Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, because of their random movement. It appears whenever an exam asks how oxygen, carbon dioxide, dissolved food or a dye moves into, out of or through something.
This lesson sits in movement across membranes and follows on from the structure of cells in cells and microscopy. It is also the base for the next lesson on osmosis.
What does “down a concentration gradient” actually mean?
Particles in a gas or a liquid are always moving in random directions. If there are more particles in one place than another, more particles will happen to move out of the crowded place than into it. The result is a net movement from high to low.
A concentration gradient is simply the difference in concentration between two places. The bigger the difference, the steeper the gradient and the faster the net movement.
Notice that no particle is “aiming” for the low concentration. Every particle moves randomly. The direction only emerges when you count all of them.
What changes the rate of diffusion?
Four factors are worth knowing, and you should be able to say why each one works.
| Factor | Effect on rate | Why |
|---|---|---|
| Steeper concentration gradient | Faster | A bigger difference means a bigger net movement |
| Higher temperature | Faster | Particles have more kinetic energy and move faster |
| Larger surface area | Faster | More particles can cross at the same time |
| Shorter distance | Faster | Particles have less far to travel |
Worked example
The data below are invented for practice. A cell respires and uses oxygen. The concentration of oxygen is 0.08 units outside the cell and 0.03 units inside it.
Question: State the direction of net movement of oxygen and explain it.
Step 1, compare the two concentrations. Outside is 0.08 and inside is 0.03, so the outside is higher.
Step 2, state the direction. Net movement of oxygen is from outside the cell to inside the cell.
Step 3, give the reason using the key terms. Oxygen particles move randomly, and because there are more of them outside, more cross inwards than outwards. The net movement is down the concentration gradient.
Step 4, say what keeps the gradient going. The cell keeps using oxygen in respiration, so the inside concentration stays low and the gradient is maintained.
A full-mark style answer would read: “Net movement is into the cell, from higher concentration outside to lower concentration inside, down the concentration gradient, because of random movement of particles.”
Now a size example.
A cube with sides of 1 cm has a surface area of 6 × 1 × 1 = 6 cm² and a volume of 1 cm³, so the ratio is 6 : 1. A cube with sides of 2 cm has a surface area of 6 × 2 × 2 = 24 cm² and a volume of 8 cm³, so the ratio is 24 : 8 = 3 : 1. The larger cube has less surface area for each unit of volume, so diffusion reaches its centre more slowly.
The mistake to watch for
A frequent answer is: “Diffusion is when particles move from high concentration to low concentration.” It sounds right, but it loses marks.
Mistaken answer: Particles move from a high concentration to a low concentration until they stop.
This suggests that every particle moves in one direction and that movement ends.
The correction is to say net movement, mention the concentration gradient and mention random movement. At equilibrium the particles keep moving, but as many cross each way, so there is no net change.
Check yourself
Answer first, then open each one.
1. Carbon dioxide is at 0.06 units inside a leaf cell and 0.04 units in the air space outside it. In which direction is the net movement?
Show answer
The concentration is higher inside the cell (0.06) than outside (0.04), so the net movement is out of the cell, down the concentration gradient.
2. A student says that diffusion has stopped because the concentrations are equal on both sides. Correct the statement.
Show answer
The net movement has stopped, but particles are still moving randomly. The same number cross in each direction, so the concentrations stay equal.
3. Give two ways to make a dye spread through water faster.
Show answer
Any two of: use warmer water (particles move faster), use a more concentrated dye (steeper gradient), or use a shorter distance or a wider container surface.
Where this leads next
Once you can explain net movement cleanly, try the special case of water in predicting water movement in an osmosis model, then compare it with energy-using movement in active transport. Practise mixed questions in the movement across membranes practice set.
Some students know the idea but lose marks because their sentence leaves out one key word. A teacher in online one-to-one Biology tuition can go through your own answers and help you tighten them.