--- trunk/tengDissertation/Lipid.tex	2006/04/24 18:49:32	2730
+++ trunk/tengDissertation/Lipid.tex	2006/04/24 18:50:41	2731
@@ -2,6 +2,45 @@
 
 \section{\label{lipidSection:introduction}Introduction}
 
+Under biologically relevant conditions, phospholipids are solvated
+in aqueous solutions at a roughly 25:1 ratio. Solvation can have a
+tremendous impact on transport phenomena in biological membranes
+since it can affect the dynamics of ions and molecules that are
+transferred across membranes. Studies suggest that because of the
+directional hydrogen bonding ability of the lipid headgroups, a
+small number of water molecules are strongly held around the
+different parts of the headgroup and are oriented by them with
+residence times for the first hydration shell being around 0.5 - 1
+ns.[14] In the second solvation shell, some water molecules are
+weakly bound, but are still essential for determining the properties
+of the system. Transport of various molecular species into living
+cells is one of the major functions of membranes. A thorough
+understanding of the underlying molecular mechanism for solute
+diffusion is crucial to the further studies of other related
+biological processes. All transport across cell membranes takes
+place by one of two fundamental processes: Passive transport is
+driven by bulk or inter-diffusion of the molecules being transported
+or by membrane pores which facilitate crossing. Active transport
+depends upon the expenditure of cellular energy in the form of ATP
+hydrolysis. As the central processes of membrane assembly,
+translocation of phospholipids across membrane bilayers requires the
+hydrophilic head of the phospholipid to pass through the highly
+hydrophobic interior of the membrane, and for the hydrophobic tails
+to be exposed to the aqueous environment.[18] A number of studies
+indicate that the flipping of phospholipids occurs rapidly in the
+eukaryotic ER and the bacterial cytoplasmic membrane via a
+bi-directional, facilitated diffusion process requiring no metabolic
+energy input. Another system of interest would be the distribution
+of sites occupied by inhaled anesthetics in membrane. Although the
+physiological effects of anesthetics have been extensively studied,
+the controversy over their effects on lipid bilayers still
+continues. Recent deuterium NMR measurements on halothane in POPC
+lipid bilayers suggest the anesthetics are primarily located at the
+hydrocarbon chain region.[16] Infrared spectroscopy experiments
+suggest that halothane in DMPC lipid bilayers lives near the
+membrane/water interface. [17]
+
+
 \section{\label{lipidSection:model}Model}
 
 \section{\label{lipidSection:methods}Methods}
