Biochemical Tests for Biological Molecules

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson covers the five core qualitative biochemical tests: Benedict's (reducing sugars), Benedict's after acid hydrolysis (non-reducing sugars), iodine in potassium iodide (starch), biuret (proteins), emulsion (lipids), plus quantitative methods using colorimetry and DCPIP (vitamin C). The lesson is the principal classroom-practical anchor of the entire module (refer to the official OCR H420 specification document for exact wording).

Biochemists and biologists use a standard set of qualitative and quantitative tests to identify biological molecules in samples. These tests form a core part of OCR A-Level Biology Practical Activity Group (PAG) requirements and are examinable both in theory papers and in written assessments of practical skills.

The reagents themselves are named after their nineteenth-century inventors and discoverers. Stanley Rossiter Benedict developed the alkaline copper-sulfate-citrate solution that bears his name in 1908 as an improvement on Fehling's solution; it remains the standard qualitative test for reducing sugars. The biuret reaction (Cu²⁺ in alkaline solution producing a purple complex with peptide groups) was named after the compound biuret (NH₂CONHCONH₂), which shows the same Cu²⁺ coordination chemistry. Iodine staining of starch was demonstrated by Bernard Courtois and Joseph Louis Gay-Lussac in the early nineteenth century, and the underlying mechanism — triiodide ion (I₃⁻) entering the amylose helix — was elucidated structurally only in the twentieth century.

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1. Overview of Biochemical Tests

Molecule	Test	Positive Result
Reducing sugars	Benedict's test	Blue → green → yellow → orange → brick-red precipitate
Non-reducing sugars	Benedict's after acid hydrolysis and neutralisation	Brick-red precipitate (after hydrolysis)
Starch	Iodine in KI solution	Yellow-brown → blue-black
Lipids	Emulsion test	Cloudy white emulsion
Proteins	Biuret test	Blue → lilac/purple

All these tests are qualitative — they tell you whether a substance is present, not precisely how much. Quantitative adaptations exist using colorimetry or reagent strips.

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2. Benedict's Test for Reducing Sugars

Principle

Reducing sugars (e.g., glucose, fructose, maltose, lactose, galactose) have a free aldehyde or ketone group that can reduce Cu²⁺ ions in Benedict's solution to Cu⁺, forming an insoluble brick-red precipitate of copper(I) oxide (Cu₂O).

graph TD
  A["Benedict's reagent (blue, Cu²⁺)"] --> C["Heat 80–90 °C"]
  B["Reducing sugar (RCHO)"] --> C
  C --> D["Cu₂O — brick-red precipitate"]
  C --> E["Oxidised sugar product"]

Procedure

1. Place approximately 2 cm³ of the sample solution into a test tube.
2. Add an equal volume of Benedict's solution (blue).
3. Heat in a water bath at 80–90 °C for 3–5 minutes.
4. Observe any colour change.

Interpretation

The intensity of colour and the amount of precipitate depend on the concentration of reducing sugar:

Colour	Approximate reducing sugar concentration
Blue	None
Green	Very low (trace)
Yellow	Low
Orange	Medium
Brick-red	High

Exam Tip: The test is semi-quantitative — the colour intensity reflects the amount of reducing sugar, but the test is not very precise. For accurate quantitative results, use colorimetry.

Quantitative Benedict's Test Using a Colorimeter

For reliable quantitative results:

1. Prepare a series of standard solutions of glucose at known concentrations (e.g., 0, 2, 4, 6, 8, 10 mg/cm³).
2. Perform Benedict's test on each standard, ensuring identical volumes, timings and temperatures.
3. Filter the Cu₂O precipitate and measure the absorbance of the remaining solution using a colorimeter with a red filter (complementary to blue).
4. As more Cu²⁺ is reduced, the blue colour fades — absorbance of red light decreases as [glucose] increases.
5. Plot a calibration curve of absorbance against glucose concentration.
6. Perform the test on the unknown sample under identical conditions and read its concentration from the calibration curve.

An alternative quantitative method centrifuges or filters the Cu₂O precipitate, dries it, and weighs it — the mass of precipitate is proportional to the reducing sugar concentration.

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3. Testing for Non-Reducing Sugars

Sucrose is the classic non-reducing sugar (both anomeric carbons are locked in the glycosidic bond). It gives a negative Benedict's test directly. To detect non-reducing sugars:

Procedure

1. Perform the Benedict's test on the sample — if it is negative, proceed.
2. Take a fresh sample and add dilute hydrochloric acid (e.g., 2 cm³ of 1 mol dm⁻³ HCl).
3. Heat in a water bath at 80 °C for 5 minutes — this hydrolyses glycosidic bonds into the component monosaccharides.
4. Neutralise with sodium hydrogencarbonate (NaHCO₃) until fizzing stops and the solution is no longer acidic (test with litmus or universal indicator). This step is critical — Benedict's reagent only works in alkaline conditions.
5. Repeat Benedict's test on the neutralised sample.
6. A positive result (brick-red) now indicates the original sample contained a non-reducing sugar.

Exam Tip: Common mistakes in this test include forgetting to neutralise after hydrolysis (Benedict's reagent is destroyed by acid), and not running a control showing the sample gives a negative direct Benedict's test first.

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4. Iodine Test for Starch

Principle

Iodine (in potassium iodide solution, forming the triiodide ion I₃⁻) binds to the helical structure of amylose in starch. The iodine molecules fit inside the helix and form a blue-black charge-transfer complex.

Procedure

1. Place a drop of the sample on a white tile or in a test tube/well of a spotting tile.
2. Add 1–2 drops of iodine in potassium iodide solution (often called "iodine solution" — yellow-brown).
3. Observe the colour.

Results

Colour	Interpretation
Yellow-brown (no change)	No starch
Blue-black	Starch present
Red-brown	Glycogen present (shorter helical segments)
Purple	Amylopectin (branched, partial helices)

The test is not quantitative in its standard form. The intensity of the blue-black colour can be measured with a colorimeter for semi-quantitative results.

Applications

• Testing leaves for starch after photosynthesis (leaf first decolourised in boiling ethanol to remove chlorophyll).
• Monitoring starch digestion in enzyme investigations (e.g., amylase activity — starch gradually disappears as the blue-black colour fades to yellow-brown).

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5. Biuret Test for Proteins

Principle

In the Biuret test, Cu²⁺ ions (in alkaline solution) form a coloured coordination complex with peptide bonds. The complex has a characteristic lilac/purple colour. Because the test detects peptide bonds, it is positive for all peptides of three or more amino acids.

Procedure

1. Add an equal volume of biuret reagent (or separately: dilute sodium hydroxide followed by dilute copper(II) sulfate solution) to the sample.
2. Mix gently — no heating is required.
3. Observe the colour after a few minutes.

Results

• Blue — no protein present.
• Lilac/purple — protein (or peptides ≥ 3 amino acids) present.

Notes

• The test is qualitative but can be made quantitative using a colorimeter. The intensity of the purple colour is proportional to protein concentration; a calibration curve with protein standards (e.g., albumin) allows unknown concentrations to be determined.
• Single amino acids and dipeptides give a negative Biuret test because they do not have enough peptide bonds.
• The test detects the peptide bond itself, so it cannot distinguish different proteins.

Quantitative Biuret Test

1. Prepare a series of protein standards at known concentrations (e.g., albumin at 0–10 mg/cm³).
2. Add the same volume of biuret reagent to each.
3. Wait 5 minutes for colour development.
4. Measure absorbance in a colorimeter at ~540 nm (green filter, complementary to purple).
5. Plot a calibration curve.
6. Measure unknown samples under identical conditions and read the concentration from the curve.

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6. Emulsion Test for Lipids

Principle

Lipids are insoluble in water but soluble in ethanol. In the emulsion test, the sample is first dissolved in absolute ethanol. Water is then added, causing the lipid to come out of solution as tiny droplets that scatter light, producing a cloudy white emulsion.

Procedure

1. Place approximately 2 cm³ of the sample in a test tube.
2. Add 2 cm³ of absolute (100%) ethanol. Shake to dissolve any lipids.
3. Add 2 cm³ of cold water and mix.
4. Observe the appearance of the solution.

Results

• Clear solution — no lipids present.
• Cloudy white emulsion — lipids present.

The test is qualitative only. Its sensitivity depends on the quantity and type of lipid.

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7. Colorimetry — Principles

A colorimeter is an instrument that measures the amount of light absorbed by a solution at a chosen wavelength. The basic principle is the Beer-Lambert law: at low concentrations, absorbance is directly proportional to concentration (and to path length).

Workflow

1. Choose the correct filter/wavelength — select the colour complementary to the colour of the solution being measured (i.e., the colour that the solution absorbs most strongly).
   • Blue solution → red filter (absorbs red)
   • Purple solution → green filter
   • Red-brown Cu₂O precipitate → blue filter for Benedict's
2. Zero the colorimeter with a blank (solvent only, or water).
3. Prepare standards at known concentrations.
4. Measure absorbance of each standard.
5. Plot a calibration curve of absorbance (y-axis) against concentration (x-axis) — ideally a straight line through the origin at low concentrations.
6. Measure the unknown and read its concentration from the curve.

Advantages of Colorimetry

• Converts qualitative tests into quantitative ones.
• Objective — removes observer bias in judging colours.
• Reproducible and precise.
• Can detect small differences in concentration.

Limitations

• Requires a calibration curve with standards.
• Only linear over a limited concentration range (Beer-Lambert deviations at high concentrations).
• Turbid samples (containing suspended particles) may scatter light, giving inaccurate absorbance readings.

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8. Summary Flow Chart

graph TD
  S[Unknown sample]
  S --> B1[Benedict's test]
  B1 -->|brick red| R1[Reducing sugar present]
  B1 -->|blue| H[Acid hydrolyse, neutralise, retest]
  H -->|brick red| R2[Non-reducing sugar present]
  H -->|blue| R3[No sugar]
  S --> I[Iodine test]
  I -->|blue-black| R4[Starch]
  I -->|yellow-brown| R5[No starch]
  S --> BI[Biuret test]
  BI -->|purple| R6[Protein]
  BI -->|blue| R7[No protein]
  S --> E[Emulsion test]
  E -->|cloudy white| R8[Lipid]
  E -->|clear| R9[No lipid]

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9. Practical Skills Summary

For OCR practical assessments, you should be able to:

• Perform each test correctly, with appropriate safety precautions (goggles, care with heated solutions, dilute acids and alkalis).
• Describe a suitable control (e.g., distilled water treated identically to verify no contamination).
• Identify potential sources of error (e.g., insufficient heating, failure to neutralise, incorrect volumes, poor timing).
• Suggest improvements (e.g., repeat measurements, use a colorimeter, standardise temperature with a water bath).
• Plot and interpret a calibration curve.
• Calculate a concentration from a calibration curve.

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Exam Tips

• For Benedict's test, state that the reagent contains Cu²⁺ and that the red precipitate is Cu₂O (copper(I) oxide).
• For non-reducing sugars, always include the neutralisation step before retesting.
• For the iodine test, name the reagent fully: "iodine in potassium iodide solution".
• For Biuret, state that the test detects peptide bonds — that is why it is positive for any protein.
• Mention colorimetry whenever a quantitative result is needed, and describe the calibration curve approach.

Common Exam Mistakes

• Calling Benedict's solution "red" (it is blue before reaction).
• Forgetting to neutralise after acid hydrolysis of non-reducing sugars.
• Saying Biuret test "detects proteins" without specifying peptide bonds.
• Writing "blue copper sulfate" as the whole biuret reagent — it also contains sodium hydroxide (alkaline conditions are essential).
• Saying iodine "stains starch" — it binds inside the amylose helix to form a blue-black complex.
• Forgetting that the emulsion test uses ethanol first, then water.
• Confusing "qualitative" and "quantitative" tests.

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10. DCPIP and Vitamin C Quantification

DCPIP (2,6-dichlorophenolindophenol) is a blue dye that is decolourised to colourless on reduction. Vitamin C (ascorbic acid) is a reducing agent that decolourises DCPIP rapidly. The titration-style quantitative test is a standard OCR PAG: titrate a known volume of DCPIP solution with the unknown sample until decolourised, and against standard vitamin-C solutions (e.g. 0–1.0 mg/cm³); the volume of unknown needed for decolourisation is inversely related to its vitamin-C concentration. Comparison with a calibration curve gives the unknown's vitamin C content. This is a classical school-level demonstration of titrimetric quantitative biochemistry. The same DCPIP reduction is also used in the Hill reaction to measure photosynthetic electron transport in isolated chloroplasts (covered in Module 5.2).

11. Chromatography for Biological Molecules

Paper chromatography and thin-layer chromatography (TLC) separate molecules by differential partitioning between a stationary phase (paper or silica) and a mobile phase (solvent mixture). Each compound has a characteristic Rf value (retention factor) given by:

$R_f = \frac{\text{distance moved by compound}}{\text{distance moved by solvent front}}$Rf​=distance moved by solvent frontdistance moved by compound​

Applications:

• Amino-acid separation — butanol/acetic-acid/water solvent on chromatography paper, visualised by ninhydrin spray (turns amino acids purple).
• Photosynthetic pigment separation — leaf extract on TLC plate, with petroleum ether or acetone solvent; chlorophyll a (blue-green), chlorophyll b (yellow-green), carotenes (yellow-orange), xanthophylls (yellow) all give distinct Rf values.
• Sugar separation — monosaccharides resolved with butanol/ethanol/water solvent.

Rf values are reproducible under identical conditions and allow positive identification of compounds by comparison with known standards. This is the OCR PAG 6 anchor.

12. Comprehensive Test-Reagent-Result Table