Diffusion, osmosis and facilitated diffusion

Diffusion and concentration gradients

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by their own random motion. The difference in concentration between two regions is called a concentration gradient; the steeper the gradient, the faster the net movement. Diffusion continues until the substance is evenly spread, a state called equilibrium, after which particles still move but there is no net change. Diffusion is passive: it needs no ATP because the particles’ own kinetic energy provides the movement. Several factors affect how quickly it happens: a shorter distance, a higher temperature, a smaller particle size and a larger surface area all speed it up. This is why gas exchange surfaces are thin and large, and why cells are usually small.

For example: Oxygen is more concentrated in the air sacs of the lungs than in the blood arriving there, so it diffuses into the blood. Carbon dioxide is more concentrated in the blood, so it diffuses the other way.

Common misconception: Diffusion does not stop when concentrations become equal; particles keep moving, but the net movement is zero at equilibrium.

Osmosis and tonicity

Osmosis is the diffusion of water across a selectively permeable membrane, from the side with more water (fewer dissolved particles) to the side with less water (more dissolved particles). A dissolved particle is a solute, and the fluid that dissolves it is the solvent; in the body, water is the main solvent. Because the membrane allows water but restricts many solutes, water moves to balance the concentration on both sides. Tonicity describes how a solution affects a cell. For tonicity, compare the dissolved particles that cannot readily cross this membrane. An isotonic solution has a similar effective concentration to the cell, so water moves in and out equally and the cell keeps its shape. In a hypotonic solution, water enters and the cell may swell and burst. In a hypertonic solution, water leaves and the cell shrinks. These effects are the reason intravenous fluids are chosen carefully.

For example: Placing a red blood cell in pure water (hypotonic) causes water to rush in and the cell to burst. Placing it in strong salt solution (hypertonic) draws water out and the cell shrivels.

Facilitated diffusion

Some substances cannot cross the lipid part of the membrane even though a gradient favours their movement. Ions are charged and cannot pass the water-avoiding lipid core; glucose is large and water-attracting. For these, the membrane provides transport proteins. In facilitated diffusion, a substance moves down its gradient through a channel protein (an open pore) or a carrier protein (which changes shape to carry it across). Water uses channel proteins called aquaporins in many cells. Facilitated diffusion is still passive: it uses no ATP, and it cannot move a substance against its gradient. It differs from simple diffusion in two useful ways: it is selective, because only substances that fit the protein can pass, and it can be saturated, because there are a limited number of proteins. Adding more solute speeds it up only until every protein is occupied.

For example: Glucose enters many cells by facilitated diffusion when its blood concentration is high, using carrier proteins that change shape to release it inside.

Comparing the three passive movements

The three processes share one rule: net movement is down a gradient and costs no ATP. They differ in what moves and whether a protein is needed. Simple diffusion moves any substance that can cross the lipid bilayer, such as oxygen, carbon dioxide and small uncharged molecules. Osmosis specifically describes water moving across a selectively permeable membrane because of a solute gradient. Facilitated diffusion moves substances that need a protein, such as ions, glucose and water through aquaporins. A useful way to remember the difference is to ask two questions: what is moving, and does it need help? Water moving because of solutes is osmosis; a solute moving through a protein is facilitated diffusion; a solute slipping through the lipid is simple diffusion. None of them can move a substance against its gradient; that requires active transport and energy.

For example: In the same cell, oxygen enters by simple diffusion, a sodium ion enters through a channel and glucose enters through a carrier — three routes, one passive principle.

Why these movements matter in care

Passive movements explain many everyday clinical observations. Oxygen reaching tissues depends on diffusion gradients, so anything that thickens the exchange surface or lowers the oxygen concentration in the air reduces it. Fluid exchange between blood and tissues reflects outward blood pressure, the inward pull from plasma proteins and lymph drainage. Low plasma protein levels weaken that inward pull, so more filtered fluid may remain in tissues and cause swelling. Intravenous fluids are chosen by tonicity: an isotonic fluid such as 0.9% saline is used to replace volume without shifting water into or out of cells, while a hypertonic fluid may be used deliberately to draw water out of swollen tissue. Knowing which way water will move lets a nurse anticipate the effect of a fluid, explain a change in a patient’s condition and recognise when a treatment is doing the opposite of what was intended.

For example: A patient with severe vomiting loses water and salts. An isotonic fluid replaces volume without causing red cells to swell or shrink; giving pure water intravenously would be dangerous.