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SAChemistrySyllabus dot point

How are the major biological molecules built from smaller units?

Describe the structure of proteins, carbohydrates and lipids in terms of their monomers and the bonds formed by condensation reactions.

How proteins, carbohydrates and lipids are built by condensation from amino acids, monosaccharides and fatty acids/glycerol, the linkages formed, hydrolysis, and worked SACE-style condensation and saturation problems.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

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What this dot point is asking
Lead worked calculation
Proteins
Carbohydrates
Lipids
The unifying theme: condensation and hydrolysis
Testing saturation
Why it matters

What this dot point is asking

SACE expects you to identify the monomers, name the linkage in each class, recognise that water is released by condensation, and link structure (such as fatty-acid saturation) to properties.

Lead worked calculation

Water released in building a polypeptide

A polypeptide is formed by condensing $50$ amino acid molecules into a single chain. (a) How many peptide bonds form and how many water molecules are released? (b) If each amino acid has an average molar mass of $110\ \text{g mol}^{-1}$ , estimate the molar mass of the polypeptide. ( $M(\text{H}_2\text{O}) = 18.0\ \text{g mol}^{-1}$ .)

Count the bonds and water molecules

Joining $n$ monomers in a chain forms $n - 1$ bonds. For $50$ amino acids that is $50 - 1 = 49$ peptide bonds, and one water molecule is released per bond, so 49 water molecules.

Estimate the molar mass

Total monomer mass $= 50 \times 110 = 5500\ \text{g mol}^{-1}$ . Subtract the mass of water lost: $49 \times 18.0 = 882\ \text{g mol}^{-1}$ .

M(\text{polypeptide}) = 5500 - 882 = 4618\ \text{g mol}^{-1}

Conclude

Because condensation removes water at each link, the polymer mass is always less than the sum of its monomer masses, by the mass of water released. This is the general rule for any condensation polymer.

Proteins

Because each amino acid is difunctional, chains can grow at both ends, and the order of amino acids (the sequence) determines the protein. Twenty amino acids combine in vast numbers of sequences.

Carbohydrates

Carbohydrates are built from monosaccharides such as glucose and fructose ( $\text{C}_6\text{H}_{12}\text{O}_6$ ). Two monosaccharides condense, with the $\text{-OH}$ groups reacting, to form a glycosidic bond and release water, giving a disaccharide (e.g. glucose plus fructose gives sucrose). Many units condense into polysaccharides such as starch and cellulose. Hydrolysis (e.g. during digestion) breaks the glycosidic bonds back into monosaccharides.

Lipids

The unifying theme: condensation and hydrolysis

All three classes are assembled by the same kind of reaction, condensation, releasing water, and broken down by the same reverse reaction, hydrolysis, consuming water. Proteins use peptide (amide) bonds, carbohydrates use glycosidic bonds, and lipids use ester bonds, but the build-up and break-down logic is identical to the esterification and hydrolysis studied earlier. This is why digestion (hydrolysis) and biosynthesis (condensation) are central to biological chemistry.

Testing saturation

The degree of unsaturation in a fat or oil can be measured by how much bromine (or iodine) it absorbs, because the halogen adds across each $\text{C}=\text{C}$ double bond (an alkene addition reaction). More double bonds means more halogen absorbed, the basis of the "iodine value" used to compare fats and oils.

Why it matters

These molecules are the chemistry of life, and they show the same condensation and hydrolysis reactions that govern synthetic polymers and esters. Linking saturation to melting point connects organic structure to the everyday difference between fats and oils, a recurring SACE application.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SACE 20215 marksTwo amino acids, glycine (

\text{H}_2\text{NCH}_2\text{COOH}

) and alanine (

\text{H}_2\text{NCH(CH}_3)\text{COOH}

), react. (a) Name the type of reaction and the small molecule released. (b) Name and draw the linkage formed. (c) Explain why two different dipeptides can form from these two amino acids.

Show worked answer →

(a) It is a condensation reaction; the small molecule released is water. (1 mark)

(b) The linkage is a peptide (amide) bond, $\text{-CO-NH-}$ , formed between the $\text{-COOH}$ of one amino acid and the $\text{-NH}_2$ of the other. (2 marks)

(c) Each amino acid has both an $\text{-NH}_2$ and a $\text{-COOH}$ group, so either can supply the acid end and either the amine end. Glycine-alanine and alanine-glycine are different dipeptides because the order of residues differs. (2 marks)

SACE 20194 marksA triglyceride is formed from glycerol and three fatty acid molecules. (a) Name the type of reaction and the linkage formed, and state how many water molecules are released. (b) Explain, in terms of structure, why a fat made from saturated fatty acids is solid at room temperature while an oil made from unsaturated fatty acids is liquid.

Show worked answer →

(a) Condensation (esterification); the linkage is an ester bond, $\text{-COO-}$ ; three water molecules are released (one per ester bond). (2 marks)

(b) Saturated fatty acid chains are straight, so they pack closely together, allowing strong dispersion forces and a higher melting point (solid). Unsaturated chains have $\text{C}=\text{C}$ double bonds that produce kinks (cis bends), so chains pack poorly, intermolecular forces are weaker, and the melting point is lower (liquid). (2 marks)