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Intermolecular Forces

Chemical Thermodynamics and the Equilibrium State

Chemical Kinetics



Biological Membranes

The ability of biological membranes to subdivide space in an aqueous environment with semipermeable barriers is one of the most crucial functions within living systems. The phospholipid bilayer is impermeable to ions and large polar molecules. The permeability of the membrane for a substance is proportional to its solubility in a nonpolar solvent relative to its solubility in water. In order to cross the membrane, the substance must separate from its solvation shell of water molecules (internal energy increase), then dissolve in the nonpolar core of the membrane (internal energy decrease), and finally, diffuse to the other side of the membrane and redissolve in water. It is the transition to the interior of the membrane that determines the possibility of meaningful permeability of the substance. For an ion or polar molecule, the change from solvation by water to the nonpolar surroundings represents a large increase in electrostatic potential energy (internal energy, enthalpy, free energy), and so there will exist a high barrier of activation energy making this transition extremely slow for such molecules and ions.



Biological Membranes

The Endocrine System

Lipid solubility determines the broad mechanism by which a hormones activate their target cells. Steroid hormones are lipids. Because they are nonpolar, steroid hormones pass directly through the plasma membrane of the target cell. Other classes of hormones, peptide hormones such as insulin and glucagon and catecholamines such as epenephrine, which are large polar molecules, usually do not enter their target cells. Instead they bind specific receptor proteins on the plasma membrane to form a hormone receptor complex. The hormone-receptor complex then activates a second messenger such as InsP3 or cAMP in a process is known as signal coupling.

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