<P> Phospholipids are optically highly birefringent, i.e. their refractive index is different along their axis as opposed to perpendicular to it . Measurement of birefringence can be achieved using cross polarisers in a microscope to obtain an image of e.g. vesicle walls or using techniques such as dual polarisation interferometry to quantify lipid order or disruption in supported bilayers . </P> <P> There are no simple methods available for analysis of phospholipids since the close range of polarity between different phospholipid species makes detection difficult . Oil chemists often use spectroscopy to determine total Phosphorus abundance and then calculate approximate mass of phospholipids based on molecular weight of expected fatty acid species . Modern lipid profiling employs more absolute methods of analysis, with nuclear magnetic resonance spectroscopy (NMR spectroscopy), particularly P - NMR, while HPLC - ELSD provides relative values . </P> <P> Phospholipid synthesis occurs in the cytosole adjacent to ER membrane that is studded with proteins that act in synthesis (GPAT and LPAAT acyl transferases, phosphatase and choline phosphotransferase) and allocation (flippase and floppase). Eventually a vesicle will bud off from the ER containing phospholipids destined for the cytoplasmic cellular membrane on its exterior leaflet and phospholipids destined for the exoplasmic cellular membrane on its inner leaflet . </P> <P> Common sources of industrially produced phospholipids are soya, rapeseed, sunflower, chicken eggs, bovine milk, fish eggs etc . Each source has a unique profile of individual phospholipid species and consequently differing applications in food, nutrition, pharmaceuticals, cosmetics and drug delivery . </P>

Phosphatidyl choline is the most common type of phospholipid