Everything in AP Biology is built from a handful of elements arranged into four kinds of large molecules, held together and shaped by the chemistry of water. This unit is short on the exam but everything later leans on it.
Water is polar: oxygen pulls the shared electrons harder than hydrogen does, so the oxygen end is partly negative and each hydrogen partly positive. Neighbouring water molecules attract through hydrogen bonds, and those bonds explain cohesion, adhesion, surface tension, high specific heat and high heat of vaporization.
Carbon, hydrogen and oxygen are in all four kinds of biological macromolecule. Nitrogen is in proteins and nucleic acids, phosphorus in nucleic acids and phospholipids, and sulfur in proteins. Organisms take these atoms from their environment.
Monomers join into polymers by dehydration synthesis, which removes the equivalent of a water molecule and forms a covalent bond. Hydrolysis breaks that bond by adding water back. A chain of n monomers has n − 1 bonds.
Monosaccharides (simple sugars) are the monomers of polysaccharides. Starch stores energy in plants, glycogen in animals, and cellulose builds plant cell walls. The way the sugars are linked and branched determines what each polymer does.
Lipids are mostly nonpolar and hydrophobic. Saturated fatty acids have only single C–C bonds and pack tightly (solid at room temperature); each double bond adds a kink, so more unsaturated fats are more liquid. Fats store energy, steroids act as hormones, cholesterol stabilizes animal membranes, and phospholipids form bilayers.
Nucleotides (five-carbon sugar + phosphate + nitrogenous base) are the monomers of DNA and RNA. Strands have a 5′ phosphate end and a 3′ hydroxyl end, and grow at the 3′ end. DNA is an antiparallel double helix paired A–T and C–G by hydrogen bonds; RNA uses ribose and uracil and is usually single-stranded.
Amino acids join by peptide bonds into polypeptides. The sequence (primary structure) and the chemistry of the R groups determine folding: hydrogen bonds in the backbone make helices and sheets (secondary), R-group interactions make the 3D shape (tertiary), and several polypeptides can combine (quaternary). Shape determines function.