The molecules of life
Living things are built, almost entirely, from four families of large carbon-based molecules: carbohydrates, proteins, lipids and nucleic acids. Each is assembled from small repeating units, and each does a distinct job. Biochemistry is where the organic chemistry of carbon meets biology.
Carbohydrates
Carbohydrates are the body's quick energy. The small units are monosaccharides — simple sugars like glucose — which join into larger chains: two make a disaccharide (sucrose), and many make a polysaccharide such as starch (energy storage in plants), glycogen (storage in animals) or cellulose (the structural fibre of plant cell walls).
They all share the same building block yet behave completely differently — starch is digestible fuel, cellulose is indigestible fibre — because of how the units are linked.
Proteins
Proteins are the workhorses. Their units are amino acids (about twenty kinds), joined by peptide bonds into long chains that fold into precise shapes. That shape is everything: it decides the protein's job.
Proteins do almost everything — enzymes catalyse reactions, others build structure (collagen, keratin), carry substances (haemoglobin carries oxygen), or act as antibodies and hormones. Heat or strong pH can denature a protein, unfolding it and destroying its function — which is why cooking changes an egg permanently and why fevers are dangerous.
Lipids
Lipids — fats and oils — are the body's long-term energy store, packing more than twice the energy per gram of carbohydrate. They are built from fatty acids and glycerol, do not dissolve in water, and also insulate the body, cushion organs and form the membranes around every cell.
Enzymes
Enzymes are biological catalysts — proteins that speed up the reactions of life without being used up, letting them run fast at body temperature. Each enzyme is specific: its shape fits only one kind of molecule (its substrate), like a lock and key.
Because they are proteins, enzymes are sensitive to conditions. Each has an optimum temperature and pH; go too far from it and the enzyme denatures and stops working. This is why body temperature is held near 37 °C and why digestive enzymes in the acidic stomach differ from those in the small intestine.
Enzymes connect back to rates
An enzyme is just a catalyst — it lowers the activation energy of a biological reaction. Everything from the rates topic applies: it speeds the reaction without being consumed, and temperature and pH change how well it works.
Building up and breaking down
The giant molecules of life are polymers built from small units, and living things assemble and dismantle them constantly. Monomers are joined by condensation — a bond forms and a small molecule (usually water) is released. The reverse, hydrolysis, splits a polymer by adding water, and it is exactly what happens when you digest food: starch to glucose, proteins to amino acids, fats to fatty acids and glycerol.
So the same few reactions, run forwards and backwards, both grow your tissues and release the nutrients you eat.
Nucleic acids and DNA
A fourth family completes the picture: the nucleic acids, DNA and RNA, built from units called nucleotides. DNA stores the genetic instructions for building every protein in an organism, and passes them from one generation to the next. Its famous double-helix structure lets it be copied faithfully when cells divide.
Food tests
Simple colour tests identify each type of nutrient — a staple of practical biochemistry.
| Nutrient | Test | Positive result |
|---|---|---|
| Starch | iodine solution | blue-black colour |
| Reducing sugar (e.g. glucose) | Benedict's solution, warmed | blue → brick-red precipitate |
| Protein | biuret solution | blue → purple/violet |
| Lipid (fat) | ethanol then water (emulsion test) | cloudy white emulsion |
A worked example
A food sample turns iodine blue-black and gives a purple colour with biuret solution, but stays blue with warm Benedict's solution. What does it contain?
- Iodine turning blue-black is the positive test for starch.Iodine only gives blue-black with starch.
- Biuret turning purple is the positive test for protein.Biuret detects the peptide bonds in proteins.
- Benedict's staying blue means no reducing sugar is present.A brick-red precipitate would indicate a reducing sugar; blue means none.
The sample contains starch and protein, but no reducing sugar.
Amino acids and zwitterions
Amino acids are the building blocks of proteins, and each carries two functional groups: a basic amino group (–NH₂) and an acidic carboxyl group (–COOH) on the same molecule. Because one is acidic and one basic, a proton transfers internally from the –COOH to the –NH₂, so the amino acid normally exists as a zwitterion — an ion with both a positive (–NH₃⁺) and a negative (–COO⁻) end but no overall charge.
The pH at which the zwitterion has zero net charge is the isoelectric point. In acid the amino acid gains H⁺ (becomes positive); in alkali it loses H⁺ (becomes negative) — so amino acids resist pH change, acting as buffers. Amino acids join through peptide bonds (a condensation reaction) to build proteins.
Two groups, one molecule
An amino acid is acid and base in one: the –COOH gives up a proton to the –NH₂, forming the zwitterion ⁺H₃N–CHR–COO⁻. This is why amino acids are solids with high melting points and dissolve well in water.