12.11
Osmosis and Osmotic Pressure of Solutions
A number of natural and synthetic materials exhibit selective permeation, meaning that only molecules or ions of a certain size, shape, polarity, char…
A semipermeable membrane allows some substances to pass through but not others. This movement of solvent molecules across a semipermeable membrane, to a solution with higher solute concentration, is called osmosis.
Consider a U-shaped tube containing pure water on the left and a sugar solution on the right separated by a semipermeable membrane.
Water molecules will flow to the sugar solution at a faster rate than the reverse to try to establish a concentration equilibrium.
As water flows to the right, the level of liquid in the two arms becomes unequal.
Eventually, the added weight of the excess water on the right creates sufficient pressure to stop osmosis.
The minimum pressure required to halt osmosis is called the osmotic pressure. It is a colligative property that is dependent on the solute concentration in the solution.
As the concentration of the solute increases, the osmotic pressure increases proportionally.
Osmotic pressure, π, can be calculated by multiplying the molarity of the solute by the temperature in kelvin and the ideal gas constant R, 0.0821 liter-atmosphere per mole kelvin.
If the concentration of the sugar solution is 1.00 molar, then at 25 °C or 298 K, the osmotic pressure will be 24.5 atmospheres.
If the osmotic pressure of the two solutions is equal, they are called isotonic.
If one solution has a lower osmotic pressure, then it is hypotonic compared to the solution with higher solute concentration.
The concentrated solution is called hypertonic compared to the dilute solution.
When red blood cells are placed in a hypertonic solution, water leaves through the pores of the semi-permeable cell membrane. This process is called crenation and it causes the cells to become shriveled.
Conversely, if the red blood cells are placed in a hypotonic solution, water moves from the outside into the cells causing the cells to swell and ultimately rupture in a process called hemolysis.
When a person is given intravenous fluids, the fluids must be isotonic with the intracellular solution of blood cells to prevent crenation or hemolysis.
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Q1: What is a semipermeable membrane and how does it work?
A semipermeable membrane allows certain substances to pass through while blocking others based on size, shape, polarity, and charge. Biological cell membranes and dialysis tubing are common examples. These selective barriers enable osmosis by permitting solvent molecules to diffuse across while restricting solute passage, creating the foundation for osmotic pressure development.
Q2: How does osmosis occur across a semipermeable membrane?
Osmosis is the diffusion-driven transfer of solvent molecules through a semipermeable membrane toward a solution with higher solute concentration. Since solvent concentration is greater on the pure solvent side, molecules diffuse faster in that direction, creating net solvent movement. This continues until osmotic pressure builds sufficiently to equalize the transfer rates.
Q3: What is osmotic pressure and how is it calculated?
Osmotic pressure is the minimum pressure required to halt osmosis, calculated using π = MRT, where M is molarity, R is 0.0821 liter-atmosphere per mole kelvin, and T is absolute temperature in kelvin. For a 1.00 molar sugar solution at 25°C, osmotic pressure equals 24.5 atmospheres. It is a colligative property that increases proportionally with solute concentration.
Q4: What are isotonic, hypotonic, and hypertonic solutions?
Isotonic solutions have equal osmotic pressure and show no net water movement across cell membranes. Hypotonic solutions have lower osmotic pressure, causing water to enter cells and potentially rupture them through hemolysis. Hypertonic solutions have higher osmotic pressure, drawing water out and causing crenation, where cells shrivel. Medical IV fluids must be isotonic to prevent these harmful effects.
Q5: Why do red blood cells crenate in hypertonic solutions?
In hypertonic solutions, the external solute concentration exceeds that inside red blood cells, creating an osmotic gradient. Water exits through the semipermeable cell membrane to equalize concentrations, causing cells to lose volume and shrivel in a process called crenation. This demonstrates how osmotic pressure differences drive water movement across biological membranes.
Q6: What happens to red blood cells in hypotonic solutions?
In hypotonic solutions, the external solute concentration is lower than inside cells, so water moves inward across the semipermeable cell membrane. Cells swell as they accumulate water and eventually rupture in a process called hemolysis. This occurs because osmotic pressure inside the cell exceeds external pressure, driving net water influx until the membrane fails.
Q7: How does reverse osmosis differ from normal osmosis?
Reverse osmosis occurs when external pressure exceeding the solution's osmotic pressure is applied, reversing normal solvent flow and pushing solvent molecules from the solution into pure solvent. This technique is used for large-scale seawater desalination and producing high-purity drinking water. Unlike spontaneous osmosis, reverse osmosis requires energy input to overcome the natural osmotic gradient.