Diffusion.

Step 1 of 5

A random walk

Drip a drop of dye into a glass of water, and don’t stir. After a few minutes the colour has spread through the whole glass. Nobody pushed it: the molecules of dye and of water are always moving, colliding with each other and changing direction at random. Spreading like that, from where there are many molecules to where there are few, is called diffusion.

The tank in the Scene has two sides, and in the middle a membrane with wide channels that both water and sugar can pass. Concentration is measured here in mOsm/L: how many dissolved particles there are in each litre. The sugar is dyed amber so you can see it; really it is colourless.

Step 2 of 5

The end

Look inside is open at Molecules: the liquids have turned clear, and you can see every molecule and the net-flow arrows. The difference between the concentrations is called the concentration gradient, and the steeper it is, the faster the diffusion.

Step 3 of 5

Still moving

Now both sides hold the same concentration, 200 mOsm/L. Choose Molecules under Look inside and watch the channels.

Step 4 of 5

Warmer

Temperature measures the energy of the molecules’ motion. Left side and Right side are fixed, and Temperature is free.

Step 5 of 5

In the lungs

Diffusion only works over short distances: an oxygen molecule diffuses one micrometre in a thousandth of a second, but one centimetre only in tens of seconds or more. So a large body cannot rely on diffusion alone, and it brings substances close to every cell, in the blood. But over the last stretch, across the membranes, it is all diffusion.