Osmolarity and Isotonicity

Osmosis Osmosis is the net movement of water across a selectively permeable membrane, driven by differences in the concentration of osmotically effective solutes. Water tends to move toward the side with the greater effective concentration of nonpenetrating solute. Osmosis is a form of passive transport, meaning it does not require cellular energy such as ATP.

A selectively permeable membrane allows certain substances to cross more easily than others. During osmosis, water can cross the membrane while some solutes cannot cross freely. Water redistributes until equilibrium is reached or until pressure opposes further movement.

Osmosis is important for:

Maintaining cell volume
Balancing body fluids
Preparing intravenous solutions
Kidney function
Preventing cell swelling or shrinking

Aquaporins Water can pass directly through the plasma membrane, but it does so slowly because water is polar and the interior of the phospholipid bilayer is hydrophobic. Aquaporins greatly increase the rate at which water can cross many cell membranes. These channels:

Form channels specifically for water
Allow rapid water movement
Do not directly require ATP, so they are a form of facilitated diffusion
Increase membrane permeability to water

Osmolarity Osmolarity is the total concentration of dissolved solute particles in one liter of solution. It measures the total concentration of all dissolved particles, including penetrating and nonpenetrating solutes, and describes the solution itself. It is commonly expressed as osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L).

A solute that dissociates into multiple particles contributes more to osmolarity. For example, NaCl dissociates into Na⁺ and Cl⁻, so 1 mole of NaCl yields approximately 2 osmoles of particles.

Low Osmolarity: Fewer dissolved particles, more water
High Osmolarity: More dissolved particles, less water

When two solutions of different osmolarities are separated by a membrane permeable only to water, water moves from the side with lower solute concentration toward the side with higher solute concentration.

Osmolarity Calculations Osmolarity can be estimated by multiplying molarity by the number of particles the solute produces in solution:

Osmolarity ≈ Molarity × van't Hoff factor (i)

Van 't Hoff Factor (i) The van 't Hoff factor (i) represents the number of dissolved particles produced per formula unit of a solute. Molecular compounds that do not dissociate, such as glucose, have an i ≈ 1. For ionic compounds, the ideal van 't Hoff factor is approximately equal to the total number of ions produced when one formula unit dissociates.

Examples:

Glucose → i = 1
NaCl → Na⁺ + Cl⁻ → i = 2
CaCl₂ → Ca²⁺ + 2 Cl⁻ → i = 3
AlCl₃ → Al³⁺ + 3 Cl⁻ → i = 4

Sample calculation: 1 M CaCl₂ × 3 = 3 Osm/L

Tonicity Tonicity describes how a solution affects the volume of a cell by causing water to move into or out of it. It predicts the effect of a solution on cell volume, depends mainly on nonpenetrating solutes, depends on membrane permeability, and describes a solution relative to a particular cell.

Tonicity depends on:

The concentration of solutes outside the cell
The concentration of solutes inside the cell
Whether the solutes can cross the membrane

Isotonic: An isotonic solution has approximately the same effective concentration of nonpenetrating solutes as the inside of the cell. Water moves in both directions, but there is no net movement of water and cell volume remains stable. 0.9% NaCl is a common isotonic IV solution. D5W (5% dextrose in water) is approximately isotonic in the bag but becomes physiologically hypotonic after the glucose is taken up and metabolized.

Hypotonic: A hypotonic solution has a lower concentration of nonpenetrating solutes than the inside of the cell. Water moves into the cell, the cell gains water, swells, and may rupture.

Hypertonic: A hypertonic solution has a greater concentration of nonpenetrating solutes than the inside of the cell. Water moves out of the cell, the cell loses water, and cell volume decreases (cell shrinks).

Osmotic Pressure Osmotic pressure is the pressure required to stop the net movement of water across a selectively permeable membrane. Higher concentrations of osmotically active particles generally produce greater osmotic pressure.

General Chemistry

Key Points