Diffusion
Exam-ready definition
The net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.
How this is marked
Examiners pay for 'net movement' and 'down a concentration gradient'. The phrase 'random movement' earns the third mark in full definitions. Common penalty: saying particles 'want to' spread out. Particles have no intention; keep the language passive.
In an exam answer
To explain gas exchange in a leaf: 'Carbon dioxide diffuses into the leaf from a region of its higher concentration outside to its lower concentration inside, down the concentration gradient.'
Full topic guide: Movement into & out of cells
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