01Monomers and polymers
Big biological molecules are built like chains: small repeating units (monomers) linked into polymers. Sugars → polysaccharides; amino acids → proteins (polypeptides); nucleotides → nucleic acids. (Lipids are the exception — they're big, but not true polymers.)
02Building up: dehydration synthesis
To join two monomers, the cell removes an –OH from one and an –H from the other. Those leave as H₂O, and a new covalent bond forms. Because water is removed, this is called dehydration synthesis (or a condensation reaction). It requires energy and is done by enzymes.
03Breaking down: hydrolysis
Hydrolysis is the reverse: water is added across the bond, splitting it. Digestion is hydrolysis — your enzymes split starch into glucose and proteins into amino acids so you can absorb them. Hydrolysis typically releases energy.
04Notebook box
Memory hook: hydro = water, lysis = split → water splits it. Dehydration = water removed → bond made.
Count the waters: joining *n* monomers into a chain removes n − 1 water molecules. A 100-amino-acid protein made by dehydration releases 99 H₂O.
Bond names to know: glycosidic (sugars), peptide (amino acids), phosphodiester (nucleotides), ester (fatty acids to glycerol).