The spreading out of the particles of a gas, or of any substance in solution.
The net movement into or out of cells depends on the concentration of particles on each side of the cell membrane.
Because the particles move randomly, there will be a net (overall) movement from an area of high concentration to an area of lower concentration.
The difference in concentration between two areas is called the concentration gradient.
The larger the difference in cnocentration, the faster the rate of diffusion.
- The diffusion of oxygen into the cells of the body from the bloodstream as cells are respiring
- The diffusion of carbon dioxide into actively photosynthesising plant cells
- The diffusion of simple sugars and amino acids from the gut through cell membranes