OCR A-Level Biology: Biological Molecules — Complete Revision Guide (H420)
OCR A-Level Biology: Biological Molecules
Biological molecules is the chemical entry point into OCR A-Level Biology A (H420) and the single most foundational course on the specification. Every later topic — from membrane transport and exchange surfaces through to respiration, photosynthesis, gene expression and homeostatic control — depends on a working command of the four macromolecule classes, the condensation and hydrolysis rules that build them, and the structure-function reasoning that connects molecular geometry to physiological role. Why does the bilayer self-assemble? Why does a single amino-acid substitution change a haemoglobin tetramer's behaviour? Why is cellulose tougher than starch despite both being glucose polymers? Every answer routes back through the molecular grammar laid down here.
Course 3 of 12 on the LearningBro OCR A-Level Biology learning path sets the vocabulary that the rest of the path will speak. It pairs structural biochemistry with the qualitative test toolkit examiners expect candidates to deploy without prompt, and it anchors three Practical Activity Groups. The other courses on the path — including Cell Structure and Microscopy, Nucleic Acids and Enzymes, Biological Membranes, Cell Division and Organisation, Exchange and Transport and Photosynthesis and Respiration — all return to the biochemistry built here. Get the molecules fluent and the rest of H420 slots into place.
Guide Overview
The Biological Molecules course is structured as twelve lessons that move from water through carbohydrates and lipids into protein structure, with inorganic ions and qualitative biochemical tests as the final two anchors.
- Water — Structure, Properties and Biological Importance
- Monosaccharides — α-Glucose, β-Glucose, Ribose and Fructose
- Disaccharides — Maltose, Sucrose, Lactose and Glycosidic Bonds
- Polysaccharides — Starch and Glycogen
- Polysaccharides — Cellulose and Plant Cell Walls
- Lipids — Triglycerides and Ester Bonds
- Phospholipids and Cholesterol
- Amino Acids and Peptide Bonds
- Protein Structure — Primary, Secondary and Tertiary
- Quaternary Structure and Protein Function
- Inorganic Ions and Their Biological Roles
- Biochemical Tests for Biological Molecules
OCR H420 Specification Coverage
This course covers OCR H420 Module 2.1.2 (biological molecules) in full. The specification organises the topic into water, carbohydrates, lipids, proteins, and inorganic ions, with qualitative tests threaded through; each is mapped here to one or more lessons (refer to the official OCR specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| Water structure and properties | OCR H420 Module 2.1.2 | Water — Structure, Properties and Biological Importance |
| Monomers and polymers; condensation and hydrolysis | OCR H420 Module 2.1.2 | Monosaccharides; Disaccharides |
| Carbohydrates — mono, di and polysaccharides | OCR H420 Module 2.1.2 | Monosaccharides; Disaccharides; Polysaccharides (Starch/Glycogen); Polysaccharides (Cellulose) |
| Lipids — triglycerides, phospholipids, cholesterol | OCR H420 Module 2.1.2 | Lipids — Triglycerides; Phospholipids and Cholesterol |
| Amino acids, peptide bonds and protein structure | OCR H420 Module 2.1.2 | Amino Acids and Peptide Bonds; Protein Structure (1°/2°/3°); Quaternary Structure |
| Inorganic ions | OCR H420 Module 2.1.2 | Inorganic Ions and Their Biological Roles |
| Qualitative tests for biological molecules | OCR H420 Module 2.1.2 | Biochemical Tests for Biological Molecules |
Module 2.1.2 is examined across all three H420 papers. Biochemistry is heavy on Paper 1 short-answer items (structure-function explanations, test predictions) and on Paper 3 as part of synoptic items combining biochemistry with cell biology, physiology or genetics. Quantitative biochemistry — Benedict's calibration curves with a colorimeter, Rf values, percentage error in volumetric work — is a reliable Paper 3 fixture.
Water — Structure, Properties and Biological Importance
The water lesson anchors the course in the physical properties of the medium in which all biochemistry happens. Water's polarity, hydrogen bonding, high specific heat capacity, high latent heat of vaporisation, cohesion, adhesion, density anomaly below 4 °C, and excellent solvent behaviour are not curiosities — each maps directly to a physiological role examined later in the path. Cohesion underwrites the transpiration stream in transport in plants. High specific heat capacity buffers cellular temperature against metabolic flux. Latent heat drives evaporative cooling in thermoregulation. Density inversion below 4 °C insulates aquatic life under ice.
A common mark-loss pattern is to treat high specific heat capacity and high latent heat of vaporisation as the same property — they are different consequences of hydrogen bonding and have distinct physiological roles. Another is to describe water as merely "a good solvent" without explaining that its polarity allows it to hydrate ionic species and to form hydrogen bonds with other polar molecules.
Monosaccharides — α-Glucose, β-Glucose, Ribose and Fructose
The monosaccharides lesson develops the structural distinction between alpha and beta glucose — the orientation of the hydroxyl group on carbon 1 — and the consequence: alpha glucose polymerises into starch and glycogen for energy storage, beta glucose polymerises into cellulose for structural rigidity. Ribose and deoxyribose (the pentose sugars in RNA and DNA, revisited in nucleotide structure) and fructose (the sweetener in sucrose) complete the catalogue.
The structure-function principle starts here: monomer geometry dictates polymer architecture, and polymer architecture dictates physiological role. The same reasoning will reappear in protein folding three lessons later. A common pitfall is to describe alpha and beta glucose as different molecules with different formulae — they share the formula C6H12O6 and differ only in the orientation of a single hydroxyl.
Disaccharides — Maltose, Sucrose, Lactose and Glycosidic Bonds
The disaccharides lesson develops condensation as the bond-forming reaction that joins two monosaccharides into a disaccharide with the elimination of water, and hydrolysis as its reverse. The three named disaccharides are maltose (two alpha glucose joined by an alpha-1,4 glycosidic bond), sucrose (alpha glucose plus fructose, the only common non-reducing sugar), and lactose (alpha glucose plus beta galactose, the sugar in mammalian milk). The non-reducing status of sucrose has an exam consequence: a Benedict's test on sucrose returns negative until acid hydrolysis followed by neutralisation cleaves the glycosidic bond and frees the reducing aldehyde of glucose.
Condensation and hydrolysis are the single most transferable reaction pair in the whole specification, and it is worth internalising the pattern once so it applies everywhere. Condensation joins two monomers with the formation of a chemical bond and the release of one water molecule — a hydroxyl group from one monomer and a hydrogen from the other combine to form that water. Hydrolysis is the exact reverse: a water molecule is added across the bond to break it, splitting a polymer back into monomers. The same logic that builds a glycosidic bond between two sugars builds a peptide bond between two amino acids (releasing water), an ester bond between glycerol and a fatty acid (releasing water), and a phosphodiester bond in a nucleic-acid backbone. A single labelled diagram of "two monomers → dimer + H2O" therefore serves for carbohydrates, proteins, lipids and nucleic acids. Examiners exploit this generality: a question that looks like it is about proteins is frequently testing whether you can transfer the condensation/hydrolysis rule you learned on sugars. When you write about digestion of any macromolecule, the operative word is hydrolysis — enzymes such as amylase, maltase, lipase and the peptidases all catalyse hydrolysis reactions that add water across a bond.
Polysaccharides — Starch and Glycogen
The starch and glycogen lesson develops the two animal-and-plant energy storage polysaccharides. Starch is the plant storage form and exists as amylose (long unbranched alpha-1,4-linked chains coiled into a helix) and amylopectin (alpha-1,4-linked chains with alpha-1,6 branch points every 24–30 residues). Glycogen is the animal storage form and is even more highly branched than amylopectin, with branch points every 8–12 residues. Branching matters because each branch terminates in a non-reducing end available for enzymatic hydrolysis by amylase or glycogen phosphorylase; more branches mean faster mobilisation.
A common mark-loss pattern is to describe glycogen as "the plant storage carbohydrate" — it is animal — or to describe amylose as "highly branched" (it is amylopectin and glycogen that are branched). Carbohydrate mobilisation returns in respiration and in blood-glucose regulation covered in neuronal and hormonal communication.
Polysaccharides — Cellulose and Plant Cell Walls
The cellulose lesson develops the structural polysaccharide of the plant cell wall. Cellulose is a long unbranched polymer of beta glucose joined by beta-1,4 glycosidic bonds. Alternating residues are flipped 180° relative to each other so the polymer assumes a straight extended chain rather than a helix, and hydroxyl groups on adjacent chains hydrogen-bond extensively to produce microfibrils with great tensile strength. The cell wall built from these microfibrils resists turgor pressure and gives the plant cell its shape (revisited in plant cell specific structures and in osmosis investigations in biological membranes).
A mark-loss pattern is to describe cellulose as "alpha-1,4 linked" rather than beta-1,4. Another is to forget that human digestive enzymes cannot hydrolyse beta-1,4 bonds — dietary fibre passes through largely undigested for that reason.
Lipids — Triglycerides and Ester Bonds
The triglycerides lesson covers the most energy-dense biological macromolecule. A triglyceride is glycerol esterified to three fatty acids through ester bonds formed by condensation — three condensation reactions releasing three water molecules. The saturated/unsaturated distinction is examined frequently: saturated fatty acids have no carbon–carbon double bonds, so their straight tails pack closely and the fat is solid at room temperature (animal fats); unsaturated fatty acids contain one or more cis double bonds that introduce kinks preventing close packing, lowering the melting point — which is why plant oils are liquid at room temperature.
The reason triglycerides make superior long-term energy stores comes down to two properties, and examiners want both. First, energy density: lipids release roughly twice the energy per gram of carbohydrate or protein because the long hydrocarbon tails are highly reduced (rich in C–H bonds), and it is the oxidation of C–H bonds that yields most of the ATP in respiration — so more C–H bonds per gram means more energy per gram. Second, lipids are insoluble in water, so large quantities can be stored in adipose tissue without affecting the water potential of the cell (unlike an equivalent mass of soluble glucose, which would draw in water osmotically and could not be stockpiled). A neat exam-ready contrast is therefore: carbohydrate (glycogen) is the short-term, rapidly mobilised store; triglyceride is the long-term, high-density store. The energy-density point is revisited in respiratory substrate questions in photosynthesis and respiration, where lipids give a higher respiratory-quotient calculation and more ATP per molecule than glucose.
Phospholipids and Cholesterol
The phospholipids and cholesterol lesson introduces the amphipathic molecules at the heart of biological membranes. A phospholipid is a triglyceride in which one fatty acid is replaced by a phosphate-substituted polar head group, giving a hydrophilic head and two hydrophobic tails. In aqueous environments phospholipids self-assemble into bilayers — the structural premise for the fluid mosaic model covered in the fluid mosaic model lesson. Cholesterol, a steroid lipid, intercalates between phospholipid tails to modulate membrane fluidity, restricting movement at high temperatures and preventing close packing at low temperatures.
Amino Acids and Peptide Bonds
The amino acids lesson develops the monomers of proteins. Every amino acid shares a central carbon bonded to an amine group, a carboxyl group, a hydrogen and a side chain R; the side chain identity (hydrophobic, hydrophilic, charged, special) determines the chemistry the residue can contribute to a folded protein. Twenty proteinogenic amino acids are encoded by the standard genetic code (revisited in the genetic code). A peptide bond is the amide bond formed by condensation between the carboxyl carbon of one amino acid and the nitrogen of the amine group on the next. A common mark-loss pattern is to draw the peptide bond between the wrong atoms.
Protein Structure — Primary, Secondary and Tertiary
The protein structure lesson develops the first three structural levels. Primary structure is the amino acid sequence linked by peptide bonds. Secondary structure is the local regular folding into alpha-helices (a right-handed helix stabilised by hydrogen bonds between every fourth residue) and beta-pleated sheets (extended strands hydrogen-bonded laterally). Tertiary structure is the overall three-dimensional fold of a single polypeptide, stabilised by hydrogen bonds, ionic interactions, hydrophobic interactions and disulfide bridges between cysteine residues.
The structure-function principle developed here is the most-reused idea in the specification. It underwrites enzyme catalysis in enzyme action. It underwrites haemoglobin's oxygen-binding story in exchange and transport. It underwrites antibody-antigen binding in communicable diseases and immunity.
Quaternary Structure and Protein Function
The quaternary structure lesson covers the assembly of multiple polypeptide subunits — and often non-protein prosthetic groups — into a functional protein. Haemoglobin (two alpha and two beta globin subunits, each bound to a haem prosthetic group with a central iron) is the canonical example, and the cooperative oxygen binding that arises from quaternary geometry will be developed in detail in exchange and transport. The lesson also distinguishes globular proteins (roughly spherical, hydrophilic exterior, metabolic and transport roles) from fibrous proteins (elongated, insoluble, structural — collagen, keratin, elastin), and conjugated proteins (with prosthetic groups — haemoglobin, glycoproteins).
Inorganic Ions and Their Biological Roles
The inorganic ions lesson covers the small charged species without which biology does not work. Hydrogen ions set pH, which gates enzyme activity. Phosphate ions appear in ATP (revisited in ATP as energy currency), in the DNA backbone, and in phospholipid heads. Iron ions sit at the haem core of haemoglobin and cytochromes. Sodium drives co-transport of glucose and amino acids in the gut. Calcium is the canonical intracellular second messenger and the cofactor for many enzymes. Each ion thus links to multiple downstream H420 modules.
Biochemical Tests for Biological Molecules
The biochemical tests lesson anchors PAG 9 (Qualitative testing of biological molecules) and threads into PAG 5 (Colorimetry) and PAG 6 (Chromatography). The four core qualitative tests, their reagents, positive results and the molecule each detects are worth committing to a single table:
| Molecule | Test | Positive result |
|---|---|---|
| Reducing sugar | Benedict's reagent, heated in a water bath | Blue → green → yellow → orange → brick-red precipitate |
| Non-reducing sugar | Boil with dilute acid, neutralise, then Benedict's | Negative first time; brick-red after hydrolysis |
| Starch | Iodine in potassium iodide solution | Orange-brown → blue-black |
| Protein (peptide bonds) | Biuret reagent | Pale blue → purple/lilac |
| Lipid | Emulsion test (dissolve in ethanol, add water) | Cloudy white emulsion |
Two of these are examined semi-quantitatively, and this is where the marks sit on Paper 3.
Worked example — a Benedict's / colorimeter calibration curve (PAG 5)
Benedict's is only qualitative on its own — "more red" is not a measurement. To quantify a reducing-sugar concentration you build a calibration curve: make a dilution series of known glucose concentrations, run Benedict's on each under identical conditions (same volume, same temperature, same heating time), then either filter and read the remaining blue colour, or use a colorimeter to record absorbance or percentage transmission. As glucose concentration rises, more blue Cu2+ is reduced to brick-red Cu2O, so the blue absorbance falls. You then plot absorbance against known concentration, read your unknown's absorbance off the axis, and interpolate its concentration from the line.
For example, if the calibration points were 0.0 mol dm−3→1.00 absorbance, 0.2→0.72, 0.4→0.48, 0.6→0.26, 0.8→0.10, and an unknown reads 0.40 absorbance, you interpolate between the 0.4 and 0.6 points to a concentration of about 0.44 mol dm−3. The marks are earned by (i) setting the colorimeter to a red filter — you measure the absorbance of the blue solution, so you select the complementary colour — (ii) calibrating to zero with a water blank, and (iii) reading the unknown from the line, not from the nearest single point. Stating that all tubes must be heated for the same time is a routine "control variable" mark.
Worked example — the Rf value in chromatography (PAG 6)
Chromatography (PAG 6) separates amino acids or pigments by their differential solubility between a mobile phase (the solvent) and a stationary phase (the paper or TLC plate). The quantitative output is the retardation factor:
Rf=distance moved by the solvent frontdistance moved by the spot
If a pigment spot travels 6.0 cm from the origin (pencil) line while the solvent front travels 8.0 cm in the same run, then
Rf=8.06.0=0.75
Rf is always between 0 and 1 (the spot cannot outrun the solvent), has no units, and is characteristic of a compound in a given solvent, so it is used to identify unknowns by comparison with reference values. The consistent mark-loss pattern is measuring to the wrong point of the spot: measure to the centre of the spot, and measure the solvent-front distance in the same lane. Two further routine marks: the origin line must be drawn in pencil (ink would dissolve and run), and the spot must start above the solvent level in the tank (or the sample washes off into the solvent rather than climbing the plate).
Linking to the Other Courses
Biological molecules is the most synoptic foundation on H420. Six sibling courses build on it directly.
Cell Structure and Microscopy houses the organelles inside which all this biochemistry happens; the cellulose cell wall, the phospholipid bilayer, the protein cytoskeleton — each is a direct application of molecular vocabulary built here.
Nucleic Acids and Enzymes takes the protein structure-function principle and applies it to catalysis, then takes the nucleotide chemistry hinted at in the monosaccharide lesson and develops DNA, RNA and the central dogma in full.
Biological Membranes, Cell Division and Organisation reuses phospholipid amphipathy and cholesterol intercalation as the structural premise for the fluid mosaic model.
Exchange and Transport reuses haemoglobin's quaternary structure and protein structure-function as the canonical worked example of cooperative binding.
Photosynthesis and Respiration reuses carbohydrate and lipid metabolism, the ATP nucleotide, and the coenzyme-as-shuttle principle.
Communicable Diseases and Immunity reuses protein-ligand specificity in antibody-antigen binding, and Neuronal and Hormonal Communication reuses it in hormone-receptor signalling and neurotransmitter-receptor interactions.
Required Practicals / PAGs
This course anchors three Practical Activity Groups: PAG 9 (Qualitative testing of biological molecules), PAG 5 (Colorimetry) and PAG 6 (Chromatography).
PAG 9 covers Benedict's (reducing and, after hydrolysis, non-reducing sugars), iodine (starch), biuret (peptide bonds) and the emulsion test (lipids). Quantitative extensions use Benedict's with a colorimeter to construct a calibration ladder against known glucose concentrations and read off unknown concentrations — this is PAG 5. PAG 6 covers chromatographic separation of amino acids or pigments and the calculation of Rf values, with the consistent mark-loss pattern of measuring to the centre of the spot rather than its leading edge. All three PAGs are reliable Paper 3 fixtures and underwrite the quantitative biochemistry of the photosynthesis content.
Common Mark-Loss Patterns
A short consolidated list of the patterns that examiners reliably penalise on Module 2.1.2 items: calling glycogen a plant storage carbohydrate, or describing cellulose as alpha-1,4 linked; drawing a peptide bond between the wrong atoms (peptide bonds form between the carboxyl carbon of one residue and the nitrogen of the amine group on the next); describing competitive enzyme inhibition as "reducing Vmax" rather than as raising the apparent Km; treating water's high specific heat capacity and high latent heat of vaporisation as the same property; failing to specify that Benedict's on sucrose is negative until acid hydrolysis followed by neutralisation cleaves the glycosidic bond; calling ATP "high-energy bonded" rather than describing hydrolysis of the terminal phosphoanhydride bond as the energy release; recording Rf values without measuring to the centre of the spot; confusing reducing and non-reducing sugars on the Benedict's pathway; and drawing amylose as "highly branched" — it is amylopectin and glycogen that are branched.
Exam Technique for Biochemistry Questions
Biochemistry questions on H420 reward a small set of habits. When you are asked to explain how a molecule's structure suits its function, always make the explicit link between a named structural feature and the consequence — "cellulose chains are held by many hydrogen bonds between adjacent chains, forming microfibrils with high tensile strength, which resists turgor pressure and supports the cell" scores every marking point because each feature is chained to a consequence. A bare list of features ("cellulose has hydrogen bonds and microfibrils") does not. When you are asked to compare two molecules (starch vs glycogen, saturated vs unsaturated, globular vs fibrous), use comparative connectives in every sentence ("whereas", "in contrast", "more highly branched than") so the examiner can see both sides of each point of comparison; parallel comparison earns marks that a two-paragraph description of each molecule in isolation does not. On bonding questions, name the bond precisely (glycosidic, peptide, ester, phosphodiester, hydrogen, ionic, disulfide) and say whether it forms by condensation and breaks by hydrolysis. And when a question mentions the effect of heat or pH on a protein, the target word is denaturation — the breaking of hydrogen and ionic bonds that changes the tertiary structure and the shape of the active site or binding site, not the breaking of peptide bonds (the primary structure is unaffected by mild heat).
Mini-FAQ
What is the difference between a reducing and a non-reducing sugar in the exam? A reducing sugar (all monosaccharides, plus maltose and lactose) has a free aldehyde or ketone group that reduces the blue Cu2+ in Benedict's to brick-red Cu2O. Sucrose is non-reducing because both anomeric carbons are locked in the glycosidic bond; it gives a negative Benedict's until you boil it with acid to hydrolyse the bond, then neutralise, then retest — at which point the freed glucose and fructose give a positive result.
Why is water described as "amphoteric" or a "metabolite," not just a solvent? Water is a reactant or product in a huge number of reactions — it is added in every hydrolysis reaction and released in every condensation reaction — so it is a metabolite, not merely the medium. That, plus its role as a solvent, temperature buffer and transport medium, is why full-mark "importance of water" answers go well beyond "it dissolves things."
Do I need to memorise the twenty amino acids or the structures of specific fatty acids? No. You need the general amino-acid structure (central carbon, amine group, carboxyl group, hydrogen, variable R group) and the fact that the R group determines the residue's chemistry; and the general fatty-acid distinction (saturated = no C=C, straight; unsaturated = one or more cis C=C, kinked). Specific names are not required at H420; the transferable structural logic is.
How is competitive enzyme inhibition described correctly? Competitive inhibitors resemble the substrate and bind the active site, so they raise the apparent Km (more substrate is needed to reach half-maximal rate) but leave Vmax unchanged, because at high substrate concentration the substrate outcompetes the inhibitor. Saying a competitive inhibitor "reduces Vmax" is the classic error — that describes non-competitive inhibition. (This is developed fully in Nucleic Acids and Enzymes.)
Closing and Next Steps
Biological molecules is the conceptual backbone of A-Level Biology. The vocabulary built here returns in every subsequent course on the OCR A-Level Biology learning path. The quickest revision win is to draw, from memory, a labelled diagram for each macromolecule class — alpha vs beta glucose; saturated vs unsaturated fatty acid; an amino acid with its four substituents; a peptide bond between two residues; the four levels of protein structure — and rehearse them weekly. Then drill the biochemical tests until the colour outcomes are automatic, because one or two marks per paper are routinely lost confusing biuret and Benedict's. Start at the Biological Molecules course and treat the structure-function principle as a single transferable habit of mind; the rest of the H420 path then becomes a series of consequences, not a list of disconnected facts.
Related Reading
- OCR A-Level Biology: Cell Structure and Microscopy — Complete Revision Guide
- OCR A-Level Biology: Biodiversity, Classification and Evolution — Complete Revision Guide
- Biological Molecules course
- Nucleic Acids and Enzymes course — applies protein structure-function to enzyme catalysis
- Biological Membranes, Cell Division and Organisation course — phospholipid amphipathy and the fluid mosaic model
- Exchange and Transport course — haemoglobin quaternary structure and cooperative binding