AQA A-Level Biology: Biological Molecules (7402)

This guide covers Topic 1 Biological molecules, the first of four sections in AQA AS Biology (7401) and the first of eight sections in A-level Biology (7402), first teaching September 2015. Sections 3.1-3.4 (including this one) form the AS subject content; sections 3.5-3.8 are A-level only.

Where this fits in 7401/7402

Biological molecules establishes the biochemical basis shared by all living organisms – the idea that a small set of carbon-based compound classes, built from monomers into polymers, underpins everything from cell structure to genetic information. This chemistry recurs throughout the rest of the specification, from cell membranes in Topic 2 to gene expression in Topic 8.

Syllabus coverage

AQA AS/A-LEVEL BIOLOGY (7401/7402) — TOPIC 1 BIOLOGICAL MOLECULES

• 3.1.1 Monomers and polymers — the general principle that monomers join to form polymers through condensation reactions (releasing water) and are broken apart by hydrolysis reactions (using water)
• 3.1.2 Carbohydrates — monosaccharides, disaccharides and polysaccharides, and the condensation and hydrolysis reactions that build and break them
• 3.1.3 Lipids — the structure and roles of lipids, including in plasma membranes, hormones and as respiratory substrates
• 3.1.4 Proteins — the general properties of proteins and their roles as structural components, enzymes, chemical messengers and blood components
• 3.1.5 Nucleic acids — the structure of DNA and RNA and the process of DNA replication
• 3.1.6 ATP — the role of ATP as an immediate energy source
• 3.1.7 Water — the properties of water relevant to living organisms
• 3.1.8 Inorganic ions — the roles of inorganic ions in living systems

How to approach it

Monomers and polymers (3.1.1) is the conceptual key to the whole topic: once condensation and hydrolysis reactions are secure, the carbohydrate, lipid and protein sub-topics become variations on the same underlying pattern rather than separate facts to memorise independently. Build a single, unified picture across 3.1.2-3.1.4 of which monomers combine into which polymers and by which bond type, since exam questions frequently ask students to compare structures across molecule classes. Nucleic acids (3.1.5) deserves particular attention to detail – know the structural differences between DNA and RNA precisely, as this groundwork is assumed without re-explanation in later topics on protein synthesis and gene expression.

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, for first teaching September 2015 — aqa.org.uk.

Monomers, polymers and the two shared reactions

Almost every large biological molecule is a polymer built from monomers by condensation (a bond forms, one water molecule is released) and broken by hydrolysis (water is added, the bond breaks). Recognising that the same two reactions run through carbohydrates, proteins and nucleic acids removes most of the memorisation from this topic.

Polymer	Monomer	Bond formed
Starch, glycogen, cellulose	alpha or beta glucose	glycosidic
Protein	amino acid	peptide
DNA, RNA	nucleotide	phosphodiester

Carbohydrates

Alpha and beta glucose are isomers differing only in the orientation of the hydroxyl group on carbon 1 — a small difference with large consequences. Alpha glucose forms starch and glycogen, which coil and branch for compact storage. Beta glucose forms cellulose, in which alternate molecules are inverted so straight chains form and hydrogen-bond into microfibrils of great tensile strength.

Starch is helical and insoluble, so it stores glucose without affecting water potential. Glycogen is more highly branched than starch, giving more free ends for rapid hydrolysis — appropriate for animals with higher metabolic rates.

Proteins

Amino acids share a central carbon bonded to an amine group, a carboxyl group, a hydrogen and a variable R group. Structure is described at four levels: primary (the sequence), secondary (alpha helices and beta pleated sheets held by hydrogen bonds), tertiary (the overall 3D shape held by hydrogen bonds, ionic bonds and disulfide bridges), and quaternary (two or more polypeptides, as in haemoglobin).

Because tertiary structure determines the shape of an enzyme’s active site, and primary structure determines tertiary structure, a single change in the amino acid sequence can abolish function.

Lipids

Triglycerides are one glycerol and three fatty acids joined by ester bonds. They are non-polar and insoluble, contain more energy per gram than carbohydrates, and release water when respired. Saturated fatty acids have no C=C double bonds; unsaturated ones do, and the resulting kinks prevent close packing, which is why unsaturated fats are liquid at room temperature. Phospholipids substitute one fatty acid for a phosphate group, producing a hydrophilic head and hydrophobic tails — the basis of the bilayer.

Food tests

Test	Reagent	Positive result
Starch	iodine in potassium iodide	orange to blue-black
Reducing sugar	Benedict’s, heated	blue to brick-red
Non-reducing sugar	boil with acid, neutralise, then Benedict’s	brick-red
Protein	biuret	blue to purple
Lipid	emulsion test with ethanol	white emulsion

Worked example

A polypeptide is 120 amino acids long. How many water molecules are released during its synthesis, and how many during complete hydrolysis?

Bonds in a chain of n monomers = n - 1 = 119

Condensation releases one water per bond formed  -> 119 released
Hydrolysis consumes one water per bond broken    -> 119 used

The number of bonds, not the number of monomers, is what the question turns on.

Common mistakes

Saying cellulose is made of alpha glucose. Describing glycogen as “the same as starch” without mentioning greater branching and why that matters. Confusing an ester bond with a glycosidic bond in triglycerides. Stating that a non-reducing sugar gives a negative Benedict’s result without explaining that hydrolysis first is what makes the second test positive. Writing that disulfide bridges hold secondary structure — they are tertiary.

Enzymes

Enzymes are globular proteins that act as biological catalysts, speeding up reactions by lowering the activation energy needed. The lock-and-key model treats the active site as a rigid, pre-shaped fit for its substrate; the more accurate induced-fit model holds that the active site changes shape slightly as the substrate binds, straining the substrate’s bonds and further lowering the activation energy. Rate of reaction rises with temperature up to an optimum (more frequent successful collisions) and then falls sharply as heat disrupts the bonds holding tertiary structure, denaturing the enzyme so the active site is no longer complementary to the substrate; extremes of pH denature enzymes the same way, by disrupting ionic and hydrogen bonds. Increasing substrate or enzyme concentration raises the rate only until the other becomes limiting, at which point the rate plateaus because all active sites are occupied at any instant. Competitive inhibitors resemble the substrate and occupy the active site directly, so their effect can be reduced by raising substrate concentration; non-competitive inhibitors bind elsewhere and change the active site’s shape, so raising substrate concentration does not reverse the effect.

Nucleic acids and DNA replication

DNA is a double helix of two antiparallel polynucleotide strands, each nucleotide made of deoxyribose, a phosphate group and one of four nitrogenous bases (adenine, thymine, cytosine, guanine). The strands are held together by hydrogen bonds between complementary base pairs — adenine with thymine (two hydrogen bonds) and cytosine with guanine (three) — so the sequence of one strand determines the sequence of the other. RNA differs from DNA in being single-stranded, containing ribose instead of deoxyribose, and using uracil in place of thymine. DNA replicates semi-conservatively: DNA helicase unwinds the double helix and breaks the hydrogen bonds between base pairs, and DNA polymerase builds a new complementary strand against each original (template) strand, so each resulting double helix contains one original and one newly synthesised strand.

ATP

ATP (adenosine triphosphate) is a nucleotide derivative made of adenine, ribose and three phosphate groups. Hydrolysis of the bond to the terminal phosphate, catalysed by ATP hydrolase, releases a usable quantity of energy and forms ADP and an inorganic phosphate; this makes ATP the immediate energy currency of the cell, used directly to drive processes such as active transport, muscle contraction and metabolic reactions, in contrast to the larger, longer-term energy stores such as glycogen or triglycerides. ATP is resynthesised from ADP and inorganic phosphate using energy released in respiration.

Water

Water is a dipolar molecule: the oxygen atom carries a slightly negative charge and each hydrogen atom a slightly positive charge, so water molecules form hydrogen bonds with each other and with other polar or charged molecules. This structure explains water’s key biological properties: a high specific heat capacity (many hydrogen bonds must be broken to raise its temperature), which buffers aquatic habitats and cells against rapid temperature change; a high latent heat of vaporisation, which allows organisms to lose large amounts of heat through the evaporation of a small volume of water, as in sweating; cohesion between water molecules, which supports the continuous water columns needed for transport in the xylem; and its effectiveness as a solvent for polar and charged substances, allowing metabolic reactions and transport of dissolved substances to occur in an aqueous medium. Water is also a reactant in hydrolysis reactions and in photosynthesis.

Inorganic ions

Inorganic ions occur in solution in the body and cytoplasm and have specific, examinable roles: hydrogen ions determine pH, which affects enzyme activity; iron ions form part of haemoglobin and bind oxygen reversibly; sodium ions drive the co-transport of glucose and amino acids across cell membranes and are essential to the generation of nerve impulses; and phosphate ions are structural components of ATP, DNA and RNA, and of phospholipids.

Quick revision checklist

• Define condensation and hydrolysis and apply both to all three polymer classes.
• Explain how alpha/beta glucose isomerism produces the different properties of starch, glycogen and cellulose.
• Describe all four levels of protein structure with the bonds involved at each.
• Relate triglyceride and phospholipid structure to their functions.
• Carry out and interpret each food test, including the non-reducing sugar sequence.
• Calculate water released or consumed from the number of bonds.