Particles in liquids and gases never stop moving. Each one wanders at random, yet the crowd as a whole spreads from where it is packed to where it is sparse. Living things use this free movement to take in oxygen, get rid of carbon dioxide and move dissolved food into cells.
Diffusion is 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. The energy comes from the kinetic energy of the moving molecules and ions themselves, so the cell spends no energy. Some substances, such as oxygen, carbon dioxide and glucose, move into and out of cells by diffusion through the cell membrane.
Fig. 1: every particle moves at random, but more move from the crowded side to the sparse side than back
Section 01
What diffusion is.
Drop dye into still water. No one stirs it, yet it spreads until the colour is even. Each dye particle moves in a random direction, bumping into water molecules. Where there are more dye particles, more of them happen to move away than move in, so the net flow is down the concentration gradient.
Net movement: particles move both ways all the time; diffusion is the overall (net) movement from higher to lower concentration.
Energy: comes from the kinetic energy of the random movement of molecules and ions, not from respiration. Diffusion is a passive process.
When it stops: once the particles are evenly spread there is no concentration gradient. Particles still move, but as many move one way as the other, so there is no net movement.
Temperature: warmer particles have more kinetic energy and move faster, so the dye spreads faster at 60 °C than at 5 °C.
Examiner trap
Do not say particles stop moving when diffusion is complete. They keep moving at random; it is the net movement that becomes zero because there is no longer a concentration gradient.
Section 02
Four factors that change the rate.
In the investigation tab particles pass from a left chamber through an opening in a wall into a right chamber. Change one factor at a time, keep the others the same, and record the rate each time.
Factor
Change that increases the rate
Why
Surface area
larger area
more particles can cross at the same time
Temperature
higher temperature
particles have more kinetic energy and move faster
Concentration gradient
steeper gradient (bigger difference)
far more particles move down the gradient than back
Distance
shorter distance (thinner barrier)
particles take less time to cross
Fair test: change only the independent variable; keep the other three factors constant.
Classic practical: agar cubes containing an indicator placed in acid. Smaller cubes have a larger surface area to volume ratio, so the colour change reaches the centre sooner.
Examiner trap
"Higher concentration" on its own is not a factor. The rate depends on the difference in concentration between the two regions, the concentration gradient.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the dye simulation, what happens once the dye is evenly spread through the water?
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Where does the energy for diffusion come from?
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In the factors tab, which change makes particles cross the wall more slowly?
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Why does raising the temperature increase the rate of diffusion?
Section 03
Why organisms need diffusion.
Gases and dissolved substances (solutes) move into and out of cells by diffusion. In each example the organism keeps the gradient steep: one side keeps supplying the substance and the other keeps using it or carrying it away.
Alveolus in the lungs: breathing keeps the air rich in oxygen and flowing blood carries oxygen away, so oxygen diffuses into the blood. Carbon dioxide diffuses the other way. The walls are one cell thick, so the distance is short.
Leaf in sunlight: photosynthesis uses up carbon dioxide inside the leaf, so it diffuses in through the stomata. Oxygen and water vapour diffuse out.
Respiring cell: respiration uses oxygen and glucose, so both diffuse in from the blood and tissue fluid; carbon dioxide diffuses out.
Solutes: digested food such as glucose and amino acids diffuses from the small intestine into the blood; urea diffuses from cells into the blood.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement best describes diffusion?
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Blood flowing past an alveolus helps oxygen diffuse into the blood because the flow:
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Agar cubes of side 1 cm, 2 cm and 3 cm containing an indicator are placed in acid. Why does the 1 cm cube change colour all the way through first?
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A student tests the effect of temperature on diffusion. Which variable must be kept constant?
Skill unlocked
Diffusion, mastered.
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