Atoms and bonds
All living things are built from atoms, mostly carbon, hydrogen, oxygen and nitrogen. Atoms join in two main ways:
- Covalent bond: atoms share electrons. Strong. It holds together the atoms inside molecules such as water.
- Ionic bond: one atom transfers electrons to another, creating charged ions that attract each other, as in table salt.
A weaker attraction, the hydrogen bond, forms between a slightly positive hydrogen and a slightly negative atom on a neighbouring molecule.
Water
Water is polar: electrons are pulled toward the oxygen, so oxygen is slightly negative and hydrogens are slightly positive. Water molecules therefore form hydrogen bonds with each other. This explains water's special features:
- Cohesion: molecules stick to each other (surface tension, water rising in plants).
- High heat capacity: it takes a lot of energy to warm water, stabilising temperatures.
- Good solvent for ions and polar molecules.
- Ice floats because hydrogen bonds hold molecules in an open lattice.
pH
pH measures hydrogen ion concentration on a logarithmic scale. Below 7 is acidic, 7 is neutral, above 7 is basic. Each unit is a tenfold change, so pH 3 is 10 × 10 × 10 = 1,000 times more acidic than pH 6.
Four classes of macromolecules
| Class | Building block | Main roles |
|---|---|---|
| Carbohydrates | Monosaccharides (e.g. glucose) | Quick energy, structure (cellulose) |
| Lipids | Fatty acids and glycerol (not true polymers) | Long-term energy, membranes, hormones |
| Proteins | Amino acids | Enzymes, structure, transport, signalling |
| Nucleic acids | Nucleotides | Store and use genetic information |
A nucleotide has a sugar, a phosphate and a nitrogenous base. Lipids are mostly hydrophobic (water-fearing).
Enzymes
Most enzymes are proteins that act as catalysts: they speed up reactions by lowering the activation energy without being used up. Each enzyme has an active site shaped to fit its substrate. Heat or extreme pH can denature an enzyme, changing its shape so it stops working.
Worked example
Protein digestion in the stomach: pepsin works well at about pH 2, but trypsin in the small intestine works best near pH 8.
- The stomach is acidic (pH 2), so pepsin's shape is correct there.
- If pepsin were moved into the intestine (pH 8), it would denature and lose activity because the charges that maintain its folded shape would change.
- Therefore enzymes are matched to their environments.
Each of these links a structure to a function, which is the central theme of biology.