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Biological molecules: proteins, carbohydrates and lipids (SACE Stage 2 Chemistry)

Syllabus dot point

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

9 min read

Quick answer

The three major biological molecules are built by condensation from small monomers, releasing water at each bond. Proteins are chains of amino acids joined by peptide (amide) bonds. Carbohydrates are monosaccharides (e.g. glucose) joined by glycosidic bonds. Lipids (triglycerides) form from glycerol and three fatty acids joined by ester bonds. Each can be broken back into its monomers by hydrolysis, the reverse reaction.

Jump to a section
  1. What this dot point is asking
  2. Lead worked calculation
  3. Proteins
  4. Carbohydrates
  5. Lipids
  6. The unifying theme: condensation and hydrolysis
  7. Testing saturation
  8. Why it matters
  9. Exam-style questions

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 5050 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 g mol−1110\ \text{g mol}^{-1}, estimate the molar mass of the polypeptide. (M(H2O)=18.0 g mol−1M(\text{H}_2\text{O}) = 18.0\ \text{g mol}^{-1}.)

Count the bonds and water molecules

Joining nn monomers in a chain forms n−1n - 1 bonds. For 5050 amino acids that is 50−1=4950 - 1 = 49 peptide bonds, and one water molecule is released per bond, so 49 water molecules.

Estimate the molar mass

Total monomer mass =50×110=5500 g mol−1= 50 \times 110 = 5500\ \text{g mol}^{-1}. Subtract the mass of water lost: 49×18.0=882 g mol−149 \times 18.0 = 882\ \text{g mol}^{-1}.

M(polypeptide)=5500−882=4618 g mol−1M(\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

Proteins and the peptide bond

Proteins are condensation polymers of amino acids, each of which has an amine group (-NH2\text{-NH}_2) and a carboxylic acid group (-COOH\text{-COOH}). When the -COOH\text{-COOH} of one reacts with the -NH2\text{-NH}_2 of another, a peptide (amide) bond (-CO-NH-\text{-CO-NH-}) forms and water is released. Long chains of amino acid residues fold into the three-dimensional shapes that give proteins their biological function.

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 (C6H12O6\text{C}_6\text{H}_{12}\text{O}_6). Two monosaccharides condense, with the -OH\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

Triglyceride structure and saturation

A triglyceride forms by condensation (esterification) of one glycerol (a triol) with three fatty acids, creating three ester bonds and releasing three water molecules. Saturated fatty acids have only single C-C\text{C-C} bonds, so their straight chains pack closely, giving strong dispersion forces and a higher melting point (solid fats). Unsaturated fatty acids contain C=C\text{C}=\text{C} double bonds that kink the chain, so packing is poorer, forces weaker, and the melting point lower (liquid oils).

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 C=C\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.

Forgetting that condensation removes water and reduces mass

A frequent SACE error is calculating a polymer's mass as the simple sum of its monomers, forgetting that each condensation bond releases a water molecule, so the product mass is less by 18.0 g mol−118.0\ \text{g mol}^{-1} per bond. Another error is naming the wrong linkage: proteins use peptide (amide) bonds, carbohydrates use glycosidic bonds, and lipids use ester bonds. Also remember nn monomers form n−1n - 1 bonds in a chain, not nn.

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 questions

Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

SACE 2021-style5 marks
Two amino acids, glycine (H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}) and alanine (H2NCH(CH3)COOH\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, -CO-NH-\text{-CO-NH-}, formed between the -COOH\text{-COOH} of one amino acid and the -NH2\text{-NH}_2 of the other. (2 marks)

(c) Each amino acid has both an -NH2\text{-NH}_2 and a -COOH\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 2019-style4 marks
A 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, -COO-\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 C=C\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)

Practise this

Sources & how we know this

Based on the official SACE Board subject outline. Not yet individually reviewed by a person; always check SACE Board and your school's materials for current rules.

ExamExplained