01 · Mechanism
Why particles move down a gradient
At any temperature above absolute zero, particles have kinetic energy and move randomly. In a region of high concentration, collisions between particles are more frequent; in a region of low concentration, particles have more space. The net result of all this random movement is a statistical drift from where particles are crowded to where they are sparse — this is diffusion.
Diffusion is an entropic process: the system moves toward maximum disorder (maximum entropy). Once particles are evenly distributed, there is no longer a net movement — though random motion continues. This equilibrium state is called dynamic equilibrium.
HIGH [C]
LOW [C]
02 · Fick's law
Quantifying the rate of diffusion
Adolf Fick (1855) described the rate of diffusion mathematically. In the form used at school level:
Rate of diffusion ∝ (Surface area × Concentration gradient) / Thickness
Additional factors not in Fick's basic law but important in biology:
- Temperature — higher temperature means greater kinetic energy, faster particle movement, steeper collision frequency.
- Molecule size — smaller molecules diffuse faster (Graham's law: rate ∝ 1/√molar mass).
- Polarity and lipid solubility — non-polar molecules cross lipid bilayers faster than polar or charged ones.
03 · Types
Simple, facilitated, and osmosis
SIMPLE DIFFUSION
Molecules move directly through the phospholipid bilayer without assistance. Works only for small, non-polar molecules: O₂, CO₂, N₂, steroid hormones, ethanol.
No protein needed. No energy needed.
FACILITATED DIFFUSION
Large or polar molecules cross via protein channels (e.g. aquaporins for water, GLUT1 for glucose) or carrier proteins that undergo a conformational change. Still down the gradient.
Protein needed. No energy needed.
OSMOSIS
Water moving specifically across a partially permeable membrane from high water potential (dilute) to low water potential (concentrated). A special case of facilitated diffusion — mainly through aquaporins.
See: Osmosis glossary entry.
ACTIVE TRANSPORT
Movement against the concentration gradient, from low to high. Requires protein pumps and ATP. Used in nerve impulses (Na⁺/K⁺ pump), root mineral uptake, and the gut.
Protein needed. ATP required. Not diffusion.
04 · Biology
Organisms adapted for efficient diffusion
| Exchange surface | What diffuses | Adaptation |
|---|---|---|
| Alveoli | O₂ in, CO₂ out | Single-cell epithelium, dense capillary network, large surface area (~70 m²) |
| Small intestine villi | Glucose, amino acids absorbed | Finger-like projections, microvilli (brush border), increase surface area ×30 |
| Root hair cells | Mineral ions absorbed | Long, thin extensions that increase surface area in contact with soil water |
| Gill lamellae (fish) | O₂ in, CO₂ out | Counter-current flow maintains steep gradient; thin epithelium, large area |
| Capillary walls | O₂, glucose to tissues; CO₂, waste out | One-cell-thick walls, short diffusion distance to cells |
The surface area to volume ratio (SA:V) is the key constraint. As an organism grows, volume increases as the cube of linear dimension but surface area only as the square. Above a certain size, simple diffusion cannot supply O₂ fast enough to the interior — which is why all large organisms evolved specialised respiratory and circulatory systems. See Cells and organelles and Cellular respiration for the downstream biology.
Diffusion is also the mechanism by which many drugs reach their target cells — their lipid solubility and molecular size determine how quickly they cross membranes, a key parameter in pharmacokinetics.
Diffusion — common questions
- Diffusion applies to any substance moving down a concentration gradient. Osmosis is specifically water moving across a partially permeable membrane. See the Osmosis glossary entry for the membrane-specific details.
- Simple diffusion requires no energy — it is driven by entropy and random thermal motion. Facilitated diffusion also requires no energy but uses protein channels or carriers. Active transport moves substances against the gradient and requires ATP. See the Cells and organelles guide.
- Fick's law states that the rate of diffusion is proportional to (surface area × concentration gradient) / thickness of membrane. Higher temperature increases kinetic energy of particles; smaller molecules diffuse faster (Graham's law). In organisms, alveoli, villi, and gill lamellae all maximise surface area.
- Facilitated diffusion uses integral membrane proteins — channel proteins (like aquaporins for water, or ion channels) or carrier proteins — to allow large or polar molecules to cross the hydrophobic lipid bilayer passively. It is still net movement from high to low concentration, so no ATP is needed.