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Key concept
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process: no energy from respiration is needed.
Diffusion is the most fundamental transport mechanism in biology. It explains how oxygen reaches your cells, how carbon dioxide leaves them, and how scent travels across a room. You must know the definition word-perfectly, understand the factors that affect its rate, and apply it to biological examples.
Definition and Key Concepts
Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Key points for exam answers:
It is a passive process: no metabolic energy (ATP) is required.
Particles move in all directions due to random kinetic energy, but the net movement is down the gradient.
Diffusion continues until equilibrium is reached: equal concentration on both sides. Particles still move, but the net movement is zero.
Only small, non-polar molecules can diffuse freely across the cell membrane (e.g. O₂, CO₂). Charged ions and large molecules require channel/carrier proteins.
Biological Examples of Diffusion
Gas exchange in the lungs: O₂ diffuses from alveolar air (high concentration) into blood (low concentration). CO₂ diffuses the opposite way.
Gas exchange in leaves: CO₂ diffuses into leaf cells through stomata for photosynthesis. O₂ diffuses out as a waste product.
Absorption in the small intestine: Digested food molecules (glucose, amino acids) diffuse from the gut lumen (high concentration) into the blood (low concentration) through the villus wall.
Removal of CO₂ from respiring cells: CO₂ is produced by respiration in the mitochondria and diffuses out of the cell into the blood.
Factors Affecting the Rate of Diffusion
The rate of diffusion is affected by:
Concentration gradient: a steeper gradient (larger difference) increases the rate. In the lungs, blood flow constantly removes O₂, maintaining a steep gradient.
Temperature: higher temperature gives particles more kinetic energy, so they move faster and diffuse more quickly.
Surface area: a larger surface area allows more particles to cross at the same time (e.g. Alveoli have a huge total surface area).
Distance / thickness of membrane: a shorter diffusion distance increases the rate. Alveolar walls are only one cell thick.
Size of molecule: smaller molecules diffuse faster than larger ones.
Try a focused question (4 marks)
Explain how the structure of the alveoli in the lungs is adapted for efficient gas exchange by diffusion. (4 marks)
Show the answer and marking guidance
Alveoli have a very large total surface area: increases the area for diffusion.
Alveolar walls are one cell thick: short diffusion distance.
A dense network of capillaries maintains a steep concentration gradient by carrying blood away.
The moist lining dissolves gases, allowing them to diffuse across more easily.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Forgetting to say "net movement": particles move in all directions, the NET movement is down the gradient.
Saying diffusion requires energy: it is passive.
Not linking structure to function in "explain" questions: always state the adaptation AND how it increases diffusion rate.