Osmosis & Tonicity
In Unit 1 you studied how solutes cross a membrane. Unit 2 will cover the movement of water. Suppose
a membrane separates two solutions with different solute concentrations. Also assume that the membrane is
impermeable to the solute (there is no way for the solute to cross the membrane), but water can freely pass
through. Water will move through the membrane from the side with the lower solute concentration to that with
the higher solute concentration. The water will "dilute" the higher concentration solution. This can also be
thought of in terms of the "water concentration". The higher the solute concentration, the lower the water
concentration. So water moves from the side with higher "water concentration" to the side with lower "water
concentration" until equilibrium is reached (no net water movement).
The concepts of osmolarity and tonicity will be presented. These are important concepts. Make sure you
The cell membrane separates the body into intracellular and extracellular compartments. The
extracellular compartment includes the interstitial fluid and blood plasma. The fluid amounts for each
compartment are given in figure 525. Note that 2/3 (~28 L) of the fluid is in the intracellular compartment, with
the remaining 1/3 (~14 L) split with 75% in the interstitial fluid and only 25% in the blood plasma.
3 Body Fluid Compartments:
• Note that plasma makes up 25% of the ECF with the interstitial fluid making up the other 75%
Intracellular Fluid (ICF) Extracellular Fluid (ECF)
(Plasma = 8%) (Interstitial Fluid = 25%)
Unit 1 explored the movement of solutes through the membrane. In this unit, you will be studying the
movement of water osmosis (figure 526). Osmolarity and tonicity are important concepts (figure 528, tables
56 and 57). Take the time to understand them.
Osmosis: The movement of water across a membrane in response to a solute concentration gradient.
• The pressure that exactly opposes a given concentration gradient
• Important factor is the # of particles in a solution, not the number of molecules
o Some molecule dissociate into ions when they dissolve into a solution (NaCl ▯Na and Cl)
• Water moves by osmosis in response to the total concentration of particles in a solution
o Increasing particles = Increasing osmotic pressure
The molarity of a solution reflects the number of molecules of a substance in a solution. Osmolarity
reflects the number of particles of a substance in a solution. If a molecule dissociates into 2 or more particles
when in solution, the osmolarity is greater than the molarity of the solution. For example, NaCl dissociates into
1 Module III
Osmosis & Tonicity
2 particles in solution Na and Cl. Therefore, the osmolarity of a 1 M solution will be 2 OsM. Glucose does
not dissociate in water, so a 1M solution of glucose will also be a 1 OsM solution.
Osmolarity: The number of particles (ions or intact molecules) per liter of solution. Measured in osmoles per
Isosmotic: 2 solutions with the same osmolarity (same concentration, same a