Testing for biological molecules

Testing for biological molecules

Testing for Carbohydrates

  • Benedict’s test: Used to identify reducing sugars. The presence of reducing sugars in a solution will turn Benedict’s solution from blue to green, yellow or red upon heating.
  • Non-reducing Sugar Test: Additional testing for sugars that do not directly react with Benedict’s reagent, such as sucrose. The sample must first be hydrolysed with dilute hydrochloric acid, then neutralized with sodium hydrogencarbonate, before the Benedict’s test can be applied.
  • Iodine test for Starch: The iodine solution reacts with starch to produce a blue-black colour. If the solution remains brown, this indicates absence of starch.

Testing for Proteins

  • Biuret Test: Detects the presence of peptide bonds in proteins. A sample is mixed with Biuret reagent (a solution of sodium hydroxide and copper sulphate). The solution turns lilac/purple in the presence of protein.

Testing for Lipids

  • Sudan III Test: Lipids can be tested by shaking the substance with Sudan III stain. The stain would dissolve in the lipids and leaves a reddish-orange colouration.
  • Emulsion Test: The substance is mixed with ethanol, then water is added. If a white emulsion forms, this indicates the presence of lipids.

Testing for Nucleic Acids

  • Dische’s Diphenylamine Test: This test is used to detect the presence of DNA. The substance is heated with Dische’s reagent. A blue colour indication confirms the presence of DNA.

Important Precautions

  • Safety gear: Always remember to put on safety gear, such as gloves and protective goggles, to protect against any possible harm from the chemicals used.
  • Identify false positives: Be aware of substances that may give false positive results. For example, Vitamin C can reduce copper in the Benedict’s test, leading to a positive result even in the absence of sugar.
  • Null result confusion: Absence of a positive test does not mean total absence of a substance. For example, a negative test for the lipid-emulsion test may not equate to the total absence of lipids, but rather the absence of triacylglycerols.
  • Accurate recording: Ensure to accurately record your observations as soon as they are made, as some colour changes may be temporary.