Unit 1: Chemistry of Life
AP Biology: 149 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 1 builds the molecular toolkit: water's chemistry, the CHNOPS elements of life, and the four macromolecule classes. Structure determines function at every level.
Water and hydrogen bonding
Water's bent shape and polar O–H bonds let each molecule donate and accept hydrogen bonds. That network produces cohesion (water sticking to water), adhesion (sticking to other surfaces), high specific heat, high heat of vaporization, expansion on freezing, and ice floating. Water is a universal solvent for polar and ionic solutes.
Elements of life
CHNOPS — carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur — dominate biomolecules. Carbon's four valence electrons allow branching skeletons and isomers. Nitrogen appears in proteins and nucleic acids; phosphorus in ATP and DNA backbones; sulfur in some amino acids.
Macromolecules
Carbohydrates store energy and provide structure (starch, glycogen, cellulose, chitin). Lipids are nonpolar hydrocarbons: fats for storage, phospholipids for membranes, steroids for signaling. Proteins are amino-acid polymers folded into structures that enzymes and transporters require. Nucleic acids (DNA, RNA) store and transmit information.
Dehydration synthesis and hydrolysis
Polymers grow by dehydration synthesis (monomers join, water released) and break by hydrolysis (water consumed). Both directions are enzyme-catalyzed.
Structure–function links
Protein shape comes from the amino-acid sequence: R-group interactions (hydrophobic packing, hydrogen bonds, disulfide bridges) fold chains into functional shapes. Nucleic-acid sequence encodes protein sequence; DNA strands pair by complementary base pairing (A–T, G–C).