OCR A Level Biology:复习笔记1.2.11 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

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