OCR AS Biology

Revision Notes

1.2.11 Biochemical Tests: Lipids

Biochemical Tests: 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

2.1 Emulsion test_2, downloadable AS & A Level Biology revision notesThe 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
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