Part 1 of 4What Diffusion Is
Particles of liquids and gases are constantly moving randomly, which gradually spreads them out.
Definition
- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
Concentration gradient
- The difference in concentration between two regions;
- particles move down this gradient until they are evenly spread.
Passive process
- Diffusion does not need energy from respiration because it relies on the random movement of the particles themselves.
Equilibrium
- Movement continues in both directions, but there is no net change once the concentration is the same everywhere.
Part 2 of 4Diffusion Across Membranes
Diffusion Across Membranes
Small molecules
- Oxygen, carbon dioxide and water move across the phospholipid layers directly down their concentration gradients.
Larger molecules
- Glucose can diffuse into cells only through transport proteins, but this is still passive if it moves down its gradient.
Gas exchange
- Oxygen diffuses from the air into the blood in the lungs, and carbon dioxide diffuses the opposite way.
Cells
- Respiring cells use up oxygen, which keeps its concentration low inside and so keeps the gradient steep.
Part 3 of 4Factors Affecting the Rate
1
A steeper concentration gradient means a greater difference between the two regions, so particles move across faster.
2
Higher temperature gives particles more kinetic energy, so they move faster and diffusion speeds up.
3
A larger surface area of membrane gives more space for particles to cross at the same time.
4
A shorter distance for particles to travel allows them to reach the other side more quickly.
Part 4 of 4Exam Checklist & Tips
1.2.3 – Key Points for the Exam
- Diffusion The net movement of particles from higher to lower concentration.
- Concentration gradient A difference in concentration between two regions.
- Passive Needing no energy from the cell.
- Net movement The overall direction in which particles are moving.
Common Mistakes
- Saying that particles stop moving at equilibrium; they keep moving but there is no net change.
- Stating that diffusion needs energy from respiration; it is a passive process.
- Writing that particles move 'from low to high' because they want to even out;
- movement is random but the net result is high to low.
- Mixing up diffusion with osmosis; osmosis refers only to water.