Biological molecules and food tests
The elements and units that build carbohydrates, proteins and fats, and how to test for each.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 4.1.1State the elements present in carbohydrates, fats and proteins and their smaller basic units.
- 4.1.2Describe the food tests for starch, reducing sugars, protein, fats and vitamin C.
5 minute read
Biological molecules and food tests
Three groups of large molecules make up most of what you eat and most of what you are.
Elements and building blocks
| Molecule | Elements | Built from |
|---|---|---|
| Carbohydrate | C, H, O | simple sugars such as glucose |
| Fat (lipid) | C, H, O | fatty acids and glycerol |
| Protein | C, H, O, N (sometimes S) | amino acids |
The one to remember is nitrogen: it appears in proteins and not in carbohydrates or fats. This is why plants need nitrate ions to make protein, and why a nitrogen-deficient plant grows poorly. Large carbohydrates are built by joining simple sugars: starch and glycogen are storage molecules, cellulose builds plant cell walls.
The food tests
Each has a reagent, a condition, and a colour change. All four are common exam questions:
- Starch — add iodine solution. Brown-orange → blue-black.
- Reducing sugar — add Benedict's solution and heat in a water bath. Blue → green → yellow → brick red (the further it goes, the more sugar).
- Protein — add biuret solution. Blue → purple/violet.
- Fat — the ethanol emulsion test: dissolve in ethanol, then add water. A white/cloudy emulsion forms.
- Vitamin C — add the food to DCPIP; the blue dye is decolourised.
Think of it like this
The food tests are four different keys for four different locks. Using Benedict's on a protein tells you nothing at all — it is not a general "food detector", each reagent answers exactly one question.
Worked examples
Method, step by step
A student tests a food sample. Iodine gives no colour change, Benedict's gives a brick-red precipitate, and biuret turns purple. What does the sample contain?
- 1No colour change with iodine means no starch present.
- 2Brick-red with Benedict's means reducing sugar is present, in a high concentration.
- 3Purple with biuret means protein is present.
The sample contains reducing sugar and protein, but no starch.
Common misconceptions
- Forgetting that Benedict's test requires heating. Without a hot water bath there is no colour change and the answer scores nothing.
- Saying the biuret test goes "blue to black" — it is blue to purple/violet.
- Thinking all sugars give a positive Benedict's result. Sucrose is a non-reducing sugar and gives no change without prior acid hydrolysis.
In the exam
- Always give the starting colour as well as the final colour. "Turns blue-black" is safer than "turns black", and many mark schemes require the change, not just the end point.
- For Benedict's, mentioning that the final colour indicates *how much* sugar is present often picks up an extra mark in a quantitative question.