AP Biology Unit 1: The Chemistry of Life

Macromolecules

• Four macromolecules (carbohydrates, proteins, nucleic acids, lipids) are crucial for biological processes.
• Each has a unique structure and function.

Carbohydrates

• Composed of carbon, hydrogen, and oxygen (1:2:1 ratio).
• Monomer: monosaccharide (e.g., glucose, fructose, galactose).
• Polymer: polysaccharide.
• Glucose structure: ring structure; chemical formula C₆H₁₂O₆.
• Dehydration reaction joins two monosaccharides to form a disaccharide (e.g., sucrose, lactose, maltose).
• Bond between monosaccharides: glycosidic linkage.
• Polysaccharides: cellulose (plant cell walls, structural), chitin (fungi cell walls, exoskeletons), starch (plant energy storage), glycogen (animal energy storage).
• Starch and cellulose differ in glycosidic linkage (alpha-1,4 for starch; beta-1,4 for cellulose) - allowing us to break one but not the other.
• Symbiotic relationships exist (termite gut microbes breaking down cellulose).

Proteins

• Composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
• Monomer: amino acid.
• Amino acid structure: central carbon bonded to an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom, and a variable R group.
• Peptide bond forms between the carboxyl group of one amino acid and the amino group of the next.
• Directionality: N-terminus (amino end) to C-terminus (carboxyl end).
• Protein structure levels:
  • Primary: linear sequence of amino acids.
  • Secondary: alpha helix or beta-pleated sheet (hydrogen bonds in the protein backbone).
  • Tertiary: 3D structure formed by various bonds (covalent, ionic, hydrogen, van der Waals, hydrophobic interactions) between R groups.
  • Quaternary: multiple polypeptides forming a larger protein (e.g., collagen, hemoglobin).
• R groups can be hydrophobic, hydrophilic, or charged.
• Hydrophobic R groups fold inward, while hydrophilic ones fold outwards.
  • Charged ones can form ionic bonds with water or other polar molecules.

Nucleic Acids

• Composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus (in phosphate groups).
• Monomer: nucleotide.
• Nucleotide structure: phosphate group, nitrogenous base (adenine, guanine, cytosine, thymine/uracil), and a pentose sugar (deoxyribose/ribose).
• Phosphodiester linkage joins nucleotides.
• Directionality: 5' end (phosphate) to 3' end (hydroxyl).
• DNA structure: double helix; antiparallel strands; base pairing (A-T with two hydrogen bonds; C-G with three hydrogen bonds).
• RNA structure: single-stranded; contains uracil instead of thymine.
• Differences between DNA and RNA: sugar type (deoxyribose vs. ribose) and strandedness.

Lipids

• Composed of carbon, hydrogen, and oxygen.
• Non-polar.
• Not polymers (no repeating monomer units).
• Types:
  • Fats (triglycerides): glycerol backbone + 3 fatty acids linked by ester bonds.
    • Saturated fats (all single bonds) are solid at room temperature.
    • Unsaturated fats (at least one double bond) are liquid at room temperature (oils).
  • Phospholipids: glycerol backbone + 2 fatty acids + phosphate group.
    • Amphipathic (polar head, non-polar tails).
    • Form bilayers in water.
  • Steroids: four fused carbon rings (e.g., cholesterol, hormones). Key roles in cell signaling.

Water Properties

• Polar covalent bonds (unequal electron sharing).
• Hydrogen bonds between water molecules.
• Cohesion: water molecules attracted to each other.
• Adhesion: water molecules attracted to other polar substances.
• Capillary action: combined effect of cohesion and adhesion.
• Surface tension: resistance to increase in surface area due to hydrogen bonds.
• Water is a universal solvent.
• High specific heat: resists changes in temperature.
• Ice is less dense than liquid water (hydrogen bonds stabilize crystal structure).
• Evaporative cooling: heat absorbed to change water from liquid to gas phase.
• pH: negative logarithm of hydrogen ion concentration (inverse relationship); pH scale (acidic/basic), significant for biological systems.