Carbohydrate Tests in Biochemistry

Carbohydrate Tests

• Biuret test: detects peptide bonds in proteins.

  • A violet color indicates a positive result for protein.
  • A blue solution indicates no protein, or the presence of carbohydrates.

• Molisch test: detects the presence of carbohydrates.

  • A violet ring forms between the solutions if carbohydrates are present.
  • The test relies on the dehydration of sugars by concentrated sulfuric acid, forming furfural or hydroxymethyl furfural, which then reacts with α-naphthol to produce a colored product.

• Iodine test: distinguishes between simple and complex sugars.

  • A yellow solution with no color change indicates a simple sugar (mono- or disaccharide).
  • A change from yellow to a deep blue color indicates the presence of starch (a complex sugar).

• Benedict test: identifies reducing sugars.

  • A red precipitate indicates a reducing sugar (maltose, lactose, monosaccharides).
  • No red precipitate indicates a non-reducing sugar (sucrose).
  • The test works by reducing soluble blue copper sulfate (Cu²⁺ ions) to insoluble red-brown copper oxide (Cu⁺) in the presence of reducing sugars.
  • Benedict’s reagent is more sensitive than Fehling’s reagent.

• Barfoed test: differentiates between mono- and disaccharides.

  • A red ring or precipitate forming within 5 minutes indicates a monosaccharide.
  • No red precipitate indicates a disaccharide.
  • The test relies on the reaction of cupric acetate in dilute acetic acid with monosaccharides, producing cuprous oxide faster than with disaccharides.

Ketone Group Tests

• Ketose test: detects the presence of a ketone group in sugars.
• Seliwanof test: another test for the presence of a ketone group, particularly fructose.
  • A deep red color indicates the presence of a ketone group, specifically fructose.
  • No deep red color indicates the absence of a ketone group, such as in glucose.
  • The tests rely on the reaction of the ketone group with the reagent.

Protein Tests

• Heat coagulation test: differentiates coagulable and non-coagulable proteins.

  • A white opacity forming in the upper part of the tube indicates a coagulable protein, like albumin.
  • No opacity indicates a non-coagulable protein, like casein or peptone.

• Heller's test: detects coagulable proteins.

  • A white ring forming at the junction of the two solutions indicates a coagulable protein, like albumin or globulin.
  • No ring indicates non-coagulable proteins, like casein or peptone.

• Acidification test: differentiates between casein and peptone.

  • A white precipitate after the addition of acetic acid indicates the presence of casein.
  • No precipitate indicates the presence of peptone.

• Litmus paper test: differentiates between casein and peptone.

  • A blue litmus paper indicates the presence of casein, as casein is alkaline and will convert red litmus paper to blue.
  • A red litmus paper indicates the presence of peptone.

This quiz covers the various tests used to identify carbohydrates in biochemistry, including the Biuret, Molisch, Iodine, and Benedict tests. Each test's methodology and resulting indicators are explained, revealing how to distinguish between different types of sugars and proteins. Test your knowledge and understanding of these essential biochemical reactions!