Revise Testing for biological molecules for Cambridge 9700 (2.1)

Biochemical tests rely on specific chemical reactions that produce easily observable changes, most commonly colour changes or precipitate formation. A reagent is a substance added to cause such a reaction. An indicator is a special type of reagent that changes colour in the presence of a specific substance or condition (like pH). For each test, you must know the correct reagents, the required conditions (e.g., temperature), and the distinct colour changes that signify a positive or negative result.

Reducing Sugars (e.g., glucose, fructose, maltose) The Benedict's test is used to detect reducing sugars. These sugars have a free aldehyde or ketone group which can donate electrons, reducing the blue copper(II) ions (Cu²⁺) in Benedict's solution to red copper(I) ions (Cu⁺), which form an insoluble precipitate of copper(I) oxide (Cu₂O).

• Reagent: Benedict's solution (an alkaline solution of copper(II) sulfate).
• Procedure:
  1. Add an equal volume of Benedict's solution to the sample.
  2. Heat the mixture in a water bath at 80-100°C for 5-10 minutes.
• Observation:
  • Negative: Stays blue (colour of Benedict's solution).
  • Positive: Colour changes from blue through green, yellow, orange, and finally to a brick-red precipitate. The final colour indicates the concentration of reducing sugar present.

Non-reducing Sugars (e.g., sucrose) Non-reducing sugars do not have a free aldehyde or ketone group and thus cannot react directly with Benedict's solution. They must first be hydrolysed (broken down) into their constituent reducing monosaccharides.

• Reagent: Dilute hydrochloric acid (HCl), sodium hydroxide (NaOH) or sodium hydrogencarbonate, Benedict's solution.
• Procedure:
  1. First, perform the standard Benedict's test to confirm the absence of reducing sugars. If it is negative (stays blue), proceed.
  2. Add a few drops of dilute HCl to a fresh sample.
  3. Heat the mixture in a water bath for 5 minutes (to hydrolyse the non-reducing sugar into reducing sugars).
  4. Neutralise the sample by adding sodium hydroxide (NaOH) or sodium hydrogencarbonate until it is alkaline (this is crucial as Benedict's reagent requires an alkaline medium to work).
  5. Perform the standard Benedict's test: add Benedict's solution and heat in a water bath at 80-100°C for 5-10 minutes.
• Observation:
  • Negative: Stays blue (if no non-reducing sugars were present or not fully hydrolysed).
  • Positive: Colour changes from blue through green, yellow, orange, and finally to a brick-red precipitate (due to the presence of the now-hydrolysed reducing sugars).

Starch Starch is a polysaccharide that forms a helical structure. Iodine molecules fit into this helix, forming a starch-iodine complex that has a characteristic blue-black colour.

• Reagent: Iodine solution in potassium iodide.
• Procedure:
  1. Add a few drops of iodine solution directly to the sample at room temperature.
• Observation:
  • Negative: Stays yellow-brown (colour of iodine solution).
  • Positive: Changes to blue-black.

The Benedict's test can be used semi-quantitatively. The observed colour provides an estimate of the concentration of reducing sugar:

• Blue: None
• Green: Trace amounts
• Yellow: Low concentration
• Orange: Medium concentration
• Brick-red: High concentration

For a fully quantitative result, a colorimeter is used. After performing the Benedict's test, the precipitate is removed (e.g., by centrifugation or filtration). The colorimeter then measures the absorbance of the remaining blue supernatant. A higher concentration of reducing sugar will produce more precipitate, leaving less blue copper(II) sulfate in the solution, resulting in a lower absorbance reading. By testing a series of known concentrations (standards), a calibration curve can be plotted (Absorbance vs. Concentration), which can then be used to determine the concentration of an unknown sample.

Lipids (e.g., fats, oils) Lipids are non-polar molecules, making them insoluble in water but soluble in organic solvents like ethanol. The emulsion test relies on this property.

• Reagent: Ethanol, Distilled water.
• Procedure:
  1. Add 2 cm³ of ethanol to a small amount of the sample in a dry, grease-free test tube.
  2. Shake thoroughly to dissolve any lipids present.
  3. Carefully decant the ethanol (leaving any undissolved material behind) into a test tube containing about 2 cm³ of distilled water.
• Observation:
  • Negative: Solution remains clear.
  • Positive: A milky white emulsion forms. This is because the lipid, which was dissolved in the ethanol, is forced out of solution when mixed with water, forming tiny, suspended droplets that scatter light and appear cloudy/milky.

Proteins The Biuret test detects the presence of peptide bonds. Since a protein is a polypeptide, it contains many peptide bonds. The test requires at least two peptide bonds to give a positive result.

• Reagents: Potassium hydroxide (or sodium hydroxide) solution, copper(II) sulfate solution.
• Procedure:
  1. Add an equal volume of potassium hydroxide solution (or NaOH) to the sample to make it alkaline.
  2. Add a few drops of dilute copper(II) sulfate solution and mix gently.
• Observation:
  • Negative: Stays blue (colour of copper(II) sulfate solution).
  • Positive: Changes to lilac or purple. The colour is due to the formation of a complex between copper(II) ions and the nitrogen atoms in the peptide bonds.

When performing these biochemical tests, it is crucial to follow safety guidelines. Wear eye protection at all times. Benedict's solution, acids (HCl), and alkalis (NaOH) are corrosive or irritant; handle them with care. When heating solutions in a water bath, use a test tube holder and point the test tube away from yourself and others. Ethanol is flammable, so keep it away from naked flames.