Polymerization and the break down of a polymer are two opposite reactions. what role does a water molecule play in both of them?

Well in polymerization or in dehydration synthesis, the individual monomer molecules contain either H or OH attached to the end of their respective molecules. When they chemically compline together both the H and OH come off or break from their respective molecules and form water as a product. In place, a covalent bond is produced connecting the monomers together. In break down of polymers, with Water, the water molecule essentially disconnects the large molecule at the same place where it would have formed a covalent bond, and causes one of the monomers to bond to either a H and or OH. Separating the 2 compounds.

Ibelieve it's temperature! i'm not 100% sure.

B) movement of materials into and out of the cell