Diffusion is the net movement of particles from a higher concentration to a lower concentration. It happens in gases and solutions.
Particles move randomly in all directions. More leave the crowded region than move back, so there is a net movement down the concentration gradient.
Diffusion does not require energy supplied by respiration.
Particles keep moving. When concentrations are equal, particles move both ways at the same rate, so there is no net movement.
Oxygen: diffuses from air in the alveoli into blood, then from blood into respiring cells.
Carbon dioxide: diffuses from cells into blood, then from blood into the alveoli to be breathed out.
Urea: diffuses from cells where it is produced into blood plasma and is carried to the kidneys for excretion.
In each case, the net movement is from higher to lower concentration.
| Change | Why the rate increases |
|---|---|
| Steeper concentration gradient | A greater difference produces more net movement. |
| Higher temperature | Particles have more kinetic energy and move faster. |
| Larger surface area | More particles can cross at the same time. |
| Shorter diffusion distance | Particles travel a shorter path. |
A thin exchange surface provides a short diffusion path. We will look at exchange surfaces in lesson 1.10.
If oxygen is continually used in respiration, the oxygen concentration inside cells stays relatively low. This maintains a gradient for oxygen to diffuse in.
Blood flow can also maintain gradients by bringing fresh substances and carrying away substances that have crossed.
Net movement: high → low concentration. Particles move randomly; no energy from respiration is needed. Evenly spread particles still move.
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