Biomolecules

The four molecular families that build every living thing — from sugars to DNA — assembled by the same simple chemistry.

High schoolIntro OrganicUni Year 1
⏱️ About 18 min

Every living thing — a bacterium, an oak, you — is built from just four families of molecule: carbohydrates, lipids, proteins and nucleic acids. Even better, the same handful of organic reactions you have just learned are what snap their building blocks together. Biology, it turns out, is organic chemistry at scale.

💡
The big idea: The four classes of biomolecule are mostly polymers: long chains built from small repeating monomers. Cells link monomers together by condensation (losing water) and break them apart by adding water back. Master that one pattern and the chemistry of life becomes readable.
🎯 By the end, you'll be able to
  • Name the four classes of biomolecule and their building blocks
  • Explain how monomers join into polymers by condensation (losing water)
  • Identify the peptide bond that links amino acids into proteins
  • Describe lipids as esters of glycerol and fatty acids
📎 Helpful to know first

Four families, one construction trick

Life runs on four classes of large molecule: carbohydrates, lipids, proteins and nucleic acids. Three of these are polymers — long chains made by linking small repeating units called monomers. And cells build almost all of them with the same move: a condensation reaction that joins two units and releases a molecule of water.

🔑 Build with water out, break with water in
Cells assemble polymers by condensation (also called dehydration synthesis): each new bond forms by expelling one water molecule. To break the polymer back into monomers, they run the reverse — hydrolysis — adding a water molecule to split each bond. One pattern, forwards and backwards.

Carbohydrates: sugars and their chains

Carbohydrates are sugars and their polymers. The monomer is a simple sugar (a monosaccharide) such as glucose, C6H12O6, rich in hydroxyl groups. Link many glucose units by condensation and you get polysaccharides — starch, which plants store as energy, and cellulose, which stiffens their cell walls.

\[ \ce{C6H12O6 + C6H12O6 -> C12H22O11 + H2O} \]
Two glucose units condense into the disaccharide maltose, releasing water — the same join-and-lose-water move as ester formation.

Lipids: fats built from glycerol and fatty acids

Lipids — fats and oils — are the odd family out: they are not polymers of a single repeating monomer. A typical fat is an ester formed from one glycerol molecule and three long fatty acid chains (a triglyceride). Each glycerol –OH condenses with a fatty acid –COOH, releasing water — ester formation you met in the reactions lesson. Their long hydrocarbon tails make lipids water-repelling, which is exactly why cell membranes are built from them.

✨ Saturated vs unsaturated fats
The saturated/unsaturated idea from hydrocarbons carries straight over. Saturated fats have fatty-acid tails with only single C–C bonds; they pack tightly and are solid at room temperature (like butter). Unsaturated fats have C=C double bonds that kink the tail, so they pack loosely and stay liquid (like olive oil).

Proteins: polymers of amino acids

Proteins are polymers of amino acids. Each amino acid carries both an amino group (–NH2) and a carboxyl group (–COOH). Those two groups react by condensation: the carboxyl of one and the amino of the next join, releasing water and forming a peptide bond. Chain many amino acids together and the sequence folds into a working protein — an enzyme, a muscle fibre, an antibody.

\[ \ce{H2N-CHR-COOH + H2N-CHR'-COOH -> H2N-CHR-CO-NH-CHR'-COOH + H2O} \]
A peptide bond (–CO–NH–) forms between two amino acids by condensation, releasing water.

Nucleic acids: the information molecules

Nucleic acids — DNA and RNA — are polymers of monomers called nucleotides. Each nucleotide has three parts: a sugar, a phosphate group, and a nitrogen-containing base. The order of the bases along the chain spells out the genetic instructions a cell reads to build its proteins. Same polymer logic, storing information instead of energy or structure.

⚠️ Lipids are the exception, not the rule
It is tempting to call all biomolecules "polymers of monomers," but lipids do not fit — a fat is a small assembly of glycerol plus a few fatty acids, not a long repeating chain. Carbohydrates, proteins and nucleic acids are the true polymers; lipids are held together by ester bonds instead.
📝 Worked example: Two amino acids join to form a dipeptide. Which functional groups react, what bond forms, and what small molecule is released?
  1. Each amino acid has an amino group (–NH2) and a carboxyl group (–COOH).
  2. The –COOH of one reacts with the –NH2 of the other — a condensation reaction.
  3. This forms a peptide bond (–CO–NH–) linking the two amino acids.
  4. As with all condensations, a molecule of water is released.
✓ The carboxyl and amino groups react, forming a peptide bond and releasing water.
✏️ Practice: A protein chain is built from 25 amino acids joined in a row. How many peptide bonds (and water molecules released) does that take?
peptide bonds
Solution
  1. Linking a chain of monomers needs one bond fewer than the number of monomers.
  2. 25 amino acids → 25 − 1 = 24 links.
  3. So 24 peptide bonds form, releasing 24 water molecules.

Check your understanding

1. How do cells link monomers into biological polymers such as proteins?
Monomers join by condensation (dehydration synthesis): each new bond forms while a water molecule is expelled. Adding water back (hydrolysis) breaks the polymer apart.
2. Which bond links amino acids together in a protein?
A peptide bond (–CO–NH–) forms by condensation between the carboxyl group of one amino acid and the amino group of the next, releasing water.
3. Which statement about lipids is correct?
Lipids are the non-polymer family: a typical fat is an ester built from glycerol and three fatty acids. Amino acids build proteins; nucleic acids store genetic information.
🔎 Related lessons
Previous: Reaction Types & Mechanisms Functional Groups Hydrocarbons
✅ Key takeaways
  • Life is built from four biomolecule families: carbohydrates, lipids, proteins and nucleic acids.
  • Carbohydrates (sugars→polysaccharides), proteins (amino acids) and nucleic acids (nucleotides) are polymers.
  • Monomers join by condensation (losing water) and split by hydrolysis (adding water).
  • Amino acids link through peptide bonds; lipids are esters of glycerol and fatty acids.
  • Saturated fats (single bonds) are solids; unsaturated fats (C=C) are liquids.
➡️ You have now traced carbon from a single C–H bond all the way to DNA. The same rules — bonding, shape, functional groups and a few reaction types — scale from methane to the molecules of life. That is the quiet power of organic chemistry.
Want to test yourself on this? Try the Chemistry practice test →