Cells and organisation · GCSE Biology
Diffusion
Define diffusion for GCSE Biology: net movement down a concentration gradient, no energy from respiration, and the factors that speed gas exchange in alveoli, leaves and villi.
Diffusion is any particle from high to low concentration. No membrane is required in the definition, and no energy from respiration is used. Steeper gradient, higher temperature, larger area, shorter path: faster rate.
The important bits
What you need to know
- 1
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient).
- 2
It is a passive process: it does not require energy from respiration. Particles move because of their own kinetic energy.
- 3
Examples: oxygen from alveoli into blood, carbon dioxide from blood into alveoli, carbon dioxide into a leaf through stomata, digested glucose from ileum into blood, urea from cells into blood.
- 4
Rate increases with a steeper concentration gradient, a higher temperature (particles move faster), a larger surface area, and a shorter diffusion distance.
- 5
Exchange surfaces are adapted to exploit this: alveoli (large surface area, thin walls one cell thick, good blood supply, ventilated), villi (microvilli, capillaries, thin wall), root hairs (large surface area).
- 6
A large surface area to volume ratio speeds exchange relative to the organism’s volume. Small unicellular organisms can rely on diffusion across the body surface; large organisms need specialised systems.
- 7
A cell membrane is partially permeable, so small molecules such as oxygen, carbon dioxide and water diffuse across it, while many larger molecules cannot.
- 8
Do not call oxygen movement into a leaf osmosis. Osmosis is water only. Do not call ion uptake from soil diffusion when the ions move against the gradient — that is active transport.
Quotations worth analysing
Short evidence. Real method.
“Diffusion is the net movement of particles from a high concentration to a low concentration.”
Include “net” if you can: particles move both ways, but more move down the gradient. Energy from respiration is not required.
“The rate of diffusion is increased by a steeper concentration gradient, higher temperature and larger surface area.”
Name the factor and the particle reason. “It happens faster” without a mechanism drops a mark.
“Alveoli have a large surface area, thin walls and a good blood supply for gas exchange by diffusion.”
Each adaptation shortens the path or maintains the gradient. Link the feature to diffusion, not to “breathing” as a vague word.
Go deeper
What does “net movement” actually mean?
Particles bounce randomly. Some travel up the gradient, but more travel down it until concentrations are equal, when net movement is zero (equilibrium). Cells never quite sit at equilibrium for oxygen because respiration keeps using it, so a gradient remains. That is why a mitochondrion-rich cell still takes in oxygen by diffusion. In the lungs, ventilation brings in fresh air (high O₂, low CO₂) and blood flow carries oxygen away, so the gradient is maintained. Write “down a concentration gradient” and name the substance. “Particles spread out” is too thin. If the question gives a diagram with 20 dots on one side of a membrane and 4 on the other, the net movement is towards the 4 until the numbers match, provided the membrane lets that particle through.
Go deeper
How do I explain a faster rate in four marks?
Pick the factor the question changes. Temperature: particles have more kinetic energy, collide with the membrane more often, so more cross per second. Gradient: a bigger difference in concentration means a higher probability that a particle leaving the crowded side is not replaced from the other side. Surface area: more membrane or more alveoli means more particles can cross at once. Distance: a thinner wall (alveolus and capillary each one cell thick) means a shorter path, so less time to cross. Fick’s law is beyond most Combined papers, but the same four ideas are the marks. In plants, air spaces in spongy mesophyll and stomata open to the air keep CO₂ diffusing to chloroplasts in the light.
Go deeper
Where do students mix diffusion up with the other two processes?
If the particle is water crossing a partially permeable membrane, the named process is osmosis (a special case of diffusion). If mineral ions move from dilute soil into a concentrated root hair, the process is active transport and needs energy. If oxygen moves from alveolus to capillary, it is diffusion. A table in your head: substance, direction relative to gradient, energy needed, membrane required? Diffusion: any substance, down gradient, no energy, membrane optional. Osmosis: water, dilute to concentrated solution, no energy, membrane required. Active transport: often ions or glucose, against gradient, energy from respiration, membrane proteins required. Name the process, then the reason.
See the idea in action
Explain how alveoli are adapted for diffusion of oxygen (4 marks). Large surface area so more oxygen molecules can cross at once. Walls one cell thick, so a short diffusion path. Good blood supply removes oxygenated blood and maintains a steep concentration gradient. Ventilation brings in air rich in oxygen, also maintaining the gradient. Oxygen therefore diffuses from alveolar air into the blood down its concentration gradient, without energy from respiration.
Exam technique
Turn knowledge into marks
Name the substance and say “down a concentration gradient” and “does not require energy from respiration”. For rate, name a factor and a particle or surface-area reason. Do not write osmosis unless the substance is water.
Common mistakes
Do not give these marks away
- 01
Saying diffusion needs energy, or calling oxygen movement osmosis.
- 02
Forgetting “net” and “concentration gradient”, or naming no example surface.
- 03
Writing that a larger organism has a larger surface area to volume ratio — the ratio falls as size increases.
Which change increases the rate of diffusion of oxygen into the blood?
AA shallower concentration gradient
BA thicker alveolar wall
CA larger alveolar surface area
DA lower temperature
Show the answer
A larger alveolar surface area. More surface means more particles can cross per second. Thicker walls, cooler temperatures and shallower gradients all slow diffusion.
Quick questions
If this is the bit you searched
What is diffusion in GCSE Biology?
The net movement of particles from a higher to a lower concentration, down a concentration gradient, without energy from respiration.
Does diffusion require energy?
No. It is passive. Particles move because of their kinetic energy. Active transport is the process that needs energy from respiration.
What factors affect the rate of diffusion?
Concentration gradient, temperature, surface area and diffusion distance. Steeper, warmer, larger area and shorter path all increase rate.
Give an example of diffusion in the human body.
Oxygen diffuses from alveoli into the blood and carbon dioxide diffuses from the blood into the alveoli. Glucose also diffuses from the small intestine into the blood.