Integrated SequencePhysics Chemistry Organic Biology

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Special points of emphasis

Electricity

The Chemical Bond

Intermolecular Forces

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Water

Lipids

Biological Membranes

The three major membrane lipids are phospholipids, glycolipids, and cholesterol. These types are all amphipathic, containing both hydrophilic and hydrophobic moeities. Phospholipids and glycolipids spontaneously form bimolecular sheets in aqueous solution. The formation of such a bilayer maximizes the access of polar molecules for other polar molecules, water for itself and water for the polar heads of the lipid molecules. London dispersion forces enable close packing of the hydrocarbon tails. In thermodynamic terms, the bilayer is a favored state of the system because it represents a minimum of electrostatic potential energy, internal energy, enthalpy, and free energy. A water molecule is released from the interior of the bilayer because the charge densities represented by its poles are in a higher potential energy state in the midst of a nonpolar environment. These thermodynamic factors minimize the exposure of hydrocarbon tales to water and enable lipid bilayers to form extensive sheets that eventually close in on themselves to prevent there being edges to the sheet. In other words, an aqueous emulsion of phospholipid tends spontaneously toward segregation into compartmentalized spaced separated by the semipermeable bilayer.



Fluid Mechanics

Intermolecular Forces

Lipids

Biological Membranes

The viscosity of a bilayer membrane depends on whether the fatty acid chains are stacked in a rigid state or exist in a relatively disordered, fluid state. In higher organisms (not bacteria), cholesterol also plays a major roll in moderating membrane fluidity. The two characteristics of the fatty acid chains that promote the rigid state are length and the degree of saturation. Long, straight hydrocarbon chains maximize Van der Waals interactions, increasing membrane viscosity (also, by related principles, increasing the melting point). Cholesterol, however, acts to decrease membrane viscosity at low temperatures, but increases it at high temperature. Fitting between the fatty acid chains, cholesterol prevents their crystallization. However, cholesterol also blocks large motions of the fatty acid chains, which, conversely makes the membrane less fluid at high temperature. In other words, cholesterol acts like a fluidity buffer for membranes.







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