Diffusion and active transport

The times come from a teaching model of the colourless layer: its depth grows with the square root of time, the same on every face and every cube, until the layers from opposite faces meet. In it a 1 cm cube clears in 12 min; real cubes vary with the agar, the acid and how accurately they are cut.

The pink edge is where the indicator changes colour, not a picture of every acid particle. The particle view is a schematic model: real hydrogen ions are far too small to see.

Changing the cube size changes the distance to the centre and the surface area together. Warmer acid and stronger acid are tested one at a time, with 1 cm cubes. Agar is not a living cell: it stands in for diffusion distances.