Chemistry of Life — AP Biology
1. Water: Properties from Polarity ★★☆☆☆ ⏱ 4 min
Water’s unique biological functions all stem from its polar molecular structure. A water molecule has one electronegative oxygen covalently bonded to two hydrogens: oxygen pulls shared electrons closer, creating a partial negative charge on oxygen and partial positive charges on hydrogens.
This polarity allows adjacent water molecules to form weak hydrogen bonds (each water can bond to up to four others), generating five biologically critical properties:
- **Cohesion and adhesion**: Cohesion (water sticking to water) creates surface tension; adhesion (water sticking to other polar molecules) enables capillary action in plant xylem.
- **High specific heat capacity**: Water absorbs/releases large amounts of heat before changing temperature, stabilizing ocean and organismal temperatures. Specific heat = $4.18 \text{ J/g°C}$.
- **High heat of vaporization**: It takes $2260 \text{ J/g}$ to evaporate water, enabling efficient evaporative cooling via sweating/panting.
- **Universal solvent**: Water dissolves polar/ionic solutes by forming hydration shells, so all cellular reactions occur in aqueous solution.
- **Ice is less dense than liquid water**: Frozen water forms a crystalline hydrogen bond lattice, so ice floats and insulates aquatic ecosystems in winter.
2. Classes of Biological Macromolecules ★★☆☆☆ ⏱ 5 min
Biological macromolecules are large carbon-based molecules that make up all living cells. Three of the four classes are polymers (long chains of repeating monomer subunits); lipids are not true polymers.
- **Carbohydrates**: Monomers = monosaccharides (simple sugars like glucose, general formula $(CH_2O)_n$). Disaccharides are two linked monosaccharides; polysaccharides are long chains: starch (plant energy storage), glycogen (animal energy storage), cellulose (plant cell wall structure), chitin (exoskeletons/fungal cell walls).
- **Lipids**: Nonpolar, hydrophobic molecules with no repeating monomer structure. Key types: triglycerides (1 glycerol + 3 fatty acids, long-term energy storage), phospholipids (amphipathic: 1 glycerol + 2 fatty acids + charged phosphate head), steroids (four fused carbon rings, e.g. cholesterol, hormones). Saturated fatty acids have no double bonds (solid at room temperature); unsaturated have double bonds (kinked, liquid at room temperature).
- **Proteins**: Monomers = 20 standard amino acids. Every amino acid has an amino group ($-NH_2$), carboxyl group ($-COOH$), hydrogen, and variable R-group that determines chemical properties. Polymers are called polypeptides, which fold into 3D functional proteins. Functions include enzyme catalysis, structural support, transport, immune defense, and cell signaling.
- **Nucleic acids**: Monomers = nucleotides, each made of a 5-carbon sugar, phosphate group, and nitrogenous base. DNA (deoxyribonucleic acid) uses deoxyribose, bases A/T/C/G, double-stranded, stores heritable information. RNA (ribonucleic acid) uses ribose, bases A/U/C/G, single-stranded, transfers genetic information for protein synthesis.
3. Dehydration Synthesis & Hydrolysis ★★★☆☆ ⏱ 3 min
All biological polymers are assembled by dehydration synthesis (condensation) reactions and broken down by hydrolysis reactions; both reactions are catalyzed by enzymes in cells.
4. Structure Determines Function ★★★☆☆ ⏱ 4 min
The core unifying theme of this unit (and a frequent FRQ topic on the AP exam) is that a biomolecule's structure directly determines its function. Even small changes to structure can eliminate biological activity.
- **Carbohydrates**: Starch uses alpha glycosidic linkages, which human amylase can break down for energy. Cellulose uses beta glycosidic linkages, which human enzymes cannot recognize, so cellulose acts as indigestible dietary fiber.
- **Lipids**: The amphipathic structure of phospholipids causes them to spontaneously form bilayers in aqueous solution, creating the selectively permeable barrier of all cell membranes. Kinks in unsaturated fatty acid tails prevent tight packing, keeping membranes fluid at low temperatures.
- **Proteins**: Protein function depends entirely on 3D shape, which forms across four structural levels: (1) primary = linear amino acid sequence; (2) secondary = alpha helices/beta sheets from backbone hydrogen bonds; (3) tertiary = 3D folded shape from R-group interactions; (4) quaternary = association of multiple polypeptide subunits. Denaturation (from pH/temperature/salt changes) disrupts weak non-covalent bonds, destroying function but leaving primary structure intact.
- **Nucleic acids**: DNA's double helix structure and complementary base pairing (A-T, C-G) allow accurate replication during cell division. RNA's single-stranded structure allows it to fold into catalytic shapes (ribozymes) or carry genetic code to ribosomes.
5. Biological Buffer Systems ★★★★☆ ⏱ 4 min
Buffer systems resist small changes in pH when acid or base is added, and are critical for maintaining homeostasis. Most cellular enzymes only function within a narrow pH range; for example, human blood pH must stay between 7.35 and 7.45 to sustain life.
CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+
When excess acid is added (increased $H^+$, lower pH), the equilibrium shifts left: $H^+$ combines with bicarbonate ($HCO_3^-$) to form carbonic acid, which breaks down into CO$_2$ and water that is exhaled. When excess base is added (decreased $H^+$, higher pH), equilibrium shifts right: carbonic acid dissociates to release more $H^+$ and restore pH.
6. Concept Check (AP Style) ⏱ 2 min
Common Pitfalls
Why: Students mix up word roots and reaction direction without connecting them to the actual process
Why: Students generalize from the other three macromolecule classes, which are all polymers
Why: Students assume all structural levels are destroyed when a protein loses function
Why: Both are bonds between glucose monomers, so students assume they are identical
Why: Students overstate the function of buffer systems