Definition · Plain-language
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
The step most authors miss
Doing CRediT right? Don’t stop at the statement.
A CRediT statement credits you inside one paper. The recognition CRediT was built for happens when those roles are tied to you, persistently. Sign in with your ORCID — free — and claim your CRediT contributions on casrai.org, the home of the standard. They become a verified, portable part of your identity, not a line that disappears into one PDF.
Free: claim your contributions, then export a journal-ready CRediT statement, schema.org structured data, JATS XML, CSV or BibTeX — and preview your public profile. A membership publishes that profile publicly and verifies the journals you serve.
Particles spreading out
Diffusion is the natural spreading of particles from where they are crowded to where they are sparse. Particles in gases and liquids are always moving randomly, and over time this random motion carries them from a region of high concentration to one of low concentration, until they are spread out evenly. The difference between the two concentrations is called the concentration gradient, and particles are said to move "down" it. A familiar example is the smell of cooking spreading through a house: the scent particles diffuse from the kitchen outwards.
Diffusion in living things
Diffusion is essential to life. It is how oxygen passes from the air in the lungs into the blood, and from the blood into cells that need it for respiration. It is also how carbon dioxide, a waste product, moves the other way to be breathed out. In the gut, digested food molecules diffuse into the blood. Because cells rely on diffusion to exchange substances with their surroundings, anything that affects its rate — a steeper concentration gradient, a larger surface area, a shorter distance or a higher temperature — speeds up these vital exchanges.
A passive process
Diffusion is passive, which means it happens on its own and the cell does not spend energy to make it occur. The energy for the movement comes from the random motion of the particles themselves. This sets diffusion apart from active transport, which moves particles against the concentration gradient — from low to high — and does require energy from respiration. Because diffusion only ever moves particles down the gradient, it continues until the concentrations are balanced and then appears to stop, though the particles keep moving randomly.
Key facts
At a glance
- Definition: net movement of particles from high to low concentration
- What moves: gases or dissolved substances
- Driven by: the concentration gradient (the difference in concentration)
- Energy: none needed — it is passive
- In the body: oxygen into cells, carbon dioxide out
- Faster when: steeper gradient, larger area, shorter distance, higher temperature
Common misconceptions
What people often get wrong
Often heard: Diffusion moves particles from low concentration to high concentration.
Actually: Diffusion moves particles the other way — from high concentration to low, down the gradient. Moving particles from low to high concentration is active transport, which needs energy. Diffusion is passive and goes down the gradient only.
Often heard: Diffusion needs the cell to use energy.
Actually: Diffusion is passive: it happens by itself from the random motion of particles, with no energy spent by the cell. Active transport is the process that requires energy, because it works against the concentration gradient.
Often heard: Diffusion stops completely once the particles are evenly spread.
Actually: The particles never stop moving; they keep moving randomly. What stops is the net movement in one direction — once concentrations are equal, just as many particles move each way, so there is no overall change.
Going deeper
