Introduction
Biomolecules apply all the structural and mechanistic principles built up through this Paper — functional groups, stereochemistry, bonding — to the specific molecules that make up living systems, setting up the metabolic pathways covered in Chapter 9.
1. Carbohydrates and their classification
Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis), classified by size: monosaccharides (single sugar units, like glucose) are the basic building blocks; disaccharides (two monosaccharides joined by a glycosidic bond, like sucrose) and polysaccharides (many monosaccharides joined together, like starch, glycogen, and cellulose) are built from them. The stereochemistry (Chapter 6) of each sugar — which specific stereoisomer it is — determines both its biological role and how enzymes recognise it, since enzymes are themselves chiral and typically process only one specific stereoisomer.
2. Amino acids, peptides, and proteins
Amino acids contain both an amine group and a carboxylic acid group (Chapter 14) on the same carbon, and exist predominantly as zwitterions (internally ionised, with a protonated amine and deprotonated carboxylate) at physiological pH, since the amine is basic enough to be protonated by the acidic carboxyl group. Amino acids link via peptide bonds (an amide linkage, Chapter 14, formed between the carboxyl of one amino acid and the amine of the next) into peptides (short chains) and proteins (long chains, folded into specific three-dimensional shapes essential for their biological function, from structural support to enzyme catalysis).
3. Lipids and membrane-related molecules
Lipids are a structurally diverse group unified by being largely nonpolar/hydrophobic (poorly soluble in water) — including fats and oils (esters of glycerol and long-chain fatty acids, Chapter 14) and phospholipids, which combine a polar (hydrophilic) head group with nonpolar (hydrophobic) fatty acid tails. This dual character drives phospholipids to spontaneously arrange into bilayers in water, with tails facing inward away from water and heads facing outward — the structural basis of every biological cell membrane.
4. Nucleic acids and their building blocks
Nucleic acids (DNA and RNA) are polymers of nucleotides, each composed of a nitrogenous base, a sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. In DNA, complementary bases on opposing strands pair via hydrogen bonding (adenine with thymine, guanine with cytosine) in a specific, predictable way that underlies both DNA’s stable double-helix structure and its ability to be accurately copied and read — the chemical basis of genetic information storage and transmission.
5. Structure–function relationships
Across all four biomolecule classes, the same principle recurs: three-dimensional structure — determined by the underlying chemistry of bonding, hydrogen bonding, and stereochemistry covered throughout this Paper — directly enables biological function, whether that is an enzyme’s active site precisely recognising a substrate, a membrane’s bilayer separating cellular compartments, or DNA’s base-pairing enabling faithful genetic replication.
Key Takeaways
- Carbohydrates range from single sugar units (monosaccharides) to large polymers (polysaccharides), with stereochemistry determining biological recognition.
- Amino acids exist as zwitterions and link via peptide bonds into proteins, whose specific three-dimensional folding (not just their sequence) determines function.
- Lipids’ dual polar/nonpolar character (especially in phospholipids) drives spontaneous bilayer formation, the structural basis of cell membranes.
- Nucleic acids are nucleotide polymers whose specific, complementary base-pairing (in DNA) enables stable storage and accurate transmission of genetic information.