AP Biology Unit 1: Chemistry of Life

Unit 1: Chemistry of Life: Quick Review

Water

Polarity: O is more electronegative → partial charges → hydrogen bonds (intermolecular, weak individually, strong collectively)
Cohesion: H-bonds between water molecules → transpiration pull in xylem
Adhesion: H-bonds between water and surfaces → capillary action
High specific heat ( $4.184 \text{ J/g·°C}$ ): resists temperature swings → stable body temp, moderate coastal climates
High heat of vaporization: evaporative cooling (sweating, transpiration)
Ice floats: crystalline lattice is less dense → insulates aquatic ecosystems
Solvent: dissolves polar/ionic substances; hydrophobic exclusion drives membrane formation

pH & Buffers

$\text{pH} = -\log[\text{H}^+]$ ; scale 0–14; each unit = 10× change
Acids donate $\text{H}^+$ ; bases accept $\text{H}^+$ (or release $\text{OH}^-$ )
Buffers = weak acid + conjugate base; minimize (not prevent) pH change
Blood buffer: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

Carbon & Functional Groups

Carbon forms 4 covalent bonds → chains, branches, rings
Dehydration synthesis: monomers join, lose $\text{H}_2\text{O}$
Hydrolysis: add $\text{H}_2\text{O}$ , break polymer bonds
Key groups: –OH (polar), –COOH (acidic), –NH₂ (basic), –PO₄ (charged, energy), –SH (disulfide bonds), –CH₃ (nonpolar, gene regulation)

Carbohydrates

Monomer: monosaccharide (glucose, fructose, galactose)
Bond: glycosidic linkage
α-linkages → starch (plants), glycogen (animals) → energy storage
β-linkages → cellulose (plants), chitin (fungi/arthropods) → structural
Same monomer, different linkage → completely different function

Lipids

NOT true polymers — grouped by hydrophobicity
Triglycerides: glycerol + 3 fatty acids; saturated (straight, solid) vs. unsaturated (kinked, liquid)
Phospholipids: amphipathic → lipid bilayer (membranes)
Steroids: 4 fused rings (cholesterol, hormones)

Proteins

Monomer: amino acid (20 types); variable R group determines properties
Bond: peptide bond
4 levels of structure:
- 1° = amino acid sequence (gene-determined)
- 2° = α-helices, β-sheets (H-bonds in backbone)
- 3° = overall 3D shape (R-group interactions)
- 4° = multiple polypeptide subunits
Denaturation = loss of shape/function (heat, pH, etc.); does NOT break peptide bonds

Enzymes

Lower activation energy ( $E_a$ ); not consumed
Induced fit model (not lock-and-key)
Competitive inhibition: blocks active site; overcome by ↑ [substrate]
Noncompetitive inhibition: binds allosteric site; NOT overcome by ↑ [substrate]

Nucleic Acids

Monomer: nucleotide = sugar + phosphate + nitrogenous base
Bond: phosphodiester bond (5' → 3' directionality)
DNA: deoxyribose, double-stranded, A=T (2 H-bonds), G≡C (3 H-bonds)
RNA: ribose, single-stranded, A=U
Chargaff's rules: %A = %T, %G = %C
ATP: adenine nucleotide + 3 phosphates → energy currency

💡 Exam Tip: For free-response questions, always follow the pattern: property → molecular mechanism (hydrogen bonding / polarity / R-groups) → specific biological example. This three-part reasoning earns full credit.

💡 Exam Tip: If given %A in DNA, calculate everything: %T = %A, then %G = %C = (100% − 2×%A) / 2. Fast points.

Key Terms

Electronegativity · Hydrogen bond · Polar covalent bond · Cohesion · Adhesion · Specific heat · Hydrophobic · Hydrophilic · pH · Buffer · Monomer · Polymer · Dehydration synthesis · Hydrolysis · Functional group · Glycosidic linkage · Peptide bond · Phosphodiester bond · Active site · Induced fit · Denaturation · Activation energy · Competitive inhibition · Noncompetitive inhibition · Amphipathic · Chargaff's rules · ATP