The absorption process can be broken down into two phases. In the first phase, ions are passively taken up into the apoplast, the 'free space' or 'outer space' of the cell. The ions are taken up into the symplast of the cells in the second phase of uptake - the "inner space.". Ion channels are transmembrane proteins that function as selective pores that allow ions to move passively into the apoplast.

The movement of ions into and out of the symplast requires metabolic energy, which is an active process. The movement of ions is known as flux; the influx of ions into the cells and the efflux of ions out of the cells.