
molecules, such as ions, proteins, and polysaccharides, while being permeable to non-polar
or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon
dioxide, nitrogen, and nitric oxide. Permeability depends on solubility, charge, or chemistry,
as well as solute size. Water molecules travel through the plasma membrane, tonoplast
membrane (vacuole) or protoplast by diffusing across the phospholipid bilayer via
aquaporins (small transmembrane proteins similar to those responsible for facilitated
diffusion and ion channels). Osmosis provides the primary means by which water is
transported into and out of cells. The turgor pressure of a cell is largely maintained by
osmosis across the cell membrane between the cell interior and its relatively hypotonic
environment.
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