What you'll learn
Substances constantly move into and out of cells, and one of the main ways they do this is by diffusion. For AQA GCSE Biology you need to understand what diffusion is, the factors that affect its rate, examples of diffusion in living organisms, and how exchange surfaces are adapted to make diffusion efficient. This guide covers the definition of diffusion, the factors affecting its rate, key biological examples such as gas exchange, and the adaptations of exchange surfaces. By the end you should be able to define diffusion, explain what speeds it up, and explain how organisms are adapted to exchange substances efficiently.
Key terms and definitions
Diffusion — The spreading out of particles from a region of higher concentration to a region of lower concentration.
Concentration gradient — The difference in concentration between two areas.
Passive — Not requiring energy from the cell.
Surface area — The total area of a surface across which diffusion can occur.
Exchange surface — A surface adapted for exchanging substances between an organism and its environment.
Partially permeable membrane — A membrane that lets some substances through but not others.
Gas exchange — The movement of oxygen and carbon dioxide across a surface by diffusion.
Alveoli — Tiny air sacs in the lungs where gas exchange occurs.
Core concepts
What diffusion is
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It happens because particles are constantly moving randomly, so over time they spread out evenly. Diffusion is a passive process, meaning it does not require energy from the cell — the particles move on their own.
Many important substances move in and out of cells by diffusion, including oxygen and carbon dioxide (in gas exchange) and dissolved food molecules such as glucose. These small molecules can diffuse across the cell membrane.
Factors affecting the rate of diffusion
Three main factors affect how quickly diffusion happens:
- Concentration gradient — The greater the difference in concentration between the two areas, the faster the rate of diffusion, because there is a bigger difference to even out.
- Temperature — The higher the temperature, the faster the diffusion, because particles have more energy and move faster.
- Surface area — The larger the surface area of the membrane, the faster the rate, because more particles can move across at once.
For a whole organism, the distance the particles must travel also matters: a shorter diffusion distance (a thinner surface) means faster diffusion.
Diffusion in living organisms
Diffusion is essential for life. Some key examples:
- In the lungs, oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the air to be breathed out.
- In the small intestine, digested food molecules such as glucose diffuse from the gut into the blood.
- In plants, carbon dioxide diffuses into the leaf for photosynthesis, and oxygen diffuses out.
In each case, substances move down their concentration gradient across a membrane or exchange surface.
Why larger organisms need exchange surfaces
Single-celled organisms can rely on diffusion across their surface because they are small and have a large surface area compared with their volume, so substances can reach all parts quickly. Larger, multicellular organisms have a smaller surface area to volume ratio, so simple diffusion across the outer surface is not fast enough to supply all their cells. They therefore need specialised exchange surfaces and transport systems to move substances efficiently.
Adaptations of exchange surfaces
Exchange surfaces, such as the alveoli in the lungs or the villi in the small intestine, are adapted to make diffusion as fast as possible. They typically have:
- A large surface area, so more diffusion can happen at once.
- A thin membrane (short diffusion distance), so substances cross quickly.
- A good blood supply (in animals), to maintain a steep concentration gradient by constantly bringing and removing substances.
- (In the lungs) being well ventilated, so fresh air keeps the concentration gradient steep.
These adaptations all increase the rate of diffusion, which is why questions often ask you to link a feature to its effect.
Diffusion compared with osmosis and active transport
It is important to see how diffusion fits alongside the other two ways substances move in and out of cells. Diffusion is the passive movement of any particles down a concentration gradient. Osmosis is a special case: it is the movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one — still passive, but specifically about water. Active transport is different again: it moves substances against the concentration gradient, from low to high concentration, which requires energy from respiration. For example, root hair cells use active transport to absorb mineral ions from the soil even when the soil is more dilute than the cell. Knowing the difference between these three processes, and that only active transport uses energy, is frequently tested.
Surface area to volume ratio in more detail
The idea of surface area to volume ratio explains why the size and shape of an organism affects diffusion. A small object has a large surface area compared with its volume, so substances can diffuse in and out fast enough to supply the whole object. As an object gets larger, its volume grows faster than its surface area, so the ratio falls and diffusion across the surface can no longer supply the interior quickly enough. This is why small organisms can rely on diffusion alone, while large organisms need exchange surfaces and transport systems such as the lungs, gut and blood. Some exchange surfaces are even folded (like villi and alveoli) specifically to increase their surface area and improve this ratio.
Worked examples
Example 1: Defining diffusion
Define diffusion. 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 that does not require energy.
Example 2: Explaining a faster rate
Explain why increasing the temperature increases the rate of diffusion. At higher temperatures, the particles have more energy and move faster. This means they spread out and cross the membrane more quickly, so the rate of diffusion increases.
Example 3: Gas exchange in the alveoli
Explain how oxygen moves from the air in the alveoli into the blood. There is a higher concentration of oxygen in the air in the alveoli than in the blood, so oxygen diffuses down its concentration gradient across the thin alveolar wall into the blood. The blood flow removes the oxygen, keeping the gradient steep.
Example 4: Surface area to volume ratio
Explain why large organisms need specialised exchange surfaces. Large organisms have a small surface area to volume ratio, so diffusion across their outer surface is too slow to supply all their cells. They need exchange surfaces with a large area, thin walls and a good blood supply to exchange substances quickly enough.
Common mistakes and how to avoid them
A very common error is saying diffusion requires energy. Diffusion is passive — it does not use energy from the cell, because the particles move on their own due to their random motion.
Students often forget the word "net" or say particles only move one way. Particles move in both directions randomly, but the net movement is from high to low concentration. Both points can matter in a full answer.
Another mistake is confusing the factors. Learn the three: concentration gradient, temperature and surface area all increase the rate, and a shorter distance also increases it.
When explaining exchange surfaces, do not just say "large surface area". Link each adaptation to its effect — large area, thin wall (short distance), and good blood supply (steep gradient) all speed up diffusion.
Finally, be careful not to confuse diffusion with osmosis or active transport. Diffusion is the passive movement of particles down a gradient; osmosis is specifically the movement of water, and active transport uses energy to move substances against a gradient.
Exam technique for "Diffusion"
Definition questions want the key phrase: net movement from high to low concentration, down a concentration gradient, as a passive process. Learn it word for word.
For rate questions, name the factor and explain its effect — for example, a steeper concentration gradient means a faster rate because there is a bigger difference to even out. Apply the same reasoning to temperature and surface area.
Exchange-surface questions expect you to link each adaptation to faster diffusion: large surface area, thin membrane and good blood supply. This structure earns full marks. Use precise terms and always mention the concentration gradient when explaining why substances move.
Quick revision summary
- Diffusion is the net, passive movement of particles from high to low concentration, down a concentration gradient.
- Rate increases with a steeper concentration gradient, higher temperature and larger surface area, and with a shorter diffusion distance.
- Examples: oxygen and carbon dioxide in the lungs, glucose in the small intestine, gases in plant leaves.
- Large organisms have a small surface area to volume ratio, so they need specialised exchange surfaces.
- Exchange surfaces are adapted with a large surface area, thin membrane and good blood supply to speed up diffusion.
- Diffusion is passive — it does not use energy, unlike active transport.