B2.1.3

• Diffusion: the spreading out of particles in liquids and gases
• More particles move from an area of higher concentration to an area of lower concentration. -> therefore: a net movement of particles from the higher to the lower concentration (a movement down the concentration gradient)
• Diffusion is a passive process.
• Simple diffusion across membranes can only happen if the phospholipid bilayer is permeable to the particles.
• Ions with positive or negative charges cannot easy pass through the membrane because the centre is hydrophobic. Polar molecules (with partial pos/neg charges) can diffuse at low rates. Small polar particles pass through more easily.