Diffusion and Osmosis

Diffusion and osmosis are important concepts that explain how water and other materials that cells need are transported across cell membranes.

Let's talk about diffusion first. It is defined as the net movement of particles from an area of high concentration to an area of lower concentration. The graduated change in concentration between the two areas is referred to as the concentration gradient. Although diffusion is net directional, the particles are constantly moving in both directions due to random motion, so even at equilibrium when the particle density is the same throughout the concentration gradient, particles continue to move in both directions at a constant exchange rate.

Similarly, water moves across cell membranes by diffusion in a process called osmosis, but not everything can freely pass through cell membranes, which is why they are referred to as semi-permeable. This is important, because it means that cells can regulate and maintain different concentrations of solutes inside versus outside their membranes. Depending on the relative solute concentrations of solutions separated by semi-permeable membranes we refer to them as hypotonic, isotonic or hypertonic. Hypotoni