Biological Molecules
The four classes of molecules that build life: carbohydrates, lipids, proteins, and the water in which they all function. Food tests with quantitative interpretation; structure-function relationships from monosaccharides up to quaternary protein architectures.
Testing for biological molecules
Four standard biochemical tests identify the principal classes of biological molecules in a sample. Each test follows the same logic: add a reagent, observe a colour change, infer presence. The 9700 syllabus (2.1) requires candidates to describe and carry out each test, plus a semi-quantitative version of Benedict's, plus a special test for non-reducing sugars.
Benedict's test for reducing sugars
Reducing sugars include all monosaccharides (glucose, fructose, galactose) and some disaccharides (maltose, lactose). They contain a free aldehyde or ketone group that can reduce Cu2+ to Cu+.
- Add roughly 2 cm³ sample to a test tube
- Add equal volume of Benedict's solution (alkaline copper(II) sulfate)
- Heat in a boiling water bath for around 5 minutes
- Observe any colour change and any precipitate that forms
- Stays blue — no reducing sugar
- Turns green — trace amount
- Turns yellow — low concentration
- Turns orange — moderate concentration
- Turns brick-red — high concentration
The reducing sugar donates electrons to Cu2+ ions, reducing them to Cu+. Cu+ ions then form an insoluble brick-red precipitate of copper(I) oxide (Cu2O). The more reducing sugar present, the more Cu+ formed, and the further the colour shifts away from the original blue.
Semi-quantitative Benedict's test
The basic test only shows whether a reducing sugar is present. To estimate how much, candidates standardise the test in one of two ways — the syllabus accepts either:
Use known concentrations of glucose to build a calibration: time how long it takes for each to first show colour change away from blue. Higher concentration → faster change. Plot calibration curve. For unknown sample, time the change and read concentration from curve.
Run Benedict's on a series of known concentrations and a sample for the same heating time (often around 5 min). Filter or centrifuge to remove precipitate, then compare filtrate colour to the calibration tubes. The standard whose colour matches gives the estimated concentration.
Saying "Benedict's is quantitative" — it is at best semi-quantitative. The estimate has wide error bars because colour judgement is subjective and reaction kinetics depend on temperature, mixing, and tube wall colour. Reliable quantification requires colorimetry (Topic 3).
Test for non-reducing sugars
Non-reducing sugars (e.g. sucrose) have their reactive groups locked in glycosidic bonds and so do not give a positive Benedict's test directly. To detect them, the bonds must first be broken by acid hydrolysis:
- Add the sample to dilute hydrochloric acid (HCl) and heat in a boiling water bath for a few minutes to hydrolyse the glycosidic bonds.
- Cool, then add an alkali (e.g. sodium hydrogencarbonate, NaHCO3) to neutralise the acid — Benedict's reagent works in alkaline conditions.
- Add Benedict's solution and heat as for the standard test.
- A positive result (colour change to brick-red) confirms a non-reducing sugar was originally present, since hydrolysis liberated reducing monomers.
Iodine test for starch
The reagent is iodine in potassium iodide solution (often called iodine solution; not pure I2). Add a few drops to the sample at room temperature.
| Original colour | Result | Interpretation |
|---|---|---|
| Yellow / orange-brown | No change | Starch absent |
| Yellow / orange-brown | Turns blue-black | Starch present |
The colour change happens because polyiodide ions slot inside the helical amylose structure, forming a coloured complex.
Emulsion test for lipids
Lipids are not water-soluble, so the test relies on dissolving them in ethanol first, then mixing with water to produce a milky emulsion of fine lipid droplets.
- Add ethanol to the sample and shake to dissolve any lipid present.
- Carefully pour the ethanol mixture into a test tube of distilled water.
- If lipid is present, the solution turns cloudy white (an emulsion of micro-droplets of lipid suspended in water).
- If no lipid is present, the solution remains clear.
Biuret test for proteins
The biuret reagent contains copper(II) sulfate in alkaline solution (often used as a single combined reagent). Cu2+ ions form a coloured complex with peptide bonds.
| Initial colour | Final colour | Interpretation |
|---|---|---|
| Pale blue | Stays pale blue | No protein |
| Pale blue | Turns purple / violet / lilac | Protein present |
The biuret colour change is produced by the peptide bonds themselves — not the amino acid side chains. So the test is positive for any peptide with at least two peptide bonds. A single amino acid in solution gives a negative result.
Quick reference: all four food tests
| Test | Reagent | Method note | Positive result |
|---|---|---|---|
| Reducing sugars | Benedict's solution | Heat in boiling water bath | Blue → brick-red precipitate |
| Non-reducing sugars | HCl, NaHCO3, Benedict's | Hydrolyse, neutralise, then Benedict's | Brick-red after second Benedict's |
| Starch | Iodine in KI solution | Drop directly onto sample at RT | Yellow/orange → blue-black |
| Lipids | Ethanol, then water | Dissolve in ethanol, pour into water | Clear → cloudy white emulsion |
| Proteins | Biuret reagent | Add at room temperature | Pale blue → purple/violet |
A student tests an unknown solution and obtains: a brick-red precipitate with Benedict's; no colour change with iodine; a cloudy white emulsion in the lipid test; a purple colour with biuret. Which classes of biological molecules are present?
- A. Reducing sugar, starch and protein only
- B. Reducing sugar, lipid and starch only
- C. Reducing sugar, lipid and protein only
- D. All four classes
A student is given a solution of an unknown sugar. Initial Benedict's test gives no colour change. The student then heats some of the solution with dilute HCl, neutralises with sodium hydrogencarbonate, and repeats Benedict's. This time it turns brick-red.
(a) Identify the type of sugar and explain how the result is consistent. [3]
(b) Suggest one further step the student could take to estimate the original concentration. [2]
(a) Type of sugar and explanation [3 marks]
The sugar is a non-reducing sugar (e.g. sucrose) [1].
Non-reducing sugars do not have a free aldehyde or ketone group available to reduce Cu2+, so the first Benedict's test is negative [1].
Boiling with dilute HCl hydrolyses the glycosidic bond, breaking the disaccharide into monosaccharides; these monosaccharides are reducing sugars and now give a positive Benedict's result after neutralisation [1].
(b) Estimating concentration [2 marks]
Run Benedict's on a series of known concentrations of the corresponding monosaccharide for the same heating time, then compare the colour of the filtrate (or precipitate) of the unknown to the standards [1]. The matching standard gives the estimated concentration of monosaccharide produced; halve this for sucrose since each sucrose hydrolysis liberates two reducing monomers [1].
- "Non-reducing sugar" alone is 1 mark; explanation of glycosidic bond hydrolysis is required for full credit
- For (b), accepting either calibration method (time to first change, OR colour comparison)
- The "halving" point is bonus — not always required for full marks
Carbohydrates and lipids
All carbohydrates fit the general formula (CH2O)n. Lipids are a more diverse class but share a defining property: they are insoluble in water. Both classes provide energy storage; carbohydrates are the dominant short-term store, lipids the dominant long-term store.
Glucose: α and β ring forms
Glucose is the most important monosaccharide in the syllabus. In aqueous solution most glucose exists as a six-membered ring rather than the open-chain form. The ring closes when the C1 carbonyl attacks the C5 hydroxyl. Depending on which face the C1 hydroxyl ends up on, glucose forms one of two stereoisomers:
Six-membered ring with the OH group on carbon-1 (the anomeric carbon) pointing downwards (axial, opposite face to the CH2OH on C5). Polymerises with α1–4 and α1–6 glycosidic bonds to form starch and glycogen.
Same six-membered ring but with the C1 OH group pointing upwards (axial, same face as the CH2OH on C5). Polymerises with β1–4 glycosidic bonds, with every alternate β-glucose flipped 180°, to form cellulose.
This single positional difference at C1 is the difference between starch (digestible energy reserve) and cellulose (indigestible structural fibre) for nearly all animals. Mammals have enzymes (amylase, maltase) that hydrolyse α-bonds, but no enzyme that hydrolyses the β1–4 bonds of cellulose — so cellulose passes through the gut as dietary fibre.
Monomers, polymers, macromolecules
| Term | Definition | Example |
|---|---|---|
| Monomer | A small repeating unit that can be joined to others by covalent bonds to form a polymer | Glucose, amino acid, nucleotide |
| Polymer | A long molecule made from many monomers covalently joined | Starch, protein, DNA |
| Macromolecule | Any very large molecule (with high relative molecular mass), commonly a polymer but not always (e.g. lipids) | Polysaccharides, proteins, nucleic acids, also large lipids |
| Monosaccharide | Single-sugar monomer of carbohydrates | Glucose, fructose, galactose, ribose |
| Disaccharide | Two monosaccharides joined by a glycosidic bond | Maltose, sucrose, lactose |
| Polysaccharide | Many monosaccharides joined by glycosidic bonds (a carbohydrate polymer) | Starch, glycogen, cellulose |
Condensation: two monomers join to form a covalent bond, releasing one molecule of water. This is how all biological polymers are built.
Hydrolysis: a covalent bond between monomers is broken by adding one molecule of water. This is how polymers are digested.
For carbohydrates, the bond formed is a glycosidic bond; for proteins, a peptide bond; for nucleic acids, a phosphodiester bond.
Disaccharides
| Disaccharide | Built from | Bond | Reducing? | Where found |
|---|---|---|---|---|
| Maltose | α-glucose + α-glucose | α1–4 | Yes | Germinating seeds; starch digestion |
| Sucrose | α-glucose + fructose | α1–2 | No | Plant transport sugar (phloem); table sugar |
| Lactose | β-galactose + α-glucose | β1–4 | Yes | Mammalian milk |
Sucrose is the only common non-reducing disaccharide because both anomeric carbons (C1 of glucose, C2 of fructose) are locked in the glycosidic bond, leaving no free reducing group.
Starch and glycogen
Both are storage polysaccharides, both made entirely of α-glucose, both broken down to release glucose for respiration. The structural details determine where they are stored and how rapidly they release glucose.
Amylose: linear chain of α-glucose joined by α1–4 bonds. Coils into a compact helix — ideal for high-density storage.
Amylopectin: α1–4 backbone with α1–6 branches every 20–30 glucose units. The branched structure exposes many free ends, allowing rapid simultaneous enzymic hydrolysis when energy is needed.
Same chemistry as amylopectin (α1–4 backbone with α1–6 branches) but more highly branched — branches every 8–12 glucose units. The denser branching gives even faster glucose release for the high metabolic rates of muscle and liver tissue.
Storage polysaccharides share four favourable properties for storing glucose: (1) compact — helical or branched coiling occupies little space; (2) insoluble — large molecules don't disturb cellular water potential; (3) not reducing — chemically unreactive in storage; (4) readily hydrolysed — many free ends allow rapid glucose release on demand.
Cellulose
Cellulose is a structural polysaccharide, found in plant cell walls. Despite being made of glucose like starch, its structure could hardly be more different.
Cellulose is made of β-glucose joined by β1–4 glycosidic bonds. To form the bond between two adjacent β-glucose units, every alternate molecule must be rotated 180°, so that the OH groups can come into alignment.
The alternating geometry produces a straight, unbranched chain rather than a helix. Many such chains can lie alongside each other in parallel.
Hydroxyl groups on adjacent parallel chains form many hydrogen bonds. Although each H-bond is weak, the cumulative effect is enormous; bundles of chains form microfibrils, which themselves bundle into macrofibrils, then fibres — remarkably high tensile strength.
Cellulose fibres run in alternating directions in successive layers of the cell wall, providing tensile strength against turgor pressure (which can exceed 1 MPa). The wall prevents the cell from bursting and dictates plant cell shape.
"Glucose monomers" is correct for both starch and cellulose, but the type of glucose differs — α for starch/glycogen, β for cellulose. Examiners often make this distinction the difference between 1 mark and 2.
Triglycerides
The simplest and most abundant lipid is the triglyceride, formed from one molecule of glycerol and three fatty acids.
Each fatty acid -COOH group reacts with one of glycerol's -OH groups in a condensation reaction. Three condensations → three ester bonds → three water molecules released. The product is a triglyceride.
| Feature | Saturated fatty acid | Unsaturated fatty acid |
|---|---|---|
| C–C bonds in tail | All single (no C=C) | One or more C=C double bonds |
| Tail shape | Straight, packs tightly | Kinked at each C=C, packs loosely |
| State at room temperature | Solid — "fat" (e.g. butter, lard) | Liquid — "oil" (e.g. olive oil) |
| Source | Predominantly animal | Predominantly plant and fish |
| Health note | High intake associated with raised LDL cholesterol | Generally regarded as healthier dietary fats |
Triglycerides yield approximately twice as much ATP per gram on respiration as carbohydrates because their hydrocarbon tails are highly reduced and contain little oxygen relative to carbon. They are also insoluble in water, so they can be stored in dense, non-osmotic adipose deposits without disturbing cell water balance — ideal for long-term storage.
Phospholipids and the bilayer
A phospholipid is structurally similar to a triglyceride, except that one of the three fatty acids is replaced by a phosphate group. This single substitution transforms the molecule from a fat into the building block of every cell membrane.
The phosphate group is charged and polar, carrying a partial negative charge. The two fatty acid tails are non-polar hydrocarbon chains.
The phosphate head is hydrophilic ("water-loving") — it interacts favourably with water and ions through hydrogen bonds.
The fatty acid tails are hydrophobic ("water-fearing") — they cannot interact with water and aggregate together to minimise contact with it.
In water, phospholipids spontaneously arrange themselves with their hydrophobic tails buried inwards (away from water) and their hydrophilic heads facing the aqueous environment outside and inside the cell. The result is a continuous double layer — the phospholipid bilayer.
The bilayer creates a hydrophobic core that blocks the movement of charged or polar substances. This selective barrier is the basis of every cell membrane and is developed further in Topic 4 (Fluid mosaic model).
Which structural feature is shared by both starch and cellulose?
- A. Branched chains of α-glucose
- B. Helical coils stabilised by hydrogen bonds within a single chain
- C. A polymer of glucose joined by glycosidic bonds, formed by condensation
- D. β1–4 glycosidic bonds and parallel chains held by inter-chain hydrogen bonds
Explain how the structure of a phospholipid is related to its role in cell membranes. [6]
Six creditable points (any six):
- A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails [1]
- This makes the molecule amphipathic / has both polar and non-polar regions [1]
- In water, phospholipids spontaneously form a bilayer with hydrophobic tails facing inward, hydrophilic heads facing outward [1]
- The hydrophobic core blocks the passage of large polar molecules and ions, providing selective permeability [1]
- Small non-polar molecules (e.g. O2, CO2, steroid hormones) can dissolve in and pass through the hydrophobic core [1]
- The bilayer is fluid (not crystalline) because individual phospholipids can move laterally; this allows membrane proteins to diffuse and the membrane to repair itself [1]
- The hydrophilic outer surfaces interact with water on either side of the membrane and provide attachment sites for peripheral proteins [1]
Mark scheme guidance: Both halves of a structure-function pair must appear — just stating "amphipathic" without explaining the consequence loses the linking mark. Six independent points; max 6.
Proteins
Proteins are the most functionally diverse class of biomolecule: enzymes, antibodies, transport proteins, receptors, channels, structural fibres, hormones, contractile filaments. The full range of behaviours arises from a single design principle — specific 3D folding determined by amino acid sequence.
Amino acids and peptide bonds
An amino acid has four groups attached to a central carbon (the α-carbon):
- An amino group (−NH2)
- A carboxyl group (−COOH)
- A hydrogen atom (−H)
- A variable R group (the side chain), which determines the chemical character of each amino acid
There are 20 different R groups in proteinogenic amino acids. R groups can be charged, polar, non-polar, large, small, aromatic, sulfur-containing, etc. The diversity of R groups is the basis for the structural diversity of proteins.
A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of an adjacent amino acid in a condensation reaction — releasing a water molecule. The resulting C–N bond is the peptide bond.
Two amino acids joined = a dipeptide. Three or more = a polypeptide. A protein is a polypeptide (or assembly of polypeptides) folded into a defined three-dimensional shape.
Levels of protein structure
The linear order of amino acids in the polypeptide chain, joined by peptide bonds. Determined by the gene encoding the protein. The primary structure ultimately determines all higher levels of folding because the R groups dictate which interactions are possible.
Local regions of the polypeptide adopt regular shapes held together by hydrogen bonds between the C=O of one peptide bond and the N–H of another peptide bond a few residues away. The two main forms are the α-helix (a coil) and the β-pleated sheet (parallel or anti-parallel strands forming a sheet).
The entire polypeptide folds into a specific three-dimensional shape, stabilised by interactions between R groups. This 3D shape is what determines the protein's function (active site for enzymes, binding pockets for receptors, surface hydrophobicity for membrane proteins).
Some proteins consist of two or more polypeptide chains (subunits) held together by the same kinds of interactions as tertiary structure. Quaternary structure does not exist for single-chain proteins. Classic example: haemoglobin (4 subunits).
Bonds stabilising tertiary and quaternary structure
| Bond | Forms between | Strength | Notes |
|---|---|---|---|
| Hydrogen bonds | Polar R groups (e.g. −OH, −NH2) on different amino acids | Weak individually | Numerous; collectively very stabilising. Disrupted by heat or extreme pH. |
| Ionic bonds | Oppositely charged R groups (e.g. −NH3+ and −COO−) | Moderate | Disrupted by changes in pH that alter R-group charges. |
| Disulfide bridges | Sulfur atoms of two cysteine residues | Strong covalent | Common in extracellular proteins (e.g. insulin, antibodies). Disrupted by reducing agents. |
| Hydrophobic interactions | Non-polar R groups clustering together away from water | Variable | Major driving force for folding; non-polar R groups end up buried in the protein interior. |
"Hydrogen bonds in secondary structure" and "hydrogen bonds in tertiary structure" sound the same, but they form between different parts of the molecule:
Secondary: H-bonds between C=O and N–H of peptide bonds (the polypeptide backbone)
Tertiary: H-bonds between R groups (the side chains)
Same chemistry, different participants. Mention which one in exam answers.
Globular proteins: haemoglobin
Globular proteins are roughly spherical, with hydrophobic R groups buried in the interior and hydrophilic R groups on the surface. They are soluble in water and typically have specific physiological functions (transport, catalysis, signalling).
Haemoglobin consists of two α-globin and two β-globin polypeptide subunits, held together by hydrophobic interactions and hydrogen bonds. Each subunit folds into a similar tertiary structure.
Each subunit contains one haem group — a non-protein component (a prosthetic group) consisting of a porphyrin ring with an iron(II) ion (Fe2+) at its centre. The Fe2+ reversibly binds one O2 molecule.
One haemoglobin molecule can carry up to four O2 molecules. Binding is cooperative: the first O2 binding promotes binding of subsequent O2 by altering the protein's shape (developed in Topic 8, Transport in mammals).
Fibrous proteins: collagen
Fibrous proteins are long, narrow, and structurally repetitive. Their R groups are largely non-polar, making them insoluble in water. They typically have structural roles — tendons, ligaments, hair, nails, blood vessel walls.
A collagen molecule consists of three polypeptide chains wound around each other in a tight triple helix. Within each chain, every third amino acid is glycine — the smallest R group — which allows the three chains to pack together closely.
The three chains are held together by hydrogen bonds. Adjacent collagen molecules in a fibril are joined end-to-end and side-to-side by covalent cross-links between specific amino acid R groups, giving collagen its characteristic high tensile strength.
Collagen fibrils assemble into fibres of remarkable tensile strength. It is the most abundant protein in mammals, providing structure to bones, cartilage, tendons, skin, and the walls of blood vessels.
| Feature | Globular (haemoglobin) | Fibrous (collagen) |
|---|---|---|
| Overall shape | Roughly spherical | Long and narrow |
| Sequence | Variable, irregular | Highly repetitive |
| Solubility in water | Soluble | Insoluble |
| Tertiary structure | Specific 3D fold per molecule | Repetitive secondary structure (triple helix here) |
| Typical role | Physiological (catalysis, transport) | Structural (support, strength) |
| Examples | Haemoglobin, enzymes, antibodies, insulin | Collagen, keratin, elastin, silk fibroin |
Which level of protein structure is determined directly by the sequence of bases in the gene encoding the protein?
- A. Primary structure
- B. Secondary structure
- C. Tertiary structure
- D. Quaternary structure
Compare the structures of haemoglobin and collagen. Give THREE differences and ONE similarity. [4 + 2]
Three differences (1 mark each)
- Haemoglobin is globular / roughly spherical; collagen is fibrous / long and narrow
- Haemoglobin is soluble in water; collagen is insoluble
- Haemoglobin contains a haem prosthetic group with Fe2+; collagen does not contain a prosthetic group
- Haemoglobin has 4 subunits (2α + 2β) of unique sequence; collagen has 3 chains of repetitive sequence
- Haemoglobin has a varied amino acid sequence; collagen has every third residue glycine
- Haemoglobin's quaternary structure is held by hydrogen bonds and hydrophobic interactions; collagen has additional covalent cross-links between fibrils
Similarity (any one for 1 mark; max 2 from list)
- Both are made of polypeptides built from amino acids joined by peptide bonds
- Both have a quaternary structure (more than one polypeptide chain)
- Both have specific 3D shape determined by primary structure
- Both are stabilised by hydrogen bonds (among other interactions)
Mark scheme guidance: Each difference point must explicitly contrast both proteins ("haemoglobin is X whereas collagen is Y"). Bare statements about one protein without the comparison typically score half.
Water
Water makes up roughly 70% of most cells. Three properties — solvent action, high specific heat capacity, high latent heat of vaporisation — underlie water's central role in life. All three trace back to one molecular feature: hydrogen bonding.
Hydrogen bonding
The water molecule has a bent shape, with two H atoms covalently bonded to one O atom. Oxygen is more electronegative than hydrogen, so the shared electrons spend more time near the O than the H. This produces a permanent dipole:
- The O atom carries a partial negative charge (δ−)
- Each H atom carries a partial positive charge (δ+)
The partial positive H of one water molecule is electrostatically attracted to the partial negative O of an adjacent water molecule. This attraction is a hydrogen bond. Each water molecule can form up to four hydrogen bonds at any moment — two via its hydrogens, two via the lone pairs on its oxygen.
A hydrogen bond is much weaker than a covalent bond but much stronger than typical van der Waals forces. Liquid water is best pictured as a continuously rearranging network of hydrogen bonds: each bond lasts only picoseconds before breaking and reforming with a different neighbour. Despite this, at any instant, almost every water molecule is hydrogen-bonded to several others. This network is the source of nearly every special property of water.
Solvent properties
Water is sometimes called the "universal solvent" — an exaggeration, but a useful one. It dissolves polar and ionic substances exceptionally well, but does not dissolve non-polar substances at all (which is exactly what allows phospholipid bilayers to exist).
When a salt such as NaCl dissolves, the δ+ hydrogens orient toward Cl− ions and the δ− oxygens orient toward Na+ ions, forming a hydration shell around each ion. This screens the ions from each other and lets them disperse through the solvent.
Polar molecules with −OH or −NH2 groups (sugars, alcohols, amino acid R groups) form hydrogen bonds with water molecules and so dissolve readily.
Water is the medium in which all metabolic reactions occur. Substrates and enzymes can move freely; ions and gases can be transported in dissolved form. Blood plasma, cytoplasm, xylem sap, and phloem sap all rely on water as the dissolving medium.
Thermal properties
To raise water's temperature, the kinetic energy of its molecules must increase — but a large fraction of the supplied energy first goes into breaking hydrogen bonds rather than increasing molecular speed. As a result, water absorbs a lot of energy for relatively small temperature change.
Biological consequence: Cells, blood plasma and large bodies of water resist sudden temperature swings, providing a stable thermal environment for enzymes and other temperature-sensitive processes.
For water to evaporate (transition from liquid to gas), the entire hydrogen-bond network around each evaporating molecule must be broken. This requires a large input of energy — energy that is removed from the remaining liquid as the molecule departs.
Biological consequence: Evaporation has a powerful cooling effect. Animals exploit this through sweating (humans), panting (dogs, birds), and licking (cats). Plants exploit it through transpiration, which also drives water transport up the xylem (Topic 7).
Which property of water is most directly responsible for its action as a coolant when sweat evaporates from skin?
- A. Hydrogen bonds make water polar
- B. High specific heat capacity
- C. High latent heat of vaporisation
- D. Water is a good solvent for polar molecules
Explain how hydrogen bonding accounts for the role of water as a transport medium in living organisms. [5]
Five creditable points (any five):
- Water molecules are polar / have a permanent dipole because oxygen is more electronegative than hydrogen [1]
- The partial charges allow hydrogen bonds to form between water molecules [1]
- Polar and ionic solutes interact with the partial charges, forming hydration shells / hydrogen bonds with water, so they dissolve [1]
- This makes water an effective solvent for transporting ions, sugars, amino acids and other polar metabolites [1]
- Liquid water is cohesive (water molecules stick to other water molecules through hydrogen bonds), allowing it to be drawn up xylem as a continuous column [1]
- Water also has adhesion to polar surfaces (e.g. xylem walls), which contributes to its movement in narrow tubes / capillary action [1]
- High specific heat capacity helps maintain a stable temperature in the transport medium (blood, sap), protecting dissolved proteins from denaturation [1]
Mark scheme guidance: The question asks specifically about transport, so the answer must connect hydrogen bonding to a transport-related consequence (solvent action, cohesion in xylem, blood as a medium). General "water is good" statements without linking to transport score zero.
Topic 2 Practice — Comprehensive
Mixed practice spanning all four sub-sections in 9700 P1/P2 style. Try each before revealing the answer.
Which combination of features describes glycogen?
- A. β-glucose monomers; straight chains; insoluble; structural
- B. α-glucose monomers; helical; unbranched; storage in plants
- C. α-glucose monomers; highly branched; storage in animals
- D. β-glucose monomers; branched; storage in animals
Which type of bond would be specifically disrupted if a protein were treated with a reducing agent that breaks −S−S− linkages?
- A. Peptide bonds
- B. Hydrogen bonds in α-helices
- C. Ionic bonds between R groups
- D. Disulfide bridges between cysteine residues
Phospholipids form bilayers in water but triglycerides do not. The most direct reason is that
- A. phospholipids have ester bonds while triglycerides do not
- B. phospholipids are amphipathic, having a hydrophilic head and hydrophobic tails, while triglycerides are entirely hydrophobic
- C. phospholipids contain saturated fatty acids and triglycerides do not
- D. phospholipid molecules are smaller than triglyceride molecules
A student investigates an unknown food sample suspected to contain a mixture of biological molecules.
(a) Outline a sequence of tests the student could perform to identify which of: starch, reducing sugar, non-reducing sugar, lipid, protein are present. State the result that would confirm each. [6]
(b) Explain why a positive Benedict's test does not, by itself, distinguish between glucose and lactose. [2]
(a) Test sequence [6 marks]
- Starch: Add iodine in KI solution at room temperature. Blue-black colour confirms starch [1]
- Reducing sugar: Add Benedict's reagent and heat in boiling water bath. Brick-red precipitate confirms reducing sugar [1]
- Non-reducing sugar: If first Benedict's was negative, hydrolyse a fresh sample with HCl, neutralise with NaHCO3, then repeat Benedict's. A brick-red colour now confirms non-reducing sugar [1]
- Lipid: Dissolve sample in ethanol, then pour into water. Cloudy white emulsion confirms lipid [1]
- Protein: Add biuret reagent at room temperature. Purple/violet colour confirms protein [1]
- Order matters: do iodine and Benedict's first because they are simplest; lipid test uses ethanol which can disrupt other tests, so it is run last [1]
(b) Why Benedict's cannot distinguish glucose from lactose [2 marks]
Both glucose and lactose are reducing sugars; both will give a positive Benedict's test by reducing Cu2+ to form a brick-red precipitate of Cu2O [1]. The Benedict's reagent reacts with the free reducing group, which is present in both molecules — the test does not differentiate between specific sugars [1].
Could be extended: Differentiating these would require chromatography or an enzyme-based assay (e.g. glucose oxidase test strip, which is specific for glucose).
Both starch and haemoglobin are made by condensation of monomer units, yet they have very different functions in cells. Compare the structures and functions of these two molecules. [6]
Six creditable points covering structure-function differences:
- Starch is a polymer of α-glucose; haemoglobin is a polymer of amino acids [1]
- Starch is held together by glycosidic bonds; haemoglobin is held together by peptide bonds (and higher-order interactions) [1]
- Starch is unbranched (amylose) or branched (amylopectin), forming compact storage molecules; haemoglobin folds into a specific globular tertiary and quaternary structure [1]
- Starch lacks a defined 3D shape with binding sites; haemoglobin has four specific haem-binding pockets, each with one Fe2+ ion [1]
- Starch's function is energy storage (insoluble in water; rapidly hydrolysed when needed); haemoglobin's function is oxygen transport (soluble in plasma; reversibly binds O2) [1]
- Starch is broken down by hydrolysis to release glucose for respiration; haemoglobin is not consumed but reused, binding O2 in the lungs and releasing it in respiring tissues [1]
Topic 2 — Common Mistakes
- 🧪Saying Benedict's is "quantitative"It is at best semi-quantitative. The standard test only confirms presence; calibration with known concentrations is required for an estimate, and that estimate has wide error bars.
- 🧬Confusing α-glucose and β-glucoseThe OH on C1 in α-glucose points down; in β-glucose it points up. This single difference dictates whether glucose polymerises into starch (digestible storage) or cellulose (indigestible structural fibre).
- 🍃Calling sucrose a reducing sugarSucrose is the only common non-reducing disaccharide because its glycosidic bond locks both anomeric carbons (C1 of glucose, C2 of fructose). Maltose and lactose are reducing.
- 🫀Treating starch and cellulose as "the same family"Both are glucose polymers, but starch has α1–4 (and α1–6) bonds and is for storage; cellulose has β1–4 bonds and is structural. Different bonds → different shape → different enzymes needed → different functions.
- 🧣Mixing up bond types in different protein levelsSecondary structure: H-bonds between peptide bond C=O and N–H (the backbone). Tertiary structure: H-bonds, ionic, disulfide, hydrophobic interactions between R groups. Same chemistry, different participants — specify which in answers.
- 🥈Saying "haemoglobin is an enzyme"Haemoglobin is a transport protein, not an enzyme. It binds and releases O2 reversibly without catalysing a reaction. Confusion arises because both are globular proteins.
- 🧘Saying triglycerides "have a phosphate head"Triglycerides have three fatty acids esterified to glycerol — no phosphate. Replacing one fatty acid with a phosphate group gives a phospholipid. The presence/absence of phosphate is the structural difference between fats and membrane lipids.
- 💧Confusing specific heat capacity with latent heat of vaporisationSpecific heat capacity = energy needed to warm the liquid; explains thermal stability. Latent heat of vaporisation = energy needed to evaporate the liquid; explains cooling by sweat. Use the right one for the right scenario.
- 🧶Forgetting that the biuret test detects peptide bonds, not amino acidsA free amino acid in solution gives a negative biuret result. The reagent forms a coloured complex specifically with the C–N peptide bond, so a positive test confirms a polypeptide.
- 🍽Saying the lipid emulsion test "produces a milky cloud because lipids dissolve in water"Reverse: lipids are insoluble in water. The cloudiness is an emulsion of fine lipid droplets dispersed in water, not a true solution. Saying "dissolves" loses the mark.
Topic 2 is the heaviest content load of all AS topics and underpins Topics 3 (enzymes), 4 (membranes), 6 (nucleic acids), 8 (haemoglobin in transport), and 11 (antibodies). Highest-yield items: the four food tests with their procedural details, the α/β glucose distinction, the structure-function relationships of starch/glycogen/cellulose, the four levels of protein structure with their specific bonds, and the role of hydrogen bonding in water's properties.