Free space Clearly, ionization of carboxyl groups creates a negative charge which makes the cell wall a cation exchange resin. Indeed much Ca2+ is associated with the pectins. In soils with high quantities of free al3+ this toxic ion is bound in large quantities to the cell wallFree space • Clearly, ionization of carboxyl groups creates a negative charge which makes the cell wall a cation exchange resin. Indeed much Ca2+ is associated with the pectins. In soils with high quantities of free Al3+, this toxic ion is bound in large quantities to the cell wall