Elements and basic units of the three molecules
Carbohydrates contain carbon, hydrogen and oxygen (C, H, O); their basic unit is a simple sugar such as glucose, and many glucose units join to make starch or glycogen. Proteins contain C, H, O AND nitrogen (N) and are built from amino acids. Lipids contain C, H and O (much less oxygen) and are built from fatty acids and glycerol (three fatty acids to one glycerol). Only proteins contain nitrogen, so nitrogen in a food means protein is present.
The four food tests — reagent plus colour change
Each test is a reagent AND a colour change — state both. Starch: add iodine solution → orange/brown to blue-black. Glucose/reducing sugar: add Benedict's solution and heat in a water bath → blue to a brick-red precipitate (it stays blue if none is present). Protein: add Biuret solution → blue to lilac/purple. Fat (lipid): the emulsion test — dissolve the sample in ethanol, then add an equal volume of water → a cloudy white emulsion.
Enzymes: catalysts affected by temperature and pH
An enzyme is a biological catalyst that speeds up a reaction and is not used up; its active site fits only its specific substrate. Raising temperature gives the enzyme and substrate more kinetic energy and more frequent collisions, so the rate rises to an optimum (about 37 °C). Above the optimum the enzyme denatures — the active-site shape changes, the substrate no longer fits, and the rate falls permanently. Each enzyme also has an optimum pH; extreme pH denatures it too.
Drawn from real examiner reports.
Enzymes denature, they are not killed
At high temperature an enzyme denatures: the active-site shape changes so the substrate no longer fits. An enzyme is a protein, not a living thing, so it cannot be "killed" — only cells and organisms die. Writing "the heat kills the enzyme", or "it denatures" with no mention of the active-site shape change, caps the mark.
"Speeds up" is only half the definition
A full enzyme definition has two elements: it is a biological catalyst AND it speeds up the rate of a reaction without being used up (so it can be reused). Writing only "something that speeds up reactions" omits the catalyst / not-used-up half and can score zero. Because it is not used up, never say the enzyme is turned into the product.
Starch test ≠ sugar test
Iodine solution → blue-black tests for starch; Benedict's solution + heat → brick-red tests for glucose/reducing sugar. Different reagents, different colours — do not swap them. A common slip is reaching for Benedict's, or writing "brick-red", for the starch test, when starch needs iodine → blue-black. Forgetting to heat Benedict's also loses the mark.
Lipid test needs ethanol AND water
The emulsion test for a fat/lipid uses two liquids: dissolve the sample in ethanol, THEN add an equal volume of water to give a cloudy white emulsion. The most common error is adding ethanol but forgetting the water, so no emulsion forms. Naming the reagent but the wrong result, or vice versa, earns only half the marks.
Describe vs explain the rate curve
If a question says explain the effect of temperature, give the reason, not just the pattern. "The rate rises then falls" is only a description. An explanation links cause to effect: more kinetic energy → more frequent enzyme–substrate collisions (rate up), then the enzyme denatures and the active-site shape changes (rate down). Read the command word first.
Optimum is a peak, not a ceiling
Above the optimum temperature or pH the rate decreases — it does not stay high or keep rising. On a graph the curve rises to a peak at the optimum, then falls as the enzyme denatures. Drawing a line that keeps climbing with temperature, or that simply plateaus, loses marks.
Nitrogen present = protein
Carbohydrates and lipids contain only carbon, hydrogen and oxygen; proteins also contain nitrogen (C, H, O, N). So nitrogen in a food means protein is present. Do not list nitrogen for a carbohydrate or lipid, and remember a lipid's basic units are fatty acids AND glycerol, not glucose.
Give both reagent and colour
For any "describe the test for X" question, give BOTH the reagent and the colour change: iodine → blue-black (starch); Benedict's + heat → brick-red (glucose); Biuret → lilac/purple (protein); ethanol + water → cloudy white (fat). Half an answer earns half the marks.
Use the increase → optimum → denature chain
For an enzyme rate-vs-temperature (or pH) graph, answer in three parts: below the optimum the rate increases (more kinetic energy, more collisions); at the optimum it peaks; above it the rate decreases as the enzyme denatures and the active-site shape changes.
Show working and give the unit
When a graph gives numbers you may be asked for a rate (amount of product ÷ time) or to compare rates as a percentage (part ÷ whole × 100). Show each stage for the method mark, and always give a rate its unit, e.g. cm³/min — a value with no unit loses a mark.
The three main biological molecules are carbohydrates, proteins and lipids (fats and oils). You need the elements each contains, its basic units (building blocks), the food test for each, and how enzymes work as biological catalysts.
Elements and basic units (two-element, mark-scheme form):
| Molecule | Elements | Basic units (building blocks) | Example larger molecule |
|---|---|---|---|
| Carbohydrate | Carbon, hydrogen, oxygen (C, H, O) | Simple sugars, e.g. glucose | Starch (plants), glycogen (animals) |
| Protein | Carbon, hydrogen, oxygen and nitrogen (C, H, O, N) | Amino acids | A protein (chain of amino acids) |
| Lipid (fat/oil) | Carbon, hydrogen, oxygen (C, H, O) | Fatty acids and glycerol (3 fatty acids + 1 glycerol) | A fat or oil |
Only proteins contain nitrogen — that is the quickest way to tell a protein from a carbohydrate or lipid.
The four food tests (reagent → colour change):
| Food substance | Reagent (and step) | Positive result (colour change) |
|---|---|---|
| Starch | Iodine solution | Orange/brown → blue-black |
| Glucose / reducing sugar | Benedict's solution, then heat in a water bath | Blue → brick-red (orange/green) precipitate |
| Protein | Biuret solution | Blue → lilac/purple (violet) |
| Fat (lipid) | Emulsion test: dissolve in ethanol, then add water | Forms a cloudy white emulsion |
Enzymes (biological catalysts):
Define an enzyme.
Complete the table for the three main biological molecules. (3 marks)
| Molecule | Basic unit (building block) |
|---|---|
| Carbohydrate | ? |
| Protein | ? |
| Lipid | ? |
