Elements — nitrogen is the protein marker
Carbohydrates and fats contain carbon, hydrogen and oxygen only — elemental analysis alone cannot tell them apart. Proteins contain those three plus nitrogen, and many also contain sulfur; nitrogen is the element examiners require. The reason is structural: proteins are chains of amino acids, and every amino acid carries an amine (–NH₂) group. (Extended) DNA adds phosphorus as well as nitrogen — phosphate in the backbone, nitrogen in the bases.
Subunits — glucose, amino acids, fatty acids and glycerol
Large molecules are assembled from small ones by condensation reactions, which release water. Starch (the plant store) and glycogen (the animal and fungal store) are both polysaccharides built entirely from glucose, differing only in branching — glycogen is the more branched, so glucose is released faster. Proteins are polymers of amino acids; there are 20 of them and their sequence fixes the protein's shape and function. A fat or oil is a triglyceride — one glycerol joined to three fatty acids — so it is built from two kinds of subunit rather than one repeating monomer.
Food tests — four reagents, four colours
Benedict's reagent, a blue solution, is heated with the sample in a water bath and gives a brick-red (orange-red) precipitate with a reducing sugar. Iodine solution, amber-brown to begin with, is added straight to the sample and turns blue-black with starch. Biuret reagent — sodium hydroxide then copper sulfate, or the ready-made solution — turns purple (violet) with protein at room temperature, with no heating. The ethanol emulsion test for fats: dissolve the food in ethanol, then pour into water, and a white cloudy emulsion forms.
Drawn from real examiner reports.
Starch and glycogen are made of glucose
Asked for the basic unit of starch or glycogen, candidates write "starch units", "glycogen" or vaguely "sugar" — none of these scores. Both are polysaccharides built from glucose monomers, and this is the single most poorly answered point in the whole topic, on Core and Extended papers alike. Starch and glycogen differ in branching and in where they are stored, never in their monomer.
Reported "very poorly known" — s22 P11/12/13/21/22/23 Q7/Q9 · s23 P11/12/13
Biuret ≠ Benedict's reagent
The two reagents are persistently swapped. Biuret detects protein and turns purple. Benedict's detects reducing sugars and gives a brick-red precipitate when heated. Writing "biuret tests for reducing sugars" or "Benedict's tests for protein" scores zero. Mnemonic: Biuret → Purple → Protein; Benedict's → Brick-red → sugar, and Benedict's is the one that is Heated.
Flagged w22 P11/12/13 Q9 · w22 P51 · w22 P61
Sucrose is not a reducing sugar
Sucrose does not reduce Benedict's solution. Heated with Benedict's it stays blue — a negative result. The reducing sugars are glucose, fructose and maltose; sucrose is the non-reducing one. Candidates routinely predict a brick-red precipitate for sucrose, or offer sucrose as the answer when a positive Benedict's result is given, and lose the mark on this distinction.
Recurring error — w22 P51 Q1bv (reducing vs non-reducing sugars)
"Blue" is not "blue-black"
The positive starch result must be written blue-black; "blue" alone was penalised. Precision matters here because iodine solution is amber-brown before the test and Benedict's is blue before its test, so "blue" is ambiguous. The colour appears because iodine molecules slot inside the helical structure of starch — which is also why glucose, having no helix, produces no colour change.
Positive starch colour given as "blue" — s22 P51 Q1b · s22 P61 Q1b
Proteins contain nitrogen, not magnesium
Two errors recur on elemental composition. Listing only C, H and O describes a carbohydrate or a fat, not a protein — nitrogen must be there, with sulfur often credited alongside it. And magnesium is not a constituent of protein at all: it is a mineral ion needed to make chlorophyll, and adding it to a protein list costs the mark.
Flagged w22 P31 Q6c ii · s22 P32 Q7c · w22 P43 Q2a i
Fat means fatty acids and glycerol
Asked for the subunits of a fat, or for the products of fat digestion, candidates write "fatty acids" and stop. Glycerol is the other half of the answer and carries its own mark. A triglyceride is one glycerol molecule joined to three fatty acid chains, so name both every time — the answer is never fatty acids alone.
Flagged s23 P33 Q3b i — "fatty acids only" given for the products of fat digestion
(Extended) DNA is not a protein
DNA was described as "a protein" and as "made of amino acids" — both wrong. DNA is a nucleic acid, a polymer of nucleotides, each one a deoxyribose sugar, a phosphate group and a base. Only proteins are made of amino acids. Recall of the structure was also only partial: two strands coiled into a double helix, complementary bases held by hydrogen bonds, A with T and C with G.
Flagged w22 P41 Q4a · s23 P23 Q8
Reagent → observation → conclusion
Every food-test answer has three separately marked parts: name the reagent exactly, state the colour change or appearance, then say what it shows. Marks are awarded for each step, so a partial answer still scores — but never stop at the colour without naming the substance it indicates.
Name the subunit, the product and the process
For "how is this large molecule made" questions, give all three parts: the subunit (glucose; amino acids; fatty acids and glycerol), the product (starch or glycogen; protein; fat or oil), and, where the question asks for it, the process — condensation.
Read the stem before naming a reagent
The stem almost always names the substance being tested for. Match the reagent to that substance instead of defaulting to Benedict's, which candidates reach for on every food-test question regardless of what was asked. One careful read saves the whole answer.
(Extended) Pair the bases, then halve
In base-percentage questions use A = T and C = G. Given one value, its partner is identical; subtract both from 100% and halve the remainder to get each of the other two. Check the four percentages sum to 100 before writing your answer — the usual slip is forgetting to halve.
Carbohydrates — contain C, H and O only. Small carbohydrates (monosaccharides such as glucose) join to form large polysaccharides (starch, glycogen, cellulose) by condensation reactions.
Fats (lipids) — also contain C, H and O only. A fat (triglyceride) is built from one glycerol molecule joined to three fatty acid chains. Fats are energy-rich stores; oils are fats that are liquid at room temperature.
Proteins — contain C, H, O and N (nitrogen); many also contain sulfur (S). Proteins are polymers of amino acids linked by peptide bonds. The sequence of amino acids determines a protein's 3-D shape and function. There are 20 different amino acids.
(Extended) DNA (deoxyribonucleic acid) — contains C, H, O, N and phosphorus (P). DNA is the molecule that carries genetic information. It is a polymer of nucleotides.
What is the monomer (basic unit) from which starch and glycogen are both made?
(a) State the chemical elements found in proteins but NOT in carbohydrates.
(b) State the monomer from which both starch and glycogen are made.
(c) Name the two types of smaller molecule from which a fat is made.
