Dehydration Synthesis and Hydrolysis

Every monomer carries -H and -OH groups sticking off its structure. To connect two monomers, the cell has to actually make room for a bond between them, and that means getting rid of one hydrogen and one hydroxyl. One monomer gives up an -OH, the other gives up an -H, and those two pieces combine into H_2O while a new covalent bond forms between the monomers. This happens every single time a bond forms, whether you're linking glucose into starch, amino acids into a protein, or nucleotides into DNA. The water isn't a side note — it's proof that a bond was just made.

Hydrolysis runs the same idea in reverse, but it isn't a mirror image for free. A water molecule splits apart into -H and -OH, and those two pieces attach to the two monomers on either side of one specific bond, breaking that bond. Only one bond breaks per water molecule added, so hydrolyzing a chain of monomers one piece at a time costs one water molecule per cut. The catch is energy: pulling water out to force a bond together (dehydration synthesis) takes energy input and almost always needs an enzyme to push it along, while letting water back in to break a bond (hydrolysis) is generally energetically downhill. They move water in opposite directions, but the energy books don't balance evenly.

This pairing is the mechanical core of Unit 1: every polysaccharide, lipid, protein, and nucleic acid is assembled and disassembled through exactly these two reactions, just with different monomers and different enzymes doing the work.