The basics
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.
- Dehydration synthesis / Hydrolysis — build polymers (lose water) vs. break polymers (add water).
- 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.