The plasma membrane of eukaryotic cells performs a multitude of tasks ranging from cell signaling to the transport of ions and other molecules from the extracellular matrix to the cytosol. The membrane is the result of the accumulation of lipids in a double layer. The plasma membrane is made up of two sections, an outer section called the exoplasmic layer and an inner section called the cytosolic layer. The two layers are composed of different lipids, with the exoplasmic layer containing mainly sphingolipids and the cytosolic layer containing phospholipids1. Steroid cholesterol is localized in both layers of the membrane, but appears to be more prevalent on the exoplasmic side of the membrane. Plasma membrane asymmetry is the result of differentiated synthesis. Outer membrane lipids such as sphingolipids are produced in the Golgi apparatus, while lipids such as phospholipids are produced in the ER membrane. Specialized membrane proteins called flipases also help maintain diversity between layers by transporting certain lipids to the exoplasmic side of the membrane. The distinction in lipid composition between the two membranes is supported by studies using the enzyme phospholipase which differentially cleaves certain membrane lipids. The enzyme cannot penetrate the cytosol and therefore only cleaves lipids from the exoplasmic layer, allowing the composition of the exoplasmic layers to be determined. In addition, the composition of the plasma membrane determines the fluidity of the membrane and therefore its ability to control the trafficking of ions and molecules. Plasma membrane fluidity has been divided experimentally into two categories known as the disordered liquid (ld) phase and the ordered liquid...... middle of paper ...... (FRET), the interaction particles can be determined in a cell2. The Fret technique is used to experimentally determine the clustering activity of a specific molecule in the plasma membrane. The GPI protein, a major constituent of lipid rafts, was crosslinked to a fluorescent marker. Upon clustering in a cell, as suspected in lipid rafts, the molecules fluoresce and a signal is recorded. This technique provides in vivo evidence of the existence of lipid rafts in cells. In vitro studies have supported the possible existence of lipid rafts. Biomembranes, made with a purified composition of lipids, proteins and steroids, have been shown to exhibit segregation into small domains. The presence of such segregation in membranes in vitro reinforces in vivo studies of the membrane and allows us to conclude that lipid rafts exist in the cell..