Diffusion, Osmosis, and Membrane Principles

Overview

This lecture covers the principles of diffusion, osmosis, membrane permeability, and tonicity, including related lab experiments and key terminology.

A solute is a substance that dissolves in a liquid (e.g., salt, sugar).
A solvent is the liquid that dissolves the solute; in biology, water is the universal solvent.
A solution is a mixture of solute dissolved in solvent.
Media refers to substances through which diffusion occurs (e.g., water, air, gels).

Diffusion is the random movement of molecules from high to low concentration until equilibrium.
A concentration gradient exists when concentrations differ across space.
The rate of diffusion increases with greater concentration gradients, lower media density (viscosity), higher temperature, higher membrane permeability, and smaller molecular size.
Heavier molecules diffuse slower than lighter molecules.

Mixing hydrochloric acid (heavy) and ammonia (light) gases forms a visible cloud closer to the acid due to its lower diffusion rate.
In agar gel, increasing crystal concentration increases the rate of diffusion.
Measure diffusion distances in mm by multiplying cm by 10.

Dialysis tubing acts as a selectively permeable membrane allowing small molecules (iodine, glucose) to diffuse but blocking larger molecules (starch).
Iodine turns purple in the presence of starch, indicating their interaction.
Glucose detected with test strips: blue (no glucose), brown (glucose present).

Osmosis is the movement of water across a selectively permeable membrane toward the saltier (hypertonic) side.
Tonicity refers to a solution’s ability to change cell water volume via osmosis.
Isotonic: equal solute concentration inside and outside—cell shape remains normal.
Hypertonic: higher solute outside—cell shrinks (crenates).
Hypotonic: higher solute inside—cell swells and may burst (lyse).
Animal cells burst in hypotonic solutions; plant cells swell but don't burst due to cell walls.

Plant (Elodia) cells shrink in hypertonic (salty) solution and swell in hypotonic (pure water).
Red blood cells: hypertonic (cells shrink), isotonic (cells normal), hypotonic (cells burst/disappear).

Viscosity — thickness or density of a liquid.
Equilibrium — uniform distribution of molecules.
Molecular weight — mass of a molecule, affects diffusion rate.
Selectively permeable membrane — allows certain molecules to pass.
Non-permeable solutes — substances that cannot cross membranes (e.g., salt).
Crenate — cell shrinking due to water loss.
Lyse — cell bursting due to water intake.
Osmolarity — concentration of solute particles in a solution.