Biochemical Tests: Lipids (OCR A Level Biology): Revision Note

Biochemical Tests for Lipids

Identifying biological molecules

• Different qualitative reagents can be used to identfy the presence of biological molecules in samples

  • Ethanol is used to identify lipids

  • Biuret reagent is used to identify proteins

  • Benedicts solution and iodine are used to identify carbohydrates

• Qualitative reagents simply determine whether or not a substance is present in a sample

• The quantity or concentration of the substance present is not determined

Practical: the emulsion test for lipids

• The emulsion test can be carried out quickly and easily in a lab to determine if a sample contains lipids

• Lipids are nonpolar molecules that do not dissolve in water but will dissolve in organic solvents such as ethanol

Apparatus

• Test tubes

• Test tube rack

• Ethanol

• Pipettes

• Food sample

• Mortar and pestle (if food sample is solid)

• Water

• Gloves

Method

• Add ethanol to the sample to be tested

• Shake to mix

• Add the mixture to a test tube of water

The Emulsion test for lipids forms a milky colour

Results

• If lipids are present, a milky emulsion will form (the solution appears ‘cloudy’); the more lipid present, the more obvious the milky colour of the solution

• If no lipid is present, the solution remains clear

Limitations

• This test is qualitative - it does not give a quantitative value as to how much lipid may be present in a sample