Diffusion (AQA GCSE Biology Combined Science): Revision Notes
Diffusion
What is diffusion?
Diffusion is how particles spread out naturally. It happens when particles of gases or substances in liquids move from where there are lots of them to where there are fewer of them.
Think of it like people in a crowded room - they naturally spread out to less crowded areas. Particles do the same thing, but they move automatically without anyone telling them to.
Net movement of particles
Particles in diffusion always move down a concentration gradient. This means:
- They move from areas with high concentration (lots of particles)
- To areas with low concentration (fewer particles)
- This continues until particles are evenly spread out
The particles themselves move randomly in all directions, but overall, more particles end up moving from the crowded area to the less crowded area.
The concentration gradient is the key driving force of diffusion. Particles will always move from high to low concentration areas until equilibrium is reached.
Factors that affect diffusion rate
Diffusion happens faster when:
Temperature increases
- Particles have more energy
- They move faster
- So they spread out quicker
Concentration difference increases
- The bigger the difference between high and low concentration areas
- The faster particles move from one area to the other
Surface area increases
- More space for particles to move through
- More particles can move at the same time
Membrane thickness decreases
- Particles have less distance to travel
- They reach the other side faster
Diffusion across cell membranes
Cell membranes allow diffusion to happen. Particles can move:
- Into cells (when there are more particles outside than inside)
- Out of cells (when there are more particles inside than outside)
The movement continues until there are equal amounts of particles on both sides of the membrane. Even then, particles keep moving randomly, but there's no overall change in concentration.
Examples of diffusion in living things
Gas exchange in lungs:
- Oxygen moves from air into blood cells by diffusion
- Carbon dioxide moves from blood cells into air by diffusion
Waste removal:
- Urea (a waste product) moves out of cells into blood plasma by diffusion
- Blood carries it away to be removed from the body
In plants:
- Gas exchange happens in leaves during photosynthesis
- Carbon dioxide diffuses in, oxygen diffuses out
Key Points to Remember:
- Diffusion is particles spreading out from high to low concentration areas
- It happens naturally - no energy is needed from the cell
- Temperature, concentration difference, surface area and membrane thickness all affect how fast it happens
- Particles move randomly but there's an overall movement down the concentration gradient
- It's essential for getting oxygen into cells and removing waste products like carbon dioxide and urea