HookThe single letter that bends a red blood cell
In 1949 the chemist Linus Pauling did something nobody had done before: he blamed a whole disease on a single molecule. Sickle-cell anaemia, he showed, was a disorder of haemoglobin itself. Seven years later Vernon Ingram narrowed the fault to one amino acid. In the beta-globin chain, at position 6 of 146, the water-loving glutamic acid is replaced by water-hating valine. One residue out of 146 — that is the entire defect, and it is enough to warp a red cell into a crescent and choke a capillary.
That one swap is a guided tour of everything in 3.1. It begins in DNA, where a single base change — the triplet GAG becoming GTG — is copied faithfully by semi-conservative replication into every red-cell precursor. It is read into the primary structure of a protein: the order of amino acids joined by peptide bonds made in condensation reactions. The rogue valine hands the folded haemoglobin a sticky hydrophobic patch, so when oxygen is unloaded the molecules polymerise into stiff fibres and buckle the cell. Change the R group of one monomer and you change the tertiary structure, the quaternary structure, the iron-carrying function, and a person's whole life. Carbohydrates, lipids, proteins, nucleic acids, ATP, water and a handful of inorganic ions — every idea in this topic obeys the same logic: small monomers, joined and broken by two reactions, fold into shapes, and shape is function.
ModelMonomers, polymers, and the two reactions that build and break them
A monomer is a small, single unit; a polymer is a large molecule made from many monomers bonded together. The whole of biochemistry runs on three monomer-to-polymer families: monosaccharides build polysaccharides, amino acids build polypeptides, and nucleotides build the polynucleotides DNA and RNA. Learn the pairing and half the vocabulary of the topic falls into place.
Just two reactions convert between them. A condensation reaction joins two molecules, forms a new chemical bond and releases a molecule of water. A hydrolysis reaction is the exact reverse: it uses a molecule of water to break a bond and split a molecule in two. Every time your body builds tissue it is running condensation; every time you digest a meal you are running hydrolysis. The same two reactions make and break glycosidic bonds in carbohydrates, ester bonds in lipids, peptide bonds in proteins and phosphodiester bonds in nucleic acids — which is why AQA can ask the same conceptual question in four different disguises.
The counting rule is worth memorising because it is examinable maths: joining n monomers into a single chain needs \(n - 1\) bonds, and each bond is made by one condensation reaction that releases one water molecule. Bonds and water always come out equal.
A single starch molecule is built from 500 alpha-glucose monomers joined in one chain. How many glycosidic bonds does it contain, and how many water molecules were released to make it? Joining \(n\) monomers needs \(n - 1\) bonds, each formed by one condensation reaction releasing one water: \[500 - 1 = 499.\] So there are 499 glycosidic bonds and 499 water molecules were released. Hydrolysing the molecule completely back to free glucose would consume the same 499 water molecules — the reaction simply run in reverse.
ModelCarbohydrates — from single sugars to storage and structure
Monosaccharides are the single-sugar monomers: glucose, fructose and galactose all share the formula C6H12O6 but differ in shape. Glucose comes in two forms that matter enormously later — alpha-glucose and beta-glucose — which differ only in the position of one hydroxyl group on carbon 1. Two monosaccharides join by a glycosidic bond in a condensation reaction to form a disaccharide: maltose (two alpha-glucose), sucrose (glucose + fructose) and lactose (glucose + galactose).
Chain many monosaccharides together and you get a polysaccharide, where structure is dictated by which monomer and which bonds. Starch is the plant store: amylose is an unbranched 1,4-linked chain that coils into a compact helix, while amylopectin adds 1,6 branches. Glycogen is the animal and fungal store — like amylopectin but even more branched, so its many free ends can be hydrolysed rapidly when respiring cells demand glucose. Both are compact and insoluble, so they store energy without dragging water into the cell by osmosis. Cellulose is the odd one out: made of beta-glucose, whose alternate monomers must flip 180 degrees, giving straight, unbranched chains that hydrogen-bond side by side into strong microfibrils — the tensile scaffolding of every plant cell wall.
You must also know the biochemical tests. For a reducing sugar, add Benedict's reagent and heat: a positive result turns the blue solution through green and orange to a brick-red precipitate. A non-reducing sugar such as sucrose gives a negative Benedict's test first; hydrolyse it by boiling with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, then repeat Benedict's — it now turns red. For starch, add iodine in potassium iodide solution: orange-brown to blue-black is positive.
ModelLipids — triglycerides and phospholipids
A triglyceride is one molecule of glycerol joined to three fatty acids. Each fatty acid attaches by an ester bond formed in a condensation reaction, so making one triglyceride releases three water molecules. Fatty acids are saturated if their hydrocarbon tail has no carbon-to-carbon double bonds (animal fats, solid at room temperature) or unsaturated if it has one or more double bonds that kink the chain (plant oils, liquid).
Structure explains why lipids are the body's premium energy store. Their long hydrocarbon tails are rich in carbon-hydrogen bonds, so on oxidation they release roughly 37 kJ per gram against about 17 kJ per gram for carbohydrate — more than double. Being insoluble in water they store energy without any osmotic cost, and they double as thermal insulation, buoyancy and physical protection around organs; their oxidation even yields useful metabolic water, which matters to desert animals.
Phospholipids swap one fatty acid for a phosphate group, and that single change builds every membrane in biology. The phosphate head is charged and hydrophilic; the two fatty-acid tails are hydrophobic. Drop phospholipids into water and they self-arrange into a bilayer, heads facing the water inside and out, tails hiding in the middle — the basic architecture you will meet again as the fluid-mosaic membrane. The test for any lipid is the emulsion test: dissolve the sample in ethanol, then add water; a cloudy white emulsion is positive.
ModelProteins — why the order of the beads decides everything
The monomer is the amino acid: a central carbon carrying an amino group, a carboxyl group, a hydrogen atom and a variable R group. There are 20 amino acids in living things and they differ only in that R group. Two amino acids join by a peptide bond in a condensation reaction (releasing water) to form a dipeptide; many form a polypeptide.
Proteins fold through four levels, and AQA tests the bonds at each. Primary structure is simply the sequence of amino acids — the sickle-cell fault lives here. Secondary structure is local folding into alpha-helices or beta-pleated sheets, held by hydrogen bonds between the carboxyl and amino groups of the backbone. Tertiary structure is the overall three-dimensional shape of one polypeptide, locked by four interactions between R groups: hydrogen bonds, ionic bonds, disulfide bridges between cysteines, and hydrophobic interactions. Quaternary structure exists only in proteins of two or more polypeptides — haemoglobin is the classic, four chains each cradling an iron-containing haem group.
Shape then dictates function. Globular proteins such as enzymes fold into precise active sites; fibrous collagen winds three chains into a rope of staggered, cross-linked strands for tensile strength; antibodies, channel proteins and hormones each earn their job from their fold. The test for protein is the biuret test: add sodium hydroxide then a few drops of copper(II) sulfate solution; a lilac-purple colour (from blue) shows peptide bonds are present.
MechanismEnzymes — lowering the hill, and everything that gets in the way
Enzymes are globular proteins that act as biological catalysts: they speed reactions by lowering the activation energy, the energy barrier a reaction must clear. They do it by binding the substrate in an active site to form an enzyme-substrate complex. AQA wants the induced-fit model, not the old lock-and-key: the active site is not a rigid mould but moulds itself around the substrate as it binds, straining the substrate's bonds so they break or form more easily. Specificity comes from the enzyme's tertiary structure, which makes the active site complementary to just one substrate shape.
Five factors change the rate, and each has a mechanism you must state. Raising temperature speeds the rate to an optimum (more kinetic energy, more successful collisions), but beyond it hydrogen and ionic bonds break, the tertiary structure changes, the active site stops being complementary, and the enzyme is denatured — permanently. pH works the same way: each enzyme has an optimum, and extremes disrupt the bonds holding the active site's shape. Raising substrate concentration increases the rate until every active site is working flat out (saturation), after which the rate plateaus at a maximum. Raising enzyme concentration raises the rate proportionally, provided substrate is in excess.
Inhibitors slow enzymes in two contrasting ways. A competitive inhibitor has a shape similar to the substrate and binds the active site itself, blocking it; because they compete, adding more substrate reduces the inhibitor's effect. A non-competitive inhibitor binds elsewhere, at an allosteric site, changing the active site's shape so the substrate no longer fits — and here adding more substrate makes no difference at all. Telling those two apart from a graph is a guaranteed exam question.
DataRequired practical 1 — clocking an enzyme's rate
The task is to investigate how a chosen variable affects the rate of an enzyme-controlled reaction. A standard system is catalase breaking hydrogen peroxide into water and oxygen, measuring the oxygen collected over time in a gas syringe or an inverted measuring cylinder; alternatives include amylase digesting starch (sampling onto iodine and timing the colour to disappear) or a protease clearing a cloudy suspension.
Design it like an examiner. The independent variable might be temperature (set with water baths), pH (set with buffers) or substrate concentration. The dependent variable is the rate — volume of oxygen per unit time, or one divided by the time to reach an end-point. The controlled variables are everything else: enzyme concentration, substrate concentration and volume, pH if it is not the variable, and the same source of enzyme throughout. Method discipline matters: equilibrate enzyme and substrate separately in the water bath first, mix, and start timing the instant they meet, sealing the bung immediately.
Sources of error are the real marks. Oxygen escapes in the moment before the bung is inserted, so early readings run low; judging a colour end-point by eye is subjective; the reaction begins on mixing, so any delay loses product; and a water bath drifts unless checked. Improvements follow directly: use a colorimeter or light gate for an objective end-point, a data logger with an oxygen or pressure sensor, a buffer to pin pH, and repeat readings to calculate a reliable mean.
At 20 degrees Celsius, 24 cm cubed of oxygen was collected in the first 60 s; at 30 degrees, 48 cm cubed in the same time. Rate is volume divided by time: \[\text{rate}_{20} = \dfrac{24}{60} = 0.40,\qquad \text{rate}_{30} = \dfrac{48}{60} = 0.80\ \text{cm}^3\,\text{s}^{-1}.\] The rate has doubled for a 10-degree rise, a temperature coefficient \(Q_{10} = 0.80 \div 0.40 = 2.0\) — the textbook value. Two subtleties earn the top marks. First, this is a mean rate over the minute; the true initial rate is higher, because hydrogen peroxide is most concentrated at the start, so for an accurate value you draw a tangent to the curve at \(t = 0\). Second, at 60 degrees you would collect almost nothing — say 3 cm cubed — not because the reaction is slow but because the catalase is denatured, its active site no longer complementary.
ModelNucleotides, the double helix, and copying it perfectly
A nucleotide has three parts: a pentose sugar, a phosphate group and a nitrogen-containing organic base. In DNA the sugar is deoxyribose and the bases are adenine, thymine, cytosine and guanine; in RNA the sugar is ribose and thymine is replaced by uracil. Nucleotides join by condensation into a phosphodiester bond, building a sugar-phosphate backbone. DNA is two antiparallel polynucleotide strands twisted into a double helix, held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine by two hydrogen bonds, cytosine with guanine by three. RNA is a single, shorter strand — messenger RNA and transfer RNA are the ones to know.
Complementary base pairing is also the key to copying. In semi-conservative replication, the enzyme DNA helicase breaks the hydrogen bonds and unwinds the helix, exposing two template strands. Free activated DNA nucleotides line up against each template by complementary base pairing, and DNA polymerase catalyses the phosphodiester bonds joining them. Each new molecule is therefore one conserved original strand plus one newly made strand — hence 'semi-conservative'. The proof is one of biology's most elegant experiments: in 1958 Meselson and Stahl grew bacteria in heavy nitrogen-15, switched them to light nitrogen-14, and used density-gradient centrifugation to show a single band of intermediate density after one generation — impossible if replication were conservative.
Because the bases pair one-to-one, their quantities are locked together — a rule called Chargaff's ratios. Suppose a sample of double-stranded DNA is found to contain 28% thymine. Adenine pairs with thymine, so adenine is also 28%, and together \(A + T = 56\%\). The remaining 44% must be shared equally between the two bases that pair with each other, cytosine and guanine, giving \(C = G = 22\%\) each. Check the total: \[28 + 28 + 22 + 22 = 100.\] The trap is to assume all four bases are 25%; they are only equal in pairs, so knowing one base fixes its partner but not the other pair.
ModelATP — the cell's rechargeable coin
ATP (adenosine triphosphate) is a phosphorylated nucleotide: the base adenine, the sugar ribose, and three phosphate groups. It is the universal immediate energy source in every cell. When the enzyme ATP hydrolase hydrolyses ATP to ADP and an inorganic phosphate group (Pi), it releases a small, usable packet of energy — about 30.5 kJ per mole. That released phosphate can also be transferred onto another molecule (phosphorylation), making it more reactive, which is how ATP powers processes such as active transport and the early steps of respiration.
ATP is not a store but a shuttle. It is resynthesised from ADP and Pi by ATP synthase in a condensation reaction, driven by respiration and photosynthesis, and the cycle turns over constantly — a typical cell remakes its own body weight of ATP each day. Why use ATP rather than releasing energy straight from glucose? Because ATP releases energy in small, manageable amounts (little is wasted as heat), does so in a single quick reaction, and is a common currency every reaction can spend. Its weaknesses are the flip side: it is unstable, so it cannot store energy long-term, and it is not transported between cells — it is made and spent on the spot.
MechanismWater and inorganic ions — the small molecules doing the heavy lifting
Water is a polar molecule: oxygen pulls the shared electrons, making it slightly negative while the hydrogens are slightly positive, so water molecules form hydrogen bonds with one another. Almost every biological property of water flows from that. It is a metabolite, taken up in hydrolysis and released in condensation, and both a raw material and a product of photosynthesis and respiration. It is an excellent solvent for ions and polar molecules, so metabolic reactions happen in solution and substances are carried in blood and xylem. Its many hydrogen bonds give it a high specific heat capacity, so it resists temperature change and buffers cells and aquatic habitats against sudden swings.
Two more properties are exam favourites. Water has a high latent heat of vaporisation: evaporating it takes a great deal of energy, so sweating and transpiration cool an organism efficiently while losing relatively little water. And cohesion — hydrogen bonds pulling molecules together — lets water form continuous columns dragged up the xylem in the transpiration stream, and produces the surface tension a pond-skater walks on. Ice being less dense than liquid water insulates the life beneath it.
Dissolved inorganic ions then do specific jobs at very different concentrations. Hydrogen ions (H+) set the pH that governs enzyme activity. Iron ions (Fe2+) sit at the centre of each haem group, binding the oxygen that haemoglobin transports. Sodium ions (Na+) drive the co-transport of glucose and amino acids across membranes and carry nerve impulses. Phosphate ions (PO4 3-) form the backbone of DNA and RNA, the terminal groups of ATP, and the hydrophilic heads of phospholipids — one ion threaded through half of this topic.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
AQA splits marks across AO1 (knowledge, roughly a third), AO2 (application to unfamiliar contexts, the largest slice) and AO3 (analysis and evaluation, including practical and maths skills). Molecule questions reward precision of language: name the bond (glycosidic, ester, peptide, phosphodiester) and name the reaction (condensation or hydrolysis) and always say a water molecule is released or used. Vague answers such as 'they join together' score nothing.
For any biochemical test, give reagent, procedure and the exact colour change of a positive result — Benedict's blue to brick-red on heating, iodine orange-brown to blue-black, biuret blue to lilac-purple — and remember the extra acid-hydrolysis step for a non-reducing sugar. Enzyme graph questions are answered through one chain every time: temperature or pH changes the bonds holding the tertiary structure, which changes the active site, which changes how complementary it is to the substrate. Distinguish competitive from non-competitive inhibition by whether adding substrate rescues the rate.
At least 10% of A-level Biology marks are maths, at a minimum of Level 2 standard, so practise the calculations here: the n-1 rule for bonds and water, initial rate from a tangent (not a crude total over time), and Chargaff base ratios. Show the formula, substitute, and give the unit. Finally, whenever a question says 'explain how the structure relates to the function', answer in explicit structure-then-consequence pairs — it is the single most common command in this paper.