Name the four classes of biomolecules, their building blocks and the bonds that join them, and say what each class does in a cell.
Why this matters
Biochemists, biological technicians and medical laboratory scientists spend their days measuring biomolecules: glucose and cholesterol in blood, proteins on a gel, DNA in a PCR tube. Materials scientists increasingly work with bio-based polymers too. Every one of these molecules belongs to one of four classes, and each class is built by the same chemistry you have already met: functional groups joined into bigger molecules.
The big idea: monomers, polymers and water
Most large biological molecules are polymers: long chains of small repeating units called monomers.
- Condensation (dehydration synthesis) joins two monomers and releases a water molecule.
- Hydrolysis breaks the link by adding water back. Digestion is mostly hydrolysis, sped up by enzymes.
monomer–OH + H–monomer -> monomer–monomer + H2O (condensation)
monomer–monomer + H2O -> monomer–OH + H–monomer (hydrolysis)
The four classes
| class | elements | building block | link between units | main jobs |
|---|---|---|---|---|
| carbohydrates | C, H, O | monosaccharides (glucose) | glycosidic bond | fast energy, structure (cellulose) |
| lipids | mostly C, H, a little O | fatty acids + glycerol (not a true polymer) | ester bond | long-term energy, membranes, hormones |
| proteins | C, H, O, N (often S) | amino acids (20 standard) | peptide (amide) bond | enzymes, structure, transport, signals |
| nucleic acids | C, H, O, N, P | nucleotides | phosphodiester bond | store and express genetic information |
Carbohydrates
Formula often close to (CH₂O)ₙ. Glucose (C₆H₁₂O₆) is the main fuel for cells.
- Monosaccharides: glucose, fructose, galactose.
- Disaccharides: two joined, such as sucrose (table sugar = glucose + fructose) and lactose.
- Polysaccharides: starch and glycogen (energy storage in plants and animals) and cellulose (plant cell walls, the most abundant organic polymer on Earth). Starch and cellulose are both made of glucose, but the links differ, which is why you can digest one and not the other.
Lipids
Lipids are grouped by being hydrophobic (they do not dissolve well in water), not by a single repeating unit.
- Triglycerides (fats and oils): one glycerol joined by ester bonds to three fatty acids. Saturated fatty acids have no C=C double bonds and pack tightly (solid butter); unsaturated ones have C=C kinks (liquid olive oil).
- Phospholipids: a water-loving phosphate head and two water-fearing tails. They form the bilayer of every cell membrane.
- Steroids: four fused rings. Cholesterol and hormones such as testosterone and oestradiol.
Proteins
Proteins are chains of amino acids. Each amino acid has an amine group, a carboxylic acid group and a side chain (R group) that gives it its character. The acid of one joins the amine of the next by a peptide bond (an amide), releasing water.
A protein's function depends on its shape, built up in levels: the sequence (primary), local folds such as helices and sheets (secondary), the full 3-D fold (tertiary), and sometimes several chains together (quaternary, as in haemoglobin). Heat, extreme pH or some chemicals can denature a protein, unfolding it so it stops working: that is what happens to egg white when you cook it. This is also why buffers matter in biology.
Enzymes are protein catalysts: they lower activation energy for specific reactions, exactly the idea from the rates lesson.
Nucleic acids
DNA and RNA are chains of nucleotides. Each nucleotide has a sugar (deoxyribose in DNA, ribose in RNA), a phosphate and a base.
- DNA bases: A, T, G, C. RNA uses U instead of T.
- Sugars and phosphates alternate along the backbone, joined by phosphodiester bonds.
- In DNA's double helix, A pairs with T and G pairs with C by hydrogen bonds.
DNA stores the instructions; RNA carries them out to build proteins. Lab methods such as PCR and sequencing all rely on this chemistry, and they are taught in supervised lab courses.
Worked example: classify the molecule
- Contains C, H, O in roughly a 1 : 2 : 1 ratio, tastes sweet. Carbohydrate.
- Contains nitrogen and sulfur, made of 300 linked units with peptide bonds. Protein.
- Contains phosphorus, has a sugar–phosphate backbone and bases A, T, G, C. Nucleic acid (DNA).
- Does not dissolve in water; glycerol plus three long chains joined by ester bonds. Lipid (a triglyceride).
Common mistakes
- Calling fats polymers. Triglycerides are assembled from parts but are not long repeating chains.
- Assuming only proteins contain nitrogen. Nucleic acids contain nitrogen too (in the bases), and phosphorus.
- Forgetting the water. Condensation releases water; hydrolysis uses it.
- Thinking denatured means broken into amino acids. Denaturing unfolds the shape; the peptide bonds usually stay intact.
Quick recap
- Four classes: carbohydrates, lipids, proteins, nucleic acids.
- Monomers join by condensation and split by hydrolysis.
- Bonds: glycosidic (sugars), ester (fats), peptide (proteins), phosphodiester (DNA/RNA).
- Shape determines protein function; DNA stores information in base sequence.
Try it
Open the Do tab and take the quiz: you will match biomolecules to their building blocks, bonds and jobs.