Cell biology substratum...blah?

Adhesive contact between cells (i.e. via adherens junctions) is mediated by calcium (a divalent cation , i.e positively charged). The chelator has a number of negative charges which bind the calcium, thus producing dissolution of the intercellular junctions. Meanwhile, the trypsin (a protease) chews up the adhesive interactions between the cells and the substratum. So, the trypsin releases the cells from the surface, while the chelator dissociates the cells from each other so they will lift up as individual cells, instead of as a sheet. This would be important, because if the next step involves counting the cells, that's hard to do when they're still stuck to each other.