What it covers: The chemical foundation of biology — water, the elements of life, and the four classes of biological macromolecules.
Exam weight: About 8–11% of the AP Biology exam.
The big question: How do the chemical structures of water and the four classes of macromolecules give rise to the properties that make life possible?
Big Ideas covered: Energetics (BI 2), Information Storage & Transmission (BI 3), Systems Interactions (BI 4).
Key topics at a glance
Water & Hydrogen Bonding
Water is polar. Hydrogen bonds between water molecules give it cohesion, adhesion, surface tension, high specific heat, and a high heat of vaporization.
Elements of Life
CHNOPS — carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur. C, H, O are most abundant. N is in proteins & nucleic acids; P in phospholipids & nucleic acids; S in some proteins.
Building Macromolecules
Dehydration synthesis joins monomers and releases water. Hydrolysis uses water to break polymers back into monomers.
Carbohydrates
Monomer: monosaccharide (e.g., glucose). Polymers: starch (plant energy storage), glycogen (animal energy storage), cellulose (plant cell walls).
Lipids
Mostly nonpolar & hydrophobic. Saturated fatty acids pack tightly (solid); unsaturated have kinks (liquid). Phospholipids have a hydrophilic head and hydrophobic tails → form membrane bilayers.
Nucleic Acids
Monomer: nucleotide (sugar + phosphate + base). DNA is an antiparallel double helix with A–T and G–C pairs. RNA uses uracil instead of thymine and is usually single-stranded.
Proteins
Monomer: amino acid (with a variable R group). Linked by peptide bonds. Four levels: primary (sequence), secondary (alpha-helix/beta-sheet), tertiary (3D shape), quaternary (multiple subunits).
Structure → Function
The shape of every macromolecule determines what it does. Change the structure (denature a protein, mutate a DNA sequence) and you change — or lose — the function.
The key terms you must know
Polar covalent bond — unequal sharing of electrons, creating partial charges. The basis of water's properties.
Hydrogen bond — weak attraction between polar molecules; collectively they give water its unique behavior.
Cohesion / Adhesion / Surface tension — water sticks to itself, sticks to polar surfaces, and forms a "skin" because of H-bonds.
Specific heat capacity — water resists temperature change; helps stabilize body temperature and habitats.
Hydrophilic vs. hydrophobic — polar/charged things mix with water; nonpolar things don't.
Phospholipid bilayer — amphipathic phospholipids self-assemble into the structure of every cell membrane.
Antiparallel double helix — DNA's two strands run in opposite 5′→3′ directions; base pairs (A–T, G–C) form between them.
Peptide bond — the covalent link between two amino acids; formed by dehydration synthesis.
Primary–quaternary structure — the four levels of protein folding; sequence determines shape determines function.
Key themes to remember
Structure determines function. The single most important rule of Unit 1 — and the rest of the course.
Water is the medium of life. Its polarity and hydrogen bonding explain almost everything about how cells work.
Polymers are built from monomers — using the same logic. Dehydration synthesis builds, hydrolysis breaks. Same idea for carbs, proteins, and nucleic acids.
Hydrophobic interactions drive membrane structure. Phospholipids spontaneously form bilayers because of water's polarity.
Sequence is information. The order of nucleotides in DNA and amino acids in proteins encodes everything an organism does.
Common exam traps
Don't confuse cohesion and adhesion. Cohesion = water to water. Adhesion = water to other polar surfaces. Both come from hydrogen bonding.
Hydrogen bonds aren't the same as covalent bonds. H-bonds are weak attractions between polar molecules. The O–H bonds inside a water molecule are polar covalent.
Lipids aren't true polymers. They don't form from a single repeating monomer the way proteins, carbs, and nucleic acids do.
Antiparallel ≠ complementary. Strands are antiparallel (run opposite 5′→3′ directions) AND complementary (A–T, G–C base pairing). Different ideas — both matter.
Primary structure determines everything. If a primary sequence question shows up, the answer almost always traces back to "the amino acid order determines the shape, which determines the function."
R groups (not the backbone) drive tertiary structure. Hydrogen bonding between backbone atoms makes secondary structure; R-group interactions make tertiary.
Phospholipid heads face water; tails do not. When asked about membrane assembly, remember: hydrophilic heads outward, hydrophobic tails sequestered in the middle.