Movement down a concentration gradient
Diffusion: the net movement of molecules or ions from a region where they are at a higher concentration to a region where they are at a lower concentration.
Particles move randomly because of their kinetic energy. When their concentration differs between two regions, this random movement results in an overall, or net, movement from the region of higher concentration towards the region of lower concentration.
Diffusion therefore occurs down a concentration gradient and is a passive process.
Diffusion through the phospholipid bilayer
The hydrophobic interior of a cell membrane limits which substances can cross directly. Small, non-polar molecules can diffuse through the phospholipid bilayer without using a membrane transport protein.
Examples include oxygen and carbon dioxide. Their net direction of movement depends on the concentration gradient across the membrane.
Ions and larger polar molecules cannot readily cross the hydrophobic region of the bilayer and therefore require another route through the membrane.
Reaching equilibrium
As diffusion proceeds, the concentration difference between the two regions decreases. Given sufficient time, the particles become evenly distributed. Random movement continues, but there is then no net movement towards either side.
Exam Tip: A complete diffusion definition should include net movement and movement from higher to lower concentration. When explaining the mechanism, state that particles move randomly rather than suggesting that every particle travels only towards the lower concentration.