Diffusion (AQA GCSE Biology): Revision Notes
Diffusion
What is diffusion?
Diffusion is when particles move and spread out. This happens with:
- Gas particles in the air
- Particles of substances dissolved in liquids
The particles move from areas where there are lots of them to areas where there are fewer of them. This movement happens naturally without any energy being needed.
Diffusion is a passive process - it occurs automatically due to the random movement of particles, with no external energy input required.
How diffusion works
Particles are always moving around randomly. When there are more particles in one area than another, more particles will move away from the crowded area than move towards it. This creates a net movement (overall movement) of particles.
This movement continues until the particles are spread out evenly. We call the difference in particle numbers between two areas a concentration gradient.
Everyday Example: Perfume in a Room
If you spray perfume in one corner of a room, the smell will eventually spread throughout the whole room as the perfume particles diffuse. The particles move from the high concentration area (where you sprayed) to lower concentration areas (rest of the room) until the scent is evenly distributed.
Factors that affect diffusion rate
Diffusion happens faster when:
- Temperature increases - hotter particles move more quickly, so they spread out faster
- Concentration difference increases - the bigger the difference between crowded and less crowded areas, the faster particles move
Understanding these factors is crucial for predicting how quickly substances will move in biological systems. Higher temperatures and steeper concentration gradients always result in faster diffusion rates.
Diffusion across cell membranes
Substances can move into and out of cells by diffusion through the cell membrane. This is how cells get the materials they need and get rid of waste products.
Two extra factors affect how fast diffusion happens across membranes:
- Surface area - bigger surface area means more space for particles to pass through, so diffusion is faster
- Membrane thickness - thinner membranes allow particles to pass through more quickly
Cell membranes are selectively permeable, meaning they only allow certain substances to pass through. This selective permeability is essential for maintaining proper cell function and homeostasis.
Examples in living things
Gas exchange in animals:
- Oxygen moves into cells by diffusion (cells need oxygen for respiration)
- Carbon dioxide moves out of cells by diffusion (it's a waste product from respiration)
Waste removal:
- Urea (a waste product) moves out of cells into the blood by diffusion
In plants:
- Gas exchange happens in leaves during photosynthesis
- Carbon dioxide moves in, oxygen moves out
Gas Exchange in the Lungs
In your lungs, oxygen diffuses from the air in the alveoli (high concentration) into your blood (lower concentration). At the same time, carbon dioxide diffuses from your blood (high concentration) into the alveoli (lower concentration) to be breathed out. This process happens continuously without you having to think about it.
Important points to remember
- Particles keep moving randomly even after diffusion is complete
- Diffusion doesn't need energy - it happens naturally
- Substances always move from high concentration to low concentration
- The process continues until concentrations are equal on both sides
Key Points to Remember:
- Diffusion is particles spreading out from high to low concentration
- No energy is needed - it happens naturally
- Temperature and concentration differences make it faster
- Thinner membranes and larger surface areas speed up diffusion across membranes
- Examples include oxygen entering cells and carbon dioxide leaving cells
- The process continues until equilibrium is reached on both sides