Monomers and polymers: carbohydrates, fats and proteins, what each does and how to test for it
By the end of this lesson you will be able to
- Define monomer and polymer, and describe condensation and hydrolysis, calculating the water released or used.
- Outline the structure of carbohydrates, fats and proteins and explain how the structure of each suits its functions in living organisms.
- Compare starch, glycogen and cellulose, and saturated with unsaturated fats.
- Describe the four food tests, with reagent, method and the negative and positive results, and interpret a set of results.
- Use a calibration curve from a colorimeter to turn a colour into a concentration.
You met this before
- Food contains carbohydrates, fats and proteins as large molecules; digestion breaks them into small soluble molecules — glucose, fatty acids and glycerol, amino acids — that can pass into the blood (Year 3, Body systems).
- Enzymes are proteins with an active site whose shape fits one substrate; heat and extreme pH change that shape and denature the enzyme (Year 3; Biology Unit 3 of this course).
- Aerobic respiration: glucose + oxygen → carbon dioxide + water, releasing energy that cells use; its equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (Year 3, Photosynthesis and respiration).
- Atoms in molecules are held by covalent bonds; a molecular formula counts the atoms: glucose is C₆H₁₂O₆, water H₂O. Relative atomic masses used here: C 12, H 1, O 16, N 14.
- Iodine solution turns blue-black when starch is present and stays orange-brown when it is absent (Year 3, digestion practical).
Name the small molecules that starch, proteins and fats are broken into by digestion, and state which of them is the main fuel for respiration in most cells.
Reveal
Starch → glucose (via maltose). Proteins → amino acids. Fats → fatty acids and glycerol. Glucose is the main respiratory substrate in most cells; the brain relies on it almost entirely under normal conditions, while heart and resting skeletal muscle also respire fatty acids.
Revise it in full: Year 3 · Body systems · Lesson 1 — Digestion: from a plate of pap to a molecule in the blood · Year 3 · Photosynthesis and respiration · Lesson 5 — Respiration: releasing the energy · Year 3 · Atomic structure and bonding · Lesson 4 — Covalent bonding
The puzzle
A lean adult carries about 0.5 kg of glycogen — the animal store of glucose — in the liver and muscles. The same person carries 10–15 kg of fat.
Glycogen is a perfectly good fuel: it releases energy fast, it can be used without oxygen, and it is made of glucose, the brain’s favourite molecule. So why is almost all of your stored energy fat?
Here is a clue. If the energy in 12 kg of fat were stored as glycogen instead, you would need to carry more than 100 kg of extra body mass. Somewhere in the structure of the two molecules is the reason — and the same kind of reasoning explains why a plant stores starch in a seed, why a camel’s hump is fat, and why your hair is protein.
One gram of fat releases about 37 kJ when respired; one gram of glycogen about 17 kJ. Glycogen also binds about 3 g of water for every gram stored, while fat is stored almost dry. Predict how many kilojoules one gram of stored glycogen (with its water) provides, and explain the 100 kg in the puzzle.
Compare your answer
Mass stored per gram of glycogen = 1 g + 3 g
Mass stored per gram of glycogen = 4 g
Energy per gram stored = 17 kJ ÷ 4 g
Energy per gram stored = 4.25 kJ g⁻¹
Energy in 12 kg of fat = 12 000 g × 37 kJ g⁻¹
Energy in 12 kg of fat = 444 000 kJ
Mass of stored glycogen needed = 444 000 kJ ÷ 4.25 kJ g⁻¹
Mass of stored glycogen needed = 104 000 g, about 104 kg
Fat stores almost nine times more energy per gram of body mass than hydrated glycogen. An animal that must carry its fuel — to walk, fly or survive a winter — stores it as fat. Glycogen is kept for what fat cannot do: release glucose within seconds.
The science — structured notes
1.1 Monomers, polymers, and the two reactions that link them
Living things are built from a small number of large molecules called macromolecules. Three of them — polysaccharides, proteins and nucleic acids — are polymers: long chains made by joining many small, similar units called monomers. Fats are large molecules too, but they are not polymers: one fat molecule is built from four smaller ones, not from a repeating chain.
The joining reaction is a condensation: two monomers bond, and a molecule of water is removed from between them. The reverse, splitting a bond by adding water, is hydrolysis (“water-splitting”). In your gut, digestive enzymes catalyse hydrolysis; inside your cells, other enzymes catalyse condensation to build new starch, glycogen and proteins.
| Macromolecule | Monomer (building block) | Bond formed by condensation | Elements |
|---|---|---|---|
| Polysaccharide (starch, glycogen, cellulose) | Monosaccharide, e.g. glucose | Glycosidic bond | C, H, O |
| Fat (triglyceride) — not a polymer | 1 glycerol + 3 fatty acids | Ester bond (three per molecule) | C, H, O (much less O) |
| Protein (polypeptide) | Amino acid (20 kinds) | Peptide bond | C, H, O, N (often S) |
| Nucleic acid (DNA, RNA) | Nucleotide | Phosphodiester bond | C, H, O, N, P |
A chain of n monomers has n − 1 bonds, so its formation releases n − 1 water molecules, and its complete hydrolysis uses n − 1 water molecules. That simple count lets you calculate masses — the kind of step the eAssessment rewards when it is shown line by line.
Glucose, C₆H₁₂O₆, has a relative molecular mass of 180; water, 18. (1) Find the relative molecular mass of maltose, made from two glucose molecules. (2) Find the relative molecular mass of a starch chain of 1 000 glucose units.
(1) Maltose = 2 glucose − 1 water
M(maltose) = (2 × 180) − 18
M(maltose) = 360 − 18
M(maltose) = 342
Check with the formula C₁₂H₂₂O₁₁: (12 × 12) + (22 × 1) + (11 × 16) = 342 ✓
(2) 1 000 glucose units are joined by 999 bonds, releasing 999 water molecules
M(chain) = (1 000 × 180) − (999 × 18)
M(chain) = 180 000 − 17 982
M(chain) = 162 018
For a long chain each glucose contributes 180 − 18 = 162, so starch is written (C₆H₁₀O₅)ₙ. Notice that the answer is a relative mass: no unit.
“Hydrolysis of a protein of 150 amino acids needs 150 water molecules.”
Reveal answer
False. A chain of 150 amino acids has 149 peptide bonds, and each bond needs one water molecule to break it, so complete hydrolysis uses 149 water molecules. The count of bonds is always one less than the count of monomers in a straight chain.
1.2 Carbohydrates and their functions
Carbohydrates contain carbon, hydrogen and oxygen, usually with twice as many hydrogen atoms as oxygen atoms, as in water. They come in three sizes. Monosaccharides are single sugar units: glucose, fructose (fruit sugar) and galactose, all C₆H₁₂O₆, and the five-carbon ribose and deoxyribose of RNA and DNA.
Disaccharides are two units joined: maltose (glucose + glucose), sucrose (glucose + fructose, table sugar) and lactose (glucose + galactose, milk sugar). Polysaccharides are chains of hundreds to many thousands of units. Sugars are sweet and dissolve in water; polysaccharides are not sweet and mostly do not dissolve.
| Polysaccharide | Where found | Structure | Function — and why the structure suits it |
|---|---|---|---|
| Starch (amylose + amylopectin) | Plants: seeds, tubers, roots — maize, sorghum, cassava, potato | α-glucose chains; amylose coiled into a helix, amylopectin branched | Energy store. Insoluble, so it does not draw water into cells by osmosis; compact coils pack a lot of glucose into little space; branches give many ends where enzymes can release glucose |
| Glycogen | Animals and fungi: liver (about 100 g) and muscle (about 400 g) in an adult | α-glucose, even more highly branched than amylopectin | Short-term energy store. So many branch ends that glucose can be released very fast, e.g. in the first minutes of a sprint or when blood glucose falls between meals |
| Cellulose | Plant cell walls — the most abundant organic molecule on Earth | β-glucose; straight chains lie side by side, held by many hydrogen bonds into strong fibres | Structure: fibres strong in tension support the plant. Humans make no enzyme to hydrolyse it, so it passes through the gut as dietary fibre |
So the functions of carbohydrates in organisms are three. They are the main respiratory substrate: glucose is respired to release energy, about 17 kJ per gram. They are energy stores: starch in plants, glycogen in animals. And they are structural: cellulose in plant walls, chitin (a polymer of a modified sugar) in insect skeletons and fungal walls, and the five-carbon sugars ribose and deoxyribose in the backbones of RNA and DNA.
A cow eats almost nothing but grass, which is mostly cellulose, yet it grows and produces milk. Humans who eat grass cannot use its energy. Suggest why, using the idea of enzyme specificity.
Reveal answer
Cellulose is a polymer of β-glucose joined by β-glycosidic bonds; hydrolysing it needs cellulase, whose active site fits those bonds. Human digestive enzymes (amylase, maltase) fit only α-bonds, so cellulose passes through us unchanged as fibre.
A cow’s rumen holds huge populations of bacteria, protists and fungi that do make cellulases. They hydrolyse the cellulose and ferment the glucose into fatty acids that the cow absorbs and respires; the microbes themselves, digested further along the gut, are also a source of protein.
1.3 Fats and their functions
Fats and oils are lipids: substances that do not dissolve in water but dissolve in organic solvents such as ethanol. The commonest lipid in food and in your body is the triglyceride: one molecule of glycerol joined by three condensation reactions to three fatty acids, forming three ester bonds and releasing three water molecules.
A fatty acid is a chain of carbon atoms, usually 16 or 18, carrying hydrogen atoms, with an acid group at one end. If every carbon–carbon bond is single, the chain holds as much hydrogen as it can: it is saturated. If there are one or more C=C double bonds, it is unsaturated (monounsaturated for one, polyunsaturated for several).
Each double bond puts a kink in the chain. Straight saturated chains pack tightly, so fats rich in them — butter, beef fat — are solid at room temperature. Kinked chains cannot pack, so oils rich in unsaturated fatty acids — sunflower, olive, marula oil — are liquid.
| Function of fats in organisms | How the structure provides it |
|---|---|
| Long-term energy store (adipose tissue in animals; oil in seeds such as groundnuts and sunflower) | Long hydrocarbon chains with little oxygen: more oxidation left to do, so about 37 kJ g⁻¹ — more than twice carbohydrate — and stored without water |
| Thermal insulation (fat under the skin; blubber in whales and seals) | Fat conducts heat poorly, reducing heat loss from the body |
| Protection of organs such as the kidneys and heart | A soft cushion that absorbs knocks |
| Cell membranes | Phospholipids (glycerol + two fatty acids + a phosphate head) form the bilayer of every membrane |
| Waterproofing (waxy leaf cuticle, oils on feathers and skin) | Non-polar chains repel water |
| Hormones and vitamins | Steroid hormones (testosterone, oestrogen) are made from cholesterol, another lipid; vitamins A, D, E and K dissolve only in fat, so fat in the diet is needed to absorb them |
| Metabolic water | Respiring 1 g of fat produces about 1.1 g of water; desert animals such as the kangaroo rat get most of their water from respiring their food and can live without drinking |
Tripalmitin is a fat made from glycerol (C₃H₈O₃, M = 92) and three molecules of palmitic acid (C₁₆H₃₂O₂, M = 256). Calculate its relative molecular mass, and the percentage of the reactants’ mass that is lost as water.
Mass of reactants = 92 + (3 × 256)
Mass of reactants = 92 + 768
Mass of reactants = 860
Water released = 3 × 18
Water released = 54
M(tripalmitin) = 860 − 54
M(tripalmitin) = 806
Percentage lost as water = (54 ÷ 860) × 100
Percentage lost as water = 6.3 %
Check with the formula C₅₁H₉₈O₆: (51 × 12) + (98 × 1) + (6 × 16) = 806 ✓
Oxygen is only 96 ÷ 806, or 12 %, of its mass; in glucose it is 96 ÷ 180, or 53 %. Less oxygen means more energy still to be released by oxidation.
A male emperor penguin fasts for up to four months in the Antarctic winter, through courtship and the whole incubation of its egg. Explain two ways in which its thick layer of fat helps it survive.
Reveal answer
Energy: during the fast the penguin respires stored fat; at about 37 kJ g⁻¹ fat releases more than twice the energy per gram of carbohydrate and is stored without water, so the store is as light as possible for the energy it holds.
Insulation: fat under the skin conducts heat poorly, so less of the heat released by respiration is lost to air at −40 °C; that means less fat must be respired to keep the body at about 38 °C. (Also acceptable: metabolic water from respiring fat, when snow is the only other water.)
A student writes: “Fats are polymers of fatty acids. Unsaturated fats have more hydrogen atoms than saturated fats, which is why they are solid. Fat is a better energy store than glycogen because it releases energy faster.”
Reveal answer
(1) A triglyceride is not a polymer: it is one glycerol joined to three fatty acids, not a long repeating chain.
(2) It is backwards: saturated chains hold the most hydrogen; unsaturated chains have fewer hydrogen atoms because of their C=C double bonds, and the kinks make them liquid (oils), not solid.
(3) Fat is a better long-term store because it holds more energy per gram and is stored without water; glycogen releases glucose faster, which is why it is kept for rapid use.
1.4 Proteins and their functions
Every amino acid has the same backbone: a central carbon atom carrying a hydrogen atom, an amino group (–NH₂), a carboxyl (acid) group (–COOH) and a side chain, the R group. Twenty different R groups make twenty amino acids, and they differ in size, charge and whether they attract water.
In a condensation reaction the –COOH of one amino acid reacts with the –NH₂ of the next, releasing water and forming a peptide bond. A chain of amino acids is a polypeptide; a protein is one or more polypeptides folded into a particular shape.
The sequence of amino acids is set by a gene. The sequence decides how the chain folds, because R groups attract or repel each other and form weak bonds (hydrogen bonds, ionic bonds) and, in some proteins, strong disulfide bridges. The result is a precise three-dimensional shape — and in proteins, shape is function.
Change one amino acid and the shape can change. In sickle-cell haemoglobin one glutamic acid (charged) is replaced by valine (non-polar) at position 6 of a chain of 146, and the molecules stick together into fibres when oxygen is low.
| Function | Example protein | How the shape serves the job |
|---|---|---|
| Catalysis | Enzymes: amylase, pepsin, lipase, catalase | An active site complementary to one substrate (Biology Unit 3) |
| Structure | Keratin (hair, nails, skin), collagen (tendons, bone, skin) | Long fibrous molecules twisted into ropes: strong in tension |
| Transport | Haemoglobin (oxygen in red blood cells); channel and carrier proteins in membranes | A binding site that holds and releases a particular molecule |
| Communication | Hormones such as insulin and glucagon; receptors on cells | A shape recognised by a matching receptor |
| Defence | Antibodies | A binding site complementary to one antigen |
| Movement | Actin and myosin in muscle | Filaments that slide past each other using energy from ATP |
| Energy (last resort) | Muscle protein broken down in starvation | About 17 kJ g⁻¹ — but the body has no protein store, so using it means losing tissue |
Your body can make eleven of the twenty amino acids from others. The remaining nine are essential amino acids: they must come from food. Animal proteins (meat, fish, eggs, milk) contain all nine in good proportions; many plant proteins are low in one — maize in lysine, beans in methionine — so combining a cereal with a legume, such as sorghum porridge with beans, supplies the full set.
The November 2022 Biology paper asked for a nutrient used as a source of energy under normal circumstances, and warned that writing “protein” alongside a correct answer was a contradiction. Explain why protein is not accepted, even though it can be respired.
Reveal answer
Under normal circumstances energy comes from carbohydrates (glucose from starch and sugars) and fats. Amino acids from dietary protein are used to build the body’s own proteins for growth and repair — enzymes, muscle, antibodies.
The body has no protein store: excess amino acids are deaminated in the liver, their nitrogen removed as urea, and only the remainder respired. Protein becomes a major fuel only in starvation, when muscle is broken down — which is not “normal circumstances”.
1.5 Testing food for its molecules
Four simple tests identify the main nutrients. Each relies on a reagent that reacts with a particular chemical group. For every test you must be able to state the reagent, the method, the negative colour (nutrient absent) and the positive colour (nutrient present). Solid food is first ground with a little distilled water in a mortar and filtered or left to settle.
| Nutrient | Reagent and method | Negative result | Positive result |
|---|---|---|---|
| Starch | Add a few drops of iodine solution (iodine in potassium iodide) to the sample or food | Stays orange-brown | Blue-black |
| Reducing sugars (glucose, fructose, maltose, lactose) | Add an equal volume of Benedict’s reagent (e.g. 2 cm³ + 2 cm³); heat in a boiling water bath for about 5 minutes | Stays blue | Green (little) → yellow → orange → brick-red precipitate (much) |
| Non-reducing sugar (sucrose) | First Benedict’s test negative; boil a fresh sample with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, then repeat Benedict’s | Stays blue after hydrolysis | Colour change after hydrolysis |
| Protein | Biuret test: add sodium hydroxide solution, then a few drops of dilute copper(II) sulfate (or one biuret reagent); do not heat | Stays blue | Lilac / purple (reacts with peptide bonds) |
| Lipid | Emulsion test: shake the sample with ethanol, then pour the liquid into an equal volume of water | Stays clear | Cloudy white emulsion |
A student tests a lemonade. Benedict’s stays blue after 5 minutes’ heating. After boiling with dilute acid and neutralising, Benedict’s gives an orange colour. Interpret both results, and state what would have happened if she had forgotten to neutralise.
Reveal answer
No reducing sugar is present in the drink as bought, so any sugar must be non-reducing — sucrose. Acid hydrolysis split sucrose into glucose and fructose, both reducing sugars, which then reduced the Benedict’s reagent (orange shows a moderate amount).
Benedict’s reagent works only in alkaline conditions. Without neutralising, the acid would have destroyed the alkalinity and the test could have stayed blue — a false negative.
1.6 From colour to number: colorimetry and the calibration curve
A food test is qualitative: it says whether a substance is present. Judging “a bit orange” against “quite orange” by eye is subjective and differs between people and in different light. To make Benedict’s test quantitative, the precipitate is removed by filtering or centrifuging and the blue solution left behind is placed in a colorimeter.
The colorimeter shines light of one colour through the sample and measures how much is absorbed. Blue copper(II) ions absorb red light, so a red filter is used. The more reducing sugar was present, the more copper(II) was used up, the paler the blue, and the lower the absorbance. The instrument is first zeroed with distilled water.
Solutions of known glucose concentration are tested in exactly the same way, and their absorbances are plotted against concentration: the calibration curve.
An unknown sample is then tested, its absorbance read on the curve, and the concentration read off the other axis. The curve is not a straight line — it flattens as the copper runs out — so it must be drawn as a smooth curve through the points and used only within the range of the standards.
Standards gave these mean absorbances: 0.0 % glucose, 0.82; 0.4 %, 0.54; 0.8 %, 0.35; 1.2 %, 0.23; 1.6 %, 0.16; 2.0 %, 0.11. A sports drink was diluted 1 cm³ of drink to 4 cm³ of water, and the diluted sample gave an absorbance of 0.29. Find the glucose concentration of the drink.
Absorbance 0.29 lies between 0.35 (0.8 %) and 0.23 (1.2 %)
Reading across from absorbance 0.29 to the smooth curve gives about 1.0 % glucose
Check by linear interpolation: 0.8 % + 0.4 % × (0.06 ÷ 0.12)
Concentration of diluted sample ≈ 1.0 %
Dilution factor = total volume ÷ volume of drink
Dilution factor = 5 cm³ ÷ 1 cm³
Dilution factor = 5
Concentration of drink = 1.0 % × 5
Concentration of drink = 5.0 % (5.0 g per 100 cm³)
Why dilute? The undiluted drink would give an absorbance of about 0.02, on the flat end of the curve where large changes in concentration give tiny changes in reading — too insensitive. Diluting moves the reading onto the steep, useful part. (Many sports drinks contain about 6 g of sugars per 100 cm³.)
Using the standards in the EXAMPLE, a milk sample digested with lactase gave an absorbance of 0.54. A second student forgot to zero the colorimeter with water; every one of her readings was 0.05 too high. Explain what concentration each student would report for the same sample, and name the type of error.
Reveal answer
First student: 0.54 matches the 0.4 % standard, so 0.4 % glucose (in the diluted sample, if it was diluted).
Second student: her reading would be 0.59, which lies on the curve between 0.0 % (0.82) and 0.4 % (0.54), at about 0.3 % — too low.
If her standards were measured with the same offset, the curve shifts too and the error largely cancels; if they were not, every unknown is reported too low. A constant offset is a systematic error (a zero error): repeating the readings does not remove it; zeroing with a blank does.
A slice of bread is chewed, and the starch in it is hydrolysed completely to glucose by amylase and maltase. The slice contains 12.0 g of starch. Calculate the mass of glucose produced, treating starch as (C₆H₁₀O₅)ₙ with very large n.
Then explain why a diabetic patient’s blood glucose rises after eating the bread even though it tasted barely sweet.
Reveal answer
Each glucose residue in starch has M = 162; each glucose molecule, M = 180
Mass of glucose = 12.0 g × (180 ÷ 162)
Mass of glucose = 12.0 g × 1.111
Mass of glucose = 13.3 g
The extra 1.3 g is the water added by hydrolysis — one H₂O per glucose unit released.
Starch is not sweet because the large polymer does not fit the sweet-taste receptors on the tongue; but amylase (in saliva, then from the pancreas) and maltase hydrolyse it to glucose in the gut, which is absorbed into the blood. Without effective insulin, that 13 g of glucose raises the blood glucose concentration as surely as 13 g of sugar would (Biology Unit 4).
Try it — Benedict’s test made quantitative
In this lab you turn a colour into a number. Glucose solutions of known concentration are heated with Benedict’s reagent, the red-brown precipitate is removed and the absorbance of the remaining blue solution is read with a red filter. You then use your calibration curve on sugars you have not measured before, and test a non-reducing sugar with and without acid hydrolysis.
Mean absorbances: 0 %, 0.82; 0.4 %, 0.53; 0.8 %, 0.35; 1.2 %, 0.24; 1.6 %, 0.16; 2.0 %, 0.11. Describe the relationship quoting values, state whether it is linear, and explain why the curve flattens.
Reveal answer
As glucose concentration rises from 0 % to 2.0 %, the absorbance falls from 0.82 to 0.11. The falls for each 0.4 % step are 0.29, 0.18, 0.11, 0.08 and 0.05, so the relationship is not linear: the gradient becomes less steep as concentration rises (a negative, curved relationship).
At low concentrations there is a large excess of copper(II) ions, so every extra bit of glucose removes blue. At high concentrations most of the copper(II) has already been reduced, so each extra 0.4 % of glucose has fewer ions left to react with and the absorbance falls less. The curve approaches a minimum set by the colour that remains when almost all the copper has precipitated.
Common mistakes to avoid
- “Fats are polymers” — a triglyceride is glycerol + three fatty acids; polysaccharides, proteins and nucleic acids are the polymers.
- Counting bonds as monomers — a chain of n monomers has n − 1 bonds and releases n − 1 water molecules.
- “Iodine tests for sugar” — iodine tests for starch; Benedict’s tests for reducing sugars.
- Heating the biuret test — only Benedict’s is heated.
- “Positive result: purple” without the starting colour — write “from blue to lilac”.
- “Sucrose gives a positive Benedict’s test” — only after acid hydrolysis and neutralisation.
- Protein named as a normal energy source — the November 2022 markscheme called it a contradiction.
- A ruled straight line through a calibration curve — it is a smooth curve; never extrapolate beyond the standards.
Study smart: how this is examined
Study routine. Today: cover the Section 1.1 table and rewrite it; then write the four food tests as “reagent — method — from … to …” from memory.
In two days: calculate the relative molecular mass of lactose, of a tripeptide of glycine (M = 75), and of a triglyceride of oleic acid (C₁₈H₃₄O₂, M = 282), with the formula check. In a week: run the lab as a full investigation, take the quiz, and explain to someone why you store fat rather than glycogen, using numbers.
How we found out — from grease to sequence
In the 1810s the French chemist Michel-Eugène Chevreul boiled animal fats with alkali — the way soap is made — and showed that every fat splits into glycerol and a set of acids he named “fatty acids”. His 1823 book on animal fats turned grease into chemistry, and the idea that a fat is glycerol joined to fatty acids has stood ever since.
Sugars were next. In 1849 Hermann von Fehling made an alkaline copper solution that turned red with sugar in urine, and around 1908–1909 the American biochemist Stanley Benedict published a more stable version, still the reagent in school laboratories. Between 1884 and 1908 Emil Fischer worked out the structures of glucose and other sugars, and joined amino acids into short chains, which he called peptides; he received the 1902 Nobel Prize in Chemistry.
Whether a protein had one exact structure was still argued about in the 1940s: many chemists thought proteins were variable mixtures. Frederick Sanger settled it. Between 1945 and 1955 he broke insulin into fragments, identified the amino acids in each, and pieced together the complete sequence of its two chains, 51 amino acids in all.
Every molecule of cattle insulin had the same sequence. That showed a protein is a defined molecule with a definite order of amino acids, and suggested that something must specify that order — a gene. Sanger received the 1958 Nobel Prize in Chemistry, and a second one in 1980 for sequencing DNA.
The folded shapes came from X-ray crystallography: Linus Pauling predicted the α-helix in 1951, and John Kendrew and Max Perutz solved the first three-dimensional protein structures, myoglobin (1958) and haemoglobin (1959–1960), sharing the 1962 Nobel Prize in Chemistry.
Explain why Sanger’s finding that every insulin molecule from one species had the same sequence was important evidence, and what it suggested about how proteins are made.
Reveal answer
If proteins were random mixtures, different molecules would have different sequences and no fixed shape; the finding that all molecules were identical showed that each protein has one defined primary structure, which explains why each has a reliable shape and function.
A precise, repeated sequence cannot arise by chance each time: it must be copied from instructions. This supported the idea — confirmed after Watson and Crick’s 1953 DNA structure — that the sequence of bases in a gene codes for the sequence of amino acids in a protein.
Where it is used now
- Staples of southern Africa. Sorghum (mabele) and maize meal are mostly starch; the morama bean of the Kalahari is unusual in being rich in both protein (roughly a third of the seed) and oil; mopane worms are more than half protein by dry mass. Pairing a cereal porridge with beans or morama supplies all nine essential amino acids.
- Diagnostic test strips. The Benedict’s reaction was used for decades to test urine for glucose in diabetes; today’s glucose meters and urine strips use enzymes such as glucose oxidase, which are specific to glucose rather than to every reducing sugar.
- Food labelling. Laboratories measure the carbohydrate, fat and protein in every packaged food so that the label can list them per 100 g — the numbers you will use in Lessons 2 and 3.
- Cellulose as a resource. Cotton is almost pure cellulose; enzymes that hydrolyse cellulose (cellulases) are used to make biofuel from crop waste and to soften denim.
- Protein medicines. Insulin, antibodies used in cancer treatment and many vaccines are proteins, made by microorganisms or cultured cells carrying the human gene — their exact amino acid sequence is checked in every batch.
Science now — and the scientist you need to be
What is happening in 2026
Predicting a protein’s shape from its sequence was one of biology’s hardest problems for fifty years. AlphaFold 2 (DeepMind, 2020–2021) largely solved it for single chains, and its database now holds predicted structures for more than 200 million proteins. The 2024 Nobel Prize in Chemistry went to Demis Hassabis and John Jumper for this work, and to David Baker for designing new proteins.
AlphaFold 3 (2024) extended prediction to proteins bound to DNA, sugars and small molecules. In 2026 protein design tools are used to invent enzymes, vaccine components and binding proteins that do not exist in nature — and they are tested in the laboratory, because a prediction is a hypothesis, not a result.
In food science, precision fermentation uses yeasts or fungi carrying animal genes to make milk and egg proteins without animals, and several such proteins are already sold in some countries. Over the next decade expect cheaper protein design, more foods built molecule by molecule, and careers such as protein engineer, food scientist, bioinformatician and nutrition analyst — all needing the monomer–polymer thinking of this lesson.
The mindshift
What you need to be able to do
Name the monomer and the bond of each macromolecule; count water in condensation and hydrolysis; explain a function by linking it to a structure; describe a food test with the colour change from … to …; and read an unknown from a calibration curve, correcting for dilution.
See an example
Peanut butter (per 100 g: fat 50 g, protein 25 g, carbohydrate 12 g of which sugars 6 g, mostly sucrose): emulsion test strongly positive; biuret positive; iodine weak (little starch); Benedict’s weak or negative without hydrolysis, because most of its sugar is sucrose.
The slip to look for in any answer, human or AI: “Benedict’s positive: contains sugar”. Sucrose gives no colour until it has been hydrolysed and neutralised.
Key terms
| Monomer | A small molecule that can join with others like it to form a polymer (glucose, amino acid, nucleotide). |
| Polymer | A large molecule made of many monomers joined in a chain (starch, glycogen, cellulose, protein, DNA). |
| Condensation | A reaction that joins two molecules with the removal of a water molecule. |
| Hydrolysis | A reaction that breaks a bond by adding a water molecule; digestion is hydrolysis. |
| Monosaccharide / disaccharide / polysaccharide | One sugar unit (glucose) / two (maltose, sucrose, lactose) / many (starch, glycogen, cellulose). |
| Triglyceride | A lipid made of one glycerol and three fatty acids joined by ester bonds. |
| Saturated / unsaturated fatty acid | No C=C double bonds (straight chain) / one or more C=C double bonds (kinked chain). |
| Amino acid; R group | Monomer of proteins with an amino group, a carboxyl group and a variable side chain (R group); 20 kinds. |
| Peptide bond; polypeptide | The bond between two amino acids formed by condensation; a chain of amino acids. |
| Essential amino acid | One of nine amino acids the body cannot make and must obtain from food. |
| Reducing sugar | A sugar that reduces copper(II) ions in Benedict’s reagent (glucose, fructose, maltose, lactose; not sucrose). |
| Calibration curve | A graph of instrument readings for standards of known concentration, used to find unknown concentrations. |
Think like a scientist
Criterion A A school nutrition club tests four unlabelled samples, A to D, known to be egg white, cooking oil, orange juice and a thin maize porridge — but not which is which.
| Sample | Iodine | Benedict’s (heated) | Biuret | Emulsion test |
|---|---|---|---|---|
| A | orange-brown | orange | blue | clear |
| B | orange-brown | blue | blue | cloudy white |
| C | blue-black | green | faint lilac | clear |
| D | orange-brown | blue | purple | clear |
(a) Identify each sample, giving the evidence for each. [4]
(b) Sample C gave a green colour with Benedict’s reagent although porridge is not sweet. Suggest why. [2]
(c) Outline how you would show whether sample A also contains sucrose. [3]
(d) The main protein in egg white, ovalbumin, has 385 amino acids. Calculate the number of water molecules needed to hydrolyse one molecule completely, and state the monomers produced. [2]
(e) Compare the structure of the starch in sample C with that of cellulose, and explain how each structure suits its function. [4]
(f) The oil in sample B is mostly unsaturated. Explain why it is liquid at room temperature while beef fat is solid. [2]
Model answer
(a) A = orange juice: Benedict’s orange shows reducing sugars (glucose and fructose), no starch, protein or lipid. B = cooking oil: only the emulsion test positive, so lipid only.
C = maize porridge: iodine blue-black shows starch. D = egg white: biuret purple shows protein, with no starch, reducing sugar or lipid. [1 each, the identification with its evidence.]
(b) Green shows a small amount of reducing sugar [1]. Amylase in the maize grain (or in saliva, if tasted) or the cooking itself may have hydrolysed a little of the starch to maltose and glucose, which are reducing sugars; too little to taste sweet [1].
(c) Take a fresh sample of A and boil it with dilute hydrochloric acid for a few minutes to hydrolyse any sucrose into glucose and fructose [1]. Neutralise with sodium hydrogencarbonate until fizzing stops, because Benedict’s needs alkaline conditions [1].
Repeat the Benedict’s test with the same volumes and heating time as the first, and compare: a stronger colour (for example red instead of orange), or a lower absorbance in a colorimeter, shows more reducing sugar after hydrolysis, so sucrose was present [1].
(d) 385 amino acids are joined by 384 peptide bonds, so 384 water molecules [1]; the monomers produced are amino acids [1].
(e) Both are polymers of glucose [1]. Starch is made of α-glucose, coiled (amylose) and branched (amylopectin), so it is compact and insoluble — it stores much glucose without affecting osmosis — and has many ends where amylase can release glucose when the plant needs energy [1].
Cellulose is made of β-glucose in straight, unbranched chains [1] lying side by side, linked by many hydrogen bonds into strong fibres, which suits its function of supporting the cell wall and the plant [1].
(f) Unsaturated fatty acids contain C=C double bonds that put kinks in their chains [1]; kinked chains cannot pack closely together, so the forces between the molecules are weaker and the oil melts below room temperature, while the straight saturated chains of beef fat pack tightly and stay solid [1].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
State a nutrient that is used as a source of energy under normal circumstances.
Reveal answer
Carbohydrate (or starch, glucose, sugars) OR fat [1]. Adding “protein” to a correct answer is a contradiction and scores 0.
Define the terms monomer and hydrolysis.
Reveal answer
Monomer: a small molecule that joins with others of the same kind to form a polymer [1]. Hydrolysis: breaking a chemical bond by the addition of water [1].
Describe how you would test a sample of milk for protein, including the result you would expect.
Reveal answer
Place about 2 cm³ of milk in a test tube and add an equal volume of sodium hydroxide solution [1]; add a few drops of dilute copper(II) sulfate solution and shake gently, without heating [1]; the colour changes from blue to lilac/purple, showing protein is present [1].
Explain two ways in which the structure of glycogen suits its function as an energy store in animals.
Reveal answer
It is highly branched, so there are many ends where enzymes can remove glucose at the same time, releasing it quickly when needed [1 + 1 for the link]. It is insoluble, so it does not affect the water potential of cells / draw water in by osmosis [1] (also acceptable: compact, so much glucose is stored in a small space). (Maximum 3.)
Calculate the relative molecular mass of a dipeptide made from glycine (M = 75) and alanine (M = 89). Show your working.
Reveal answer
M = 75 + 89 − 18
M = 146 [2]
1 mark for subtracting the water; 164 (no water removed) scores 0.
Outline four functions of lipids in the human body.
Reveal answer
Any four: long-term energy store in adipose tissue [1]; thermal insulation under the skin [1]; protection/cushioning of organs such as the kidneys [1]; phospholipids form cell membranes [1]; steroid hormones are made from cholesterol [1]; absorption and transport of fat-soluble vitamins A, D, E, K [1]; source of metabolic water [1]. (Maximum 4.)
7. When 1.00 g of fat is respired, about 1.07 g of water is produced; 1.00 g of glucose produces 0.60 g. Using the equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, show that the glucose figure is correct, and explain why fat produces more water per gram.
Reveal answer
1 mol glucose (180 g) → 6 mol water (6 × 18 = 108 g)
Water per gram of glucose = 108 g ÷ 180 g
Water per gram of glucose = 0.60 g [2]
A fat molecule is mostly carbon and hydrogen with very little oxygen (tripalmitin, C₅₁H₉₈O₆, is only 12 % oxygen by mass against 53 % for glucose), so each gram contains far more hydrogen atoms [1]; during respiration all this hydrogen ends up in water, combining with oxygen taken in from the air, so more water forms per gram — the same reason fat releases more energy per gram [1].
8. Sickle-cell haemoglobin differs from normal haemoglobin by a single amino acid in each β-chain of 146: valine (non-polar R group) replaces glutamic acid (negatively charged R group). Explain how one change can alter the function of the whole protein, using the terms primary structure, R group and shape.
Reveal answer
The primary structure — the sequence of amino acids — determines how the chain folds, because the R groups interact through weak bonds [1]. Replacing a charged, water-attracting R group on the surface with a non-polar one changes how that part of the molecule interacts with water and with other molecules [1].
The non-polar valine forms a sticky patch that fits a pocket on a neighbouring haemoglobin molecule, so when oxygen is low the molecules bind together into long fibres [1]; the fibres distort red blood cells into rigid sickle shapes that carry oxygen poorly and block capillaries — a change in shape and interaction causes a change in function [1].
Summary
- Polysaccharides, proteins and nucleic acids are polymers of monosaccharides, amino acids and nucleotides; fats are glycerol + three fatty acids. Condensation joins monomers and releases water; hydrolysis breaks bonds by adding water; a chain of n monomers has n − 1 bonds.
- Carbohydrates are fuel (glucose, about 17 kJ g⁻¹), stores (starch in plants, glycogen in animals — branched, compact, insoluble) and structure (cellulose — straight β-glucose chains in fibres; indigestible to us, so it is dietary fibre).
- Fats store about 37 kJ g⁻¹ without water, insulate, protect organs, build membranes, make steroid hormones and carry vitamins A, D, E, K; unsaturated chains are kinked, so oils are liquid.
- Proteins are chains of 20 kinds of amino acid joined by peptide bonds; the sequence decides the folded shape, and the shape decides the function — enzymes, structure, transport, hormones, antibodies, movement. Nine amino acids are essential.
- Tests: iodine (orange-brown → blue-black, starch); Benedict’s, heated (blue → brick-red, reducing sugar); biuret (blue → lilac, protein); emulsion (clear → cloudy, lipid). A colorimeter and calibration curve make Benedict’s quantitative.
Next time → A balanced diet: the seven nutrient groups, what vitamins and minerals do, and the diseases that appear when one is missing.