Diffusion, Osmosis & Active Transport

What diffusion is

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient

Several things to notice about the definition:

• Net movement, not just movement. Individual particles bounce around randomly in all directions. The reason a substance appears to spread out is that, on average, more particles happen to move from the crowded side to the empty side than the other way around.
• Down a concentration gradient, which means from a high concentration to a low concentration. Diffusion never moves particles "uphill" against the gradient.
• It is a passive process, so it does not need any energy from the cell. The particles' own kinetic energy does all the work.
• Diffusion stops being net once the concentrations on the two sides are equal. The particles keep bouncing around after that, but with no net flow.

Why it happens

Every particle in a fluid is in constant, random motion. The more crowded a region is, the more particles in it bouncing around per second, and the more particles per second that happen to bump across into the neighbouring (less crowded) region. The reverse traffic is smaller because fewer particles live in the lower-concentration region in the first place. Over time, the imbalance evens out.

Diffusion across cell membranes

The cell membrane is partially permeable (sometimes called selectively permeable). It lets some substances cross freely but blocks others:

• Small, uncharged molecules like oxygen, carbon dioxide, water, urea and small lipid-soluble molecules diffuse across the membrane with no help
• Large molecules like starch, glucose and proteins cannot diffuse across on their own
• Charged ions also struggle to cross the lipid membrane without help

Important examples of diffusion in living organisms