C, H, O in all; only protein has N
All three food groups contain carbon, hydrogen and oxygen (C, H, O). Proteins ALSO contain nitrogen (N) and sometimes sulfur (S) — the only group that always has nitrogen. Basic units: carbohydrates (starch, glycogen) are chains of simple sugars such as glucose, ; proteins are chains of amino acids; a lipid is one glycerol with three fatty acids. Only protein supplies the nitrogen to build new protein.
The four food tests and colours
Four tests, reagent → positive colour. Reducing sugar: Benedict's solution + heat → brick-red. Starch: iodine solution → blue-black. Protein: Biuret reagent → purple/lilac. Lipid: dissolve in ethanol, then add water → cloudy white emulsion (Sudan III → red layer also accepted). Note: iodine is blue-black, not just 'blue'; the emulsion needs BOTH ethanol AND water.
Enzymes, active site and denaturation
An enzyme is a protein biological catalyst — it speeds up a reaction without being used up. Its active site is complementary to the substrate, forming an enzyme-substrate complex. Heating raises kinetic energy → more collisions → faster rate, until above the optimum the enzyme denatures: the active site changes shape so the substrate no longer fits (irreversible). Extreme pH also changes the active-site shape; optimum pH varies (pepsin 2, amylase 7).
Drawn from real examiner reports.
Starch test: iodine, blue-black
The starch test uses iodine solution, which turns blue-black. Naming Benedict's, or giving the colour as 'brown-black', 'blue' alone, or 'brick-red', loses the mark. Brick-red is the positive for a different test — Benedict's for reducing sugar — so mixing the two costs marks in any question that examines both tests together.
June 2024 Paper 1BR Q4(b)(ii) — weaker answers named Benedict's instead of iodine, or gave the positive colour as brick-red rather than blue-black.
Emulsion test needs ethanol AND water
The lipid emulsion test has two steps: dissolve the sample in ethanol, then pour into water — a cloudy white emulsion shows lipid is present. A very common error is naming ethanol but omitting the water; ethanol alone gives no visible emulsion, so the procedural mark is lost. Sudan III (a red-stained layer) is an accepted alternative method.
June 2024 Paper 2B Q6(a)(ii) — a very common error was naming ethanol without water for the emulsion test.
Denature alone caps the mark
On a denaturation question, saying only 'the enzyme denatures' scores one of two. The full answer has two parts: the active-site shape changes, so the substrate can no longer fit and no enzyme-substrate complexes form. Add that the change is irreversible for a temperature question. 'The enzyme is killed' is wrong — enzymes are molecules, not living things.
June 2024 Paper 1BR Q4(c)(iii) — saying only 'denature', without the active-site shape change and loss of substrate binding, capped the mark.
Describe vs explain: enzyme rate
'The rate gets faster' is a description and scores nothing on an explain question. The chain is: higher temperature → molecules gain kinetic energy → they collide more frequently → more enzyme-substrate complexes → faster rate. Mark schemes want at least 'kinetic energy' and 'more frequent collisions', not a restatement that heat speeds it up.
June 2024 Paper 1BR Q4(c)(ii) — weak answers described the rate change without the kinetic-energy / collision-frequency explanation.
Enzymes denature; organisms die
A high temperature denatures enzymes — it does not 'kill' them, because enzymes are molecules, not living things. Whole organisms such as bacteria and yeast are what die at high temperature. Examiners reject 'high temperature kills the enzymes' or 'denatures the bacteria' — pair the verb to the noun.
June 2024 Paper 2B Q3(a) and Paper 1BR Q8(b)(ii) — recurring misconception that heat kills enzymes or denatures bacteria; enzymes denature, organisms die.
Biuret tests protein, not sugar
Both reagents start blue, but they test different molecules and give different positives. Biuret reagent tests protein and turns purple/lilac with no heating. Benedict's solution tests reducing sugar and turns brick-red on heating. Confusing the two names, or swapping their colours, is a common way to lose a food-test mark.
Food tests: reagent and colour
Each food test earns one mark for the reagent and one for the positive colour. Give both together: 'Add iodine solution — it turns blue-black if starch is present.' The reagent alone, or the colour alone, is worth at most one of the two marks, so never write just one half.
Chain the enzyme-rate answer
For enzyme-rate explains, chain: kinetic energy → collisions → enzyme-substrate complexes → faster rate. Above the optimum the active site changes shape → substrate cannot fit → reaction stops (add 'permanently' to show it is irreversible).
Match the command word
Read the command word first. 'State/name' wants the term; 'describe' wants what happens; 'explain' wants the mechanism (kinetic-energy or active-site reasoning); 'calculate' wants working and a number. Answering 'explain' with only a description is the commonest lost mark.
All three major food molecules contain carbon (C), hydrogen (H) and oxygen (O). Proteins additionally contain nitrogen (N) and sometimes sulfur (S).
| Molecule | Elements | Built from (basic units) |
|---|---|---|
| Carbohydrate (starch, glycogen) | C, H, O | Simple sugars (e.g. glucose) |
| Protein | C, H, O, N (sometimes S) | Amino acids |
| Lipid (fat or oil) | C, H, O | Fatty acids + glycerol (3 fatty acids + 1 glycerol) |
Starch (in plants) and glycogen (in animals and fungi) are large molecules made from many glucose units joined in a chain. Proteins are large molecules made from many amino acids joined in a specific sequence — the sequence determines the protein's shape and function. A lipid is formed from one glycerol molecule bonded to three fatty acid chains.
An enzyme is a biological catalyst (a protein) that speeds up the rate of a metabolic reaction and is not used up in the reaction.
Each enzyme has an active site — a specifically shaped region where the substrate molecule fits. The fit between the substrate and the active site is described as complementary (sometimes called the "lock-and-key" model). The substrate binds, reacts, and products leave; the active site is unchanged and free to accept the next substrate molecule.
When an enzyme is denatured, the shape of its active site changes so the substrate can no longer fit; the enzyme loses its catalytic activity. Denaturation is irreversible (permanent) and is caused by temperatures above the optimum or by extreme pH values.
| Food substance | Test reagent | Positive result | Negative result |
|---|---|---|---|
| Reducing sugar (e.g. glucose) | Benedict's solution + heat | Brick-red precipitate | Solution stays blue |
| Starch | Iodine solution | Blue-black colour | Stays orange-brown |
| Protein | Biuret reagent | Purple / lilac colour | Stays blue |
| Lipid (fat/oil) | Ethanol, then add water | Cloudy white emulsion | Stays clear |
Define the term enzyme.
State the chemical elements present in:
(a) carbohydrates (1 mark)
(b) proteins (2 marks)
