<P> The plasma membranes of cells contain combinations of glycosphingolipids and protein receptors organised in glycolipoprotein microdomains termed lipid rafts . These specialised membrane microdomains compartmentalise cellular processes by serving as organising centers for the assembly of signaling molecules, influencing membrane fluidity and membrane protein trafficking, and regulating neurotransmission and receptor trafficking . Lipid rafts are more ordered and tightly packed than the surrounding bilayer, but float freely in the membrane bilayer . Although more common in plasma membrane, lipid rafts have also been reported in other parts of the cell, such as the Golgi apparatus and lysosomes . </P> <P> One key difference between lipid rafts and the plasma membranes from which they are derived is lipid composition . Research has shown that lipid rafts generally contain 3 to 5-fold the amount of cholesterol found in the surrounding bilayer . Also, lipid rafts are enriched in sphingolipids such as sphingomyelin, which is typically elevated by 50% compared to the plasma membrane . To offset the elevated sphingolipid levels, phosphatidylcholine levels are decreased which results in similar choline - containing lipid levels between the rafts and the surrounding plasma membrane . Cholesterol interacts preferentially, although not exclusively, with sphingolipids due to their structure and the saturation of the hydrocarbon chains . Although not all of the phospholipids within the raft are fully saturated, the hydrophobic chains of the lipids contained in the rafts are more saturated and tightly packed than the surrounding bilayer . Cholesterol is the dynamic "glue" that holds the raft together . Due to the rigid nature of the sterol group, cholesterol partitions preferentially into the lipid rafts where acyl chains of the lipids tend to be more rigid and in a less fluid state . One important property of membrane lipids is their amphipathic character . Amphipathic lipids have a polar, hydrophilic head group and a non-polar, hydrophobic region . The figure to the right shows the inverted cone - like shape of sphingomyelin and the cone - like shape of cholesterol based on the area of space occupied by the hydrophobic and hydrophilic regions . It should be noted that cholesterol has the ability to pack in between the lipids in rafts, serving as a molecular spacer and filling any voids between associated sphingolipids . </P> <P> Rietveld & Simons related lipid rafts in model membranes to the immiscibility of ordered (Lo phase) and disordered (Ld or Lα phase) liquid phases . The cause of this immiscibility is uncertain, but the immiscibility is thought to minimize the free energy between the two phases . Studies have shown there is a difference in thickness of the lipid rafts and the surrounding membrane which results in hydrophobic mismatch at the boundary between the two phases . This phase height mismatch has been shown to increase line tension which may lead to the formation of larger and more circular raft platforms to minimize the energetic cost of maintaining the rafts as a separate phase . Other spontaneous events, such as curvature of the membrane and fusing of small rafts into larger rafts, can also minimize line tension . </P>

Name or list three components of a lipid raft
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