OxfordAQA International A Level Biology
OxfordAQA International A Level Biology 3.1.1 Biological Molecules Exam Questions
Free OxfordAQA International A Level Biology revision for 3.1.1 Biological Molecules: what you need to know, common mistakes and exam-style questions with answers. Covers monomers and polymers, condensation and hydrolysis, carbohydrates, lipids, proteins and the biochemical tests used to identify them.
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About OxfordAQA International A Level Biology 3.1.1 Biological Molecules
Biological molecules opens Unit 1 of the OxfordAQA International AS and A Level Biology course. You need to know how monomers join by condensation and are split by hydrolysis, and how the structures of carbohydrates, lipids and proteins relate to their functions. This topic is the foundation for enzymes, membranes, nucleic acids, digestion and respiration later in the course. Examiners often ask you to complete a condensation reaction from a given structure, to explain how starch, cellulose or a triglyceride suits its role, and to describe food tests, including the extra steps needed to detect a non-reducing sugar.
What you need to know
- Condensation and hydrolysis A condensation reaction joins two molecules with a chemical bond and releases a molecule of water. Hydrolysis breaks a bond using a molecule of water. Glycosidic, ester and peptide bonds all form by condensation, and you may be asked to apply this to unfamiliar molecules.
- Carbohydrates Alpha and beta glucose differ in the position of the OH group on carbon 1. Maltose is two alpha glucose molecules; sucrose is glucose and fructose. Starch (amylose and amylopectin) is built from alpha glucose and is a compact, insoluble store, while cellulose is built from beta glucose and forms straight chains linked by hydrogen bonds into strong microfibrils.
- Lipids A triglyceride is one glycerol joined to three fatty acids by ester bonds. Saturated fatty acids have no C=C double bonds in the R group; unsaturated fatty acids have at least one. In a phospholipid one fatty acid is replaced by a phosphate-containing group, giving a hydrophilic head and two hydrophobic tails.
- Proteins Amino acids have an amine group, a carboxyl group and a variable R group, and 20 are common in proteins. Peptide bonds join them into polypeptides. Primary structure is the amino acid sequence, secondary structure is held by hydrogen bonds, tertiary structure by hydrogen bonds, ionic bonds and disulfide bridges, and quaternary structure involves more than one polypeptide.
- Biochemical tests Benedict's reagent heated with a reducing sugar gives a brick-red precipitate. For a non-reducing sugar, boil with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate and repeat the Benedict's test. Iodine in potassium iodide turns blue-black with starch, biuret reagent turns purple with protein, and the emulsion test gives a milky white emulsion with lipid.
Common mistakes in 3.1.1 Biological Molecules
- Saying hydrolysis releases water. Condensation releases water; hydrolysis uses water to break a bond.
- Writing that cellulose is made of alpha glucose. Cellulose is made of beta glucose, so alternate monomers are inverted, which gives straight, unbranched chains.
- Claiming heat breaks peptide bonds when a protein denatures. Denaturation breaks hydrogen and ionic bonds so the tertiary structure changes; the peptide bonds stay intact.
- Describing the emulsion test in the wrong order. The sample is shaken with ethanol first and then mixed with water, which turns milky white if lipid is present.
Try three 3.1.1 Biological Molecules exam-style questions
1. Describe how you would test a solution for the presence of a non-reducing sugar. (3 marks)
Show the mark scheme answer
Heat with Benedict's reagent first and show it stays blue (no reducing sugar); heat a fresh sample with dilute hydrochloric acid, then neutralise with sodium hydrogencarbonate; heat with Benedict's reagent; brick-red precipitate shows a non-reducing sugar was present.
2. Explain how the structure of starch makes it suited to its function as a storage molecule in plant cells. (3 marks)
Show the mark scheme answer
Insoluble, so it does not affect water potential or osmosis; helical / coiled (amylose), so it is compact and a lot can be stored in a small space; branched (amylopectin), so it has many ends for rapid hydrolysis to glucose; large molecule, so it cannot leave the cell.
3. A polypeptide contains 152 amino acids. State how many peptide bonds it contains and how many water molecules are needed to hydrolyse it completely. (2 marks)
Show the mark scheme answer
151 peptide bonds; 151 water molecules (one for each peptide bond broken).
3.1.1 Biological Molecules FAQs
What does OxfordAQA International A Level Biology 3.1.1 Biological Molecules cover?
It covers monomers and polymers, condensation and hydrolysis, carbohydrates, lipids, proteins and the biochemical tests used to identify them.
Which unit is 3.1.1 Biological Molecules examined in?
It is part of Unit 1, The diversity of living organisms, which is examined at International AS. The Unit 5 synoptic paper at A2 can also draw on this content.
How are saturated and unsaturated fatty acids different?
Saturated fatty acids have only single carbon to carbon bonds in their R group. Unsaturated fatty acids have one or more C=C double bonds, which put kinks in the chain and lower the melting point of the lipid.
