Open a jar of something smelly at one end of a room, and soon you smell it at the other end. No fan is needed. Molecules are always moving and bumping, and that random motion alone spreads them out. You met the idea in Diffusion and Osmosis; here we look at it across the cell membrane.
Key idea
In diffusion, particles spread from where they are more concentrated to where they are less concentrated, down the concentration gradient, until they are evenly mixed. It needs no energy from the cell. Small nonpolar molecules such as O₂ and CO₂ diffuse straight through the membrane.
Why it works
Nothing pushes the particles in one direction. Each particle moves at random. But if there are 30 particles on the left and 6 on the right, many more particles happen to cross from left to right than from right to left, simply because there are more of them there to cross. The net movement is down the gradient. When the two sides are equal, as many cross each way as the other: there is still movement, but no net change. That balance is dynamic equilibrium.
- A steeper gradient (a bigger difference) means faster net diffusion.
- Higher temperature means faster-moving particles and faster diffusion.
- Smaller particles diffuse faster; a shorter distance takes less time. That is why lung and capillary walls are only one cell thick.
Worked example: Net movement
A membrane separates 30 oxygen molecules on the left from 10 on the right. What happens, and how many molecules end up on each side at equilibrium?
- More oxygen is on the left, so the net movement is left to right.
- The total is $30 + 10 = 40$; even sides means $40 \div 2 = 20$ each.
- So a net 10 molecules move from left to right.
Answer: 20 on each side (a net 10 moved left to right)
Worked example: Into the cell
A cell uses oxygen all the time in cellular respiration. Which way does oxygen diffuse, and why does it keep going?
- The cell uses up its oxygen, so the concentration inside stays low.
- Outside it stays higher, so the gradient points into the cell.
- As long as the cell keeps using oxygen, the gradient never disappears.
Answer: Into the cell, because the cell keeps the inside concentration low.
Common mistake
Thinking diffusion stops at equilibrium. The particles never stop moving; they keep crossing both ways. At equilibrium the two directions are equal, so there is no net change. Watch the animation: particles still cross after the sides are even.