Chemistry of Life
What you must understand
Life is built from a small set of elements (C, H, O, N, P, S) arranged into four macromolecule classes: carbohydrates, lipids, proteins, and nucleic acids. Each class is a polymer (or, for lipids, an assembled structure) of monomers joined by dehydration synthesis and broken by hydrolysis. Know the direction of water in each reaction: water is removed to build a bond and added to break one.
Properties of water flow from its polarity and hydrogen bonding: cohesion, adhesion, high specific heat, high heat of vaporization, and its role as a solvent. pH matters because H+ concentration changes protein shape and enzyme activity.
Proteins have four structural levels, and shape determines function. Denaturation destroys function without breaking peptide bonds. Nucleic acids store information in the sequence of nitrogenous bases; complementary base pairing (A–T, G–C in DNA; A–U in RNA) underlies replication and transcription.
Traps
- A question may ask which bond holds the two strands of DNA together (hydrogen bonds) versus which bonds form the sugar-phosphate backbone (covalent phosphodiester bonds).
- "Hydrolysis" is often mislabeled as a building reaction. Building = dehydration.
- Lipids are not true polymers, and phospholipids are amphipathic, not purely hydrophobic.
- An enzyme denatured by heat loses activity but its primary structure is intact.
Rule of thumb
Match the macromolecule to its monomer before answering: sugar→monosaccharide, protein→amino acid, nucleic acid→nucleotide, lipid→glycerol + fatty acids (not a repeating polymer).
Worked example
A polypeptide is heated and loses function. Which level of structure is disrupted first? Tertiary and secondary structure (hydrogen bonds, ionic interactions) unfold before the covalent primary sequence changes.