The Plasma Membrane of Eukaryotic Cells

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The plasma membrane of eukaryotic cells performs a multitude of tasks ranging from cell signaling to transport of ions and other molecules from the extracellular matrix into the cytosol. The membrane is the result of lipid packing into a bi-layer. The plasma membrane is composed of two sections, an outer section known as the exoplasmic layer, and an inner section known as the cytosolic layer. The two layers are composed of differing lipids with the exoplasmic layer containing primarily sphingolipids and the cytosolic layer containing phospholipids1. The steroid cholesterol is located throughout both layers of the membrane, yet appears to be more prevalent on the exoplasmic face of the membrane. The asymmetry of the plasma membrane is the result of differentiated synthesis. Outer membrane lipids such as sphingolipids are produced in Golgi apparatus while lipids such as phospholipids are produced in the ER membrane. Specialized proteins in the membrane known as flipases also help maintain the diversity between the layers by transporting certain lipids to the exoplasmic face of the membrane. The distinction in lipid composition between the two membranes is supported through studies using the enzyme phospholipase which differentially cleaves certain membrane lipids. The enzyme cannot permeate into the cytosol and thus only cleaves the exoplasmic layer lipids, allowing for the determination of the exoplasmic layers composition. Moreover the composition of the plasma membrane determines the fluidity of the membrane and thus its ability to control the trafficking of ions and molecules. The fluidity of the plasma membrane has been experimentally divided into two categories known as the liquid disordered(ld) phase and the liquid ordered... ... middle of paper ... ... (FRET), particle interaction can be determined within a cell2. The Fret technique is used to experimentally determine the clustering activity of a specific molecule in the plasma membrane. GPI protein, a major constituent of lipid rafts, was cross-linked to a fluorescent marker. Upon clustering within a cell, as suspected within lipid rafts, the molecules will fluoresce and a signal will be recorded. This technique provides in vivo evidence for the existence of lipid rafts in cells. In vitro studies have provided support for the possible existence of lipid rafts. Biomembranes, manufactured with purified lipid, protein and steroid composition, have been shown to show segregation into small domains. The presence of such segregation in in vitro membranes reinforces the in vivo studies of the membrane and helps lead to the conclusion that lipid rafts exist in the cell

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