Laboratory Two
Osmosis, because it is different in plants!
Introduction
Osmosis
is the movement of solvent molecules through a selectively
permeable membrane into a region of higher solute concentration, aiming to equalize
the solute concentrations on the two sides. The term osmosis may also be used to
describe a physical process in which any solvent moves, without input of energy,
across a semi-permeable membrane (permeable to the solvent, but not the solute)
separating two solutions of different concentrations. Although osmosis does not
require input of energy, it does use kinetic energy and can be made to do work.
The first documented observation of osmosis was made by Jean-Antoine
Nollet (19th November 1700 – 25th April 1770) in 1748, who was a French
clergyman and physicist.
Osmosis is essential 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 and/or hydrophobic molecules like lipids as well as to small molecules like
oxygen, carbon dioxide, nitrogen, nitric oxide.
Tonicity
is the osmolarity of a solution--the amount of
solute
in a solution. A
Solute is any dissolved substance in a solution, such as sugars and salts. The term
Tonicity is commonly used when describing the response of cells immersed in an
external solution. Like osmotic pressure, tonicity is influenced only by solutes that
cannot cross the membrane, as only these exert an osmotic pressure. Solutes able
to freely cross the membrane do not affect tonicity because they will always be in
equal concentrations on both sides of the membrane.
There are two things to
always
remember about osmoses and tonicity:
• Tonicity is always in comparison to a cell.
• The cell has a specific amount of sugar and salt
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