Next time you fill a glass of water to the brim, look closely at the top. The water bulges slightly above the rim without spilling. That small dome is Unit 1 in action: water molecules holding on to each other.
Water: a small molecule that does a lot
A water molecule is polar. The oxygen pulls the shared electrons towards itself, so the oxygen end is slightly negative and the two hydrogen ends slightly positive. Neighbouring molecules attract each other through hydrogen bonds. That one fact explains most of water's useful properties:
- Cohesion: water sticks to water. It gives water surface tension, and helps pull water up a plant's xylem.
- Adhesion: water sticks to other polar surfaces, which is what lets it climb thin tubes.
- High specific heat: water takes a lot of energy to warm up, so it keeps cells and climates steady.
- Ice floats: hydrogen bonds lock ice into an open lattice that is less dense than liquid water, so lakes freeze from the top down and life survives underneath.
The four macromolecules
Living things build big molecules from small ones. Dehydration synthesis joins monomers by removing water; hydrolysis splits them by adding water back.
| Macromolecule | Monomer | Elements | Main jobs |
|---|---|---|---|
| Carbohydrates | Monosaccharides (e.g. glucose) | C, H, O | Energy (starch, glycogen), structure (cellulose) |
| Lipids | Not true polymers | Mostly C, H, O; phospholipids add P | Energy storage, membranes, hormones |
| Proteins | Amino acids | C, H, O, N (some S) | Enzymes, structure, transport, signalling |
| Nucleic acids | Nucleotides | C, H, O, N, P | Store and pass on genetic information |
A protein's shape decides its job. The amino acid sequence (primary structure) folds into helices and sheets (secondary), then a full 3D shape held by interactions between R groups (tertiary). Some proteins join several chains together (quaternary).
Common mistakes students make
- Calling lipids polymers. Fats are built from glycerol and fatty acids, not long chains of repeating monomers.
- Mixing up secondary and tertiary structure. Secondary structure comes from hydrogen bonds in the backbone; tertiary from R-group interactions.
- Forgetting where nitrogen and phosphorus go. Nitrogen is in proteins and nucleic acids; phosphorus is in nucleic acids and phospholipids. Carbohydrates have neither.
- Writing "hydrolysis builds polymers". It's the other way round: hydrolysis breaks them.
Try it
Before you flip each card, say the answer out loud. Then ask yourself: what would you expect to see if you put a drop of water and a drop of cooking oil on a sheet of wax paper?
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