Kaplan MCAT General Chemistry Review

One of the most important roles of the cell membrane is to maintain the unique differences between the intracellular and extracellular compartments. Both of these compartments are aqueous solutions separated by the cell membrane, which functions as a semipermeable barrier. Channels and pumps in the membrane serve to create and maintain concentration gradients of various solutes on either side of the membrane. A delicate balance of water influx and efflux must be established in order to prevent the cell from becoming dehydrated and shrinking or waterlogged and possibly bursting. The extracellular compartment itself is in continuity with the plasma compartment of the blood. A balance between these two compartments is dependent upon two pressures: the hydrostatic pressure of the blood generated by the contraction of the heart and the osmotic pressure of the plasma compartment, primarily established by the concentration of plasma proteins such as albumin. (Since the osmotic pressure of the plasma compartment is based on the concentration of plasma protein, it is also called oncotic pressure.) The hydrostatic pressure tends to push fluid volume out of the vascular compartment into the extracellular compartment, while the osmotic pressure tends to pull fluid volume back into the vascular compartment from the extracellular compartment.

Consider a container separated into two compartments by a semipermeable membrane (which, by definition, selectively permits the passage of certain molecules). One compartment contains pure water, while the other contains water with dissolved solute. The membrane allows water but not solute to pass through. Because substances tend to flow, or diffuse, from higher to lower concentration (which results in an increase in entropy), water will diffuse from the compartment containing pure water into the compartment containing the water-solute mixture. This net flow will cause the water level in the compartment containing the solution to rise above the level in the compartment containing pure water.

Because the solute cannot pass through the membrane, the concentrations of solute in the two compartments can never be equal. However, the hydrostatic pressure exerted by the water level in the solute-containing compartment will eventually oppose the influx of water; thus, the water level will rise only to the point at which it exerts a sufficient pressure to counterbalance the tendency of water to flow across the membrane. This pressure, defined as the osmotic pressure (? ) of the solution, is given by this formula:

? = iM RT

where M is the molarity of the solution, R is the ideal gas constant, T is the absolute temperature (in Kelvin), and i is the van’t Hoff factor. The equation clearly shows that osmotic pressure is directly proportional to the molarity of the solution. Thus, osmotic pressure, like all colligative properties, depends only upon the presence of the solute, not its chemical identity.

Bridge

Osmosis explains how many biological systems regulate their fluid levels.

MCAT Expertise

On the MCAT, remember always to use Kelvin temperatures when T is in an equation!

One application of osmotic pressure is a particular method of water purification called reverse osmosis (RO). In reverse osmosis, impure water is placed into one container separated from another container by a semipermeable membrane. High pressure is applied to the impure water, which forces it to diffuse across the membrane, filling the compartment on the other side of the membrane with purified water. Because the water is being forced across the membrane in the direction opposite its concentration gradient (that is, the water is being forced from the compartment with the lower concentration of water to the compartment with the higher concentration of water), large pressures (higher than the solution’s osmotic pressure) are needed to accomplish the purification.

Key Concept

Water will move toward the chamber with either greater molarity or (if the molarity is the same) to the chamber with higher temperature.

Conclusion