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 from amino acids, monosaccharides and fatty-acid/glycerol units through condensation reactions forming peptide, glycosidic and ester bonds.
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What this dot point is asking
You must describe each biomolecule's monomers and the condensation bond that links them.
A common theme: condensation and hydrolysis
All three classes of biomolecule are polymers (or, for lipids, large assembled molecules) built by condensation - joining units with the loss of water - and broken down by hydrolysis - splitting with the addition of water. Only the monomer and the type of bond differ.
Proteins
Two amino acids join when the -COOH of one condenses with the -NH of the next, forming a peptide bond (-CONH-) and releasing water. Many amino acids form a polypeptide; one or more polypeptide chains folded into a specific shape make a protein.
Carbohydrates
The monomers are monosaccharides such as glucose (). Two monosaccharides condense - an -OH on each reacts - to form a glycosidic bond and release water, giving a disaccharide (e.g. maltose). Many monosaccharides form a polysaccharide such as starch or cellulose.
Lipids
A triglyceride (fat or oil) forms when one molecule of glycerol (a tri-alcohol) condenses with three fatty-acid molecules (long-chain carboxylic acids). Each -OH of glycerol reacts with a -COOH of a fatty acid to form an ester bond, releasing three molecules of water in total.
- Saturated fats have fatty-acid chains with only single C-C bonds; they pack closely and are usually solid at room temperature.
- Unsaturated fats (oils) have one or more C=C double bonds, which kink the chains so they pack less well and are usually liquid.
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.
2022 SACE Stage 22 marksGlucose and fructose are the monosaccharide units of sucrose, shown as ring structures. State why both glucose and fructose are classified as carbohydrates.Show worked answer →
A carbohydrate is a compound that contains carbon, hydrogen, and oxygen, in which the hydrogen and oxygen are present in the ratio 2:1 (the same ratio as in water), and which contains multiple hydroxyl groups together with a carbonyl (aldehyde or ketone) group.
Both glucose and fructose fit this definition: each has the molecular formula C6H12O6 (H:O ratio of 2:1) and each carries several hydroxyl groups and a carbonyl group. One mark for the elements/ratio, one for the characteristic functional groups (poly-hydroxy aldehyde or ketone).
2024 SACE Stage 24 marksTwo triglycerides A and B are shown. Referring to the structures of these triglycerides, explain which of A or B is more likely to be found as a solid at room temperature.Show worked answer →
Solid versus liquid depends on the degree of saturation of the fatty-acid chains.
A saturated triglyceride has fatty-acid chains with only single C-C bonds, so the chains are straight and can pack closely together.
Close packing maximises the contact between chains, giving stronger dispersion (intermolecular) forces, so more energy is needed to melt it and it is solid at room temperature.
An unsaturated triglyceride has C=C double bonds that create kinks/bends in the chains, preventing close packing.
The weaker dispersion forces in the unsaturated triglyceride give a lower melting point, so it is liquid. The triglyceride drawn with no (or fewer) C=C double bonds (the more saturated one) is more likely to be solid. One mark per linked point.
2024 SACE Stage 22 marksDraw the structural formula of a section of a protein chain containing two glutamine units.Show worked answer →
Two amino acids join by a condensation reaction that forms a peptide (amide) bond, with loss of a water molecule.
The required structure shows two glutamine residues joined through a -CO-NH- peptide linkage between the carboxyl group of one and the amino group of the next: ...-NH-CH(R)-CO-NH-CH(R)-CO-... where R is the glutamine side chain (-CH2-CH2-CO-NH2). One mark for a correct peptide (amide) linkage between the units, one mark for correctly retaining both glutamine side chains and the chain continuing at each end.