Describe and be able to do chemical tests for: (a) starch (iodine solution) (b) glucose and maltose (Benedict’s solution) (c) protein (biuret test) (d) lipids (ethanol emulsion test)

Sourav PanNovember 2, 2024

Answered

To test for the presence of specific macromolecules in biological samples, various chemical tests can be employed. Here’s a detailed description of the tests for starch, glucose and maltose, proteins, and lipids, including their procedures and expected results.

(a) Starch Test (Iodine Solution)

Principle: Iodine solution reacts with starch to produce a blue-black color.Procedure:

1. Take a small sample of the substance to be tested (e.g., a piece of food or a solution).
2. Add a few drops of iodine solution (iodine dissolved in potassium iodide) directly onto the sample.
3. Observe any color change.

Expected Results:

• Positive Result: A blue-black color indicates the presence of starch.
• Negative Result: No color change (remains brown or yellow) indicates that starch is absent.

(b) Glucose and Maltose Test (Benedict’s Solution)

Principle: Benedict’s solution detects reducing sugars, such as glucose and maltose, by changing color when heated.Procedure:

1. Place 2 mL of the sample solution in a test tube.
2. Add 2 mL of Benedict’s solution to the test tube.
3. Heat the mixture in a boiling water bath for about 5 minutes.
4. Allow it to cool and observe any color change.

Expected Results:

• Positive Result: The solution changes color from blue to green, yellow, orange, or red, depending on the concentration of reducing sugars present.
• Negative Result: No color change; the solution remains blue indicates that reducing sugars are absent.

(c) Protein Test (Biuret Test)

Principle: The Biuret test detects proteins by forming a purple complex with copper ions in an alkaline solution.Procedure:

1. Prepare your sample by dissolving it in distilled water if necessary.
2. Add an equal volume of Biuret reagent (which contains sodium hydroxide and copper(II) sulfate) to the sample.
3. Mix gently and let it stand for about 5 minutes.
4. Observe any color change.

Expected Results:

• Positive Result: A color change to purple indicates the presence of peptide bonds (and thus proteins).
• Negative Result: No color change; the solution remains blue indicates that proteins are absent.

(d) Lipid Test (Ethanol Emulsion Test)

Principle: The ethanol emulsion test identifies lipids based on their ability to form an emulsion with ethanol.Procedure:

1. Take a small amount of the sample and place it in a clean test tube.
2. Add about 2-3 mL of ethanol to the sample and shake vigorously for about 1 minute to dissolve any lipids present.
3. After shaking, add an equal volume of distilled water to the test tube without shaking.
4. Observe any changes in appearance.

Expected Results:

• Positive Result: A milky emulsion forms at the interface between water and ethanol, indicating the presence of lipids.
• Negative Result: The solution remains clear; no emulsion forms indicates that lipids are absent.