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Chemistry for Biologists - Water and living organisms
Overall discussion of the role of water in living systems.



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Special points of emphasis

Work, Energy, and Power

Electricity

Intermolecular Forces

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Water

Solutions

In addition to electrolytes, water dissolves polar substances. Why? To understand solubility, it is always helpful to review the thermochemistry of the solution process. To conceptualize the enthalpy change in the process of something like sucrose, a polar substance, dissolving in water, imagine the initial state (pure sucrose and pure water) and the final state (a sucrose solution). Imagine pulling the sucrose molecules from each other. They are attracted by hydrogen bonding, so this takes work. Imagine pulling the water molecules from each other. They are also attracted by hydrogen bonding, so this takes work as well. Now we have our imaginary state, all of the water and sucrose molecules separated. The increase in enthalpy, so far, has been identical to vaporization. Now, imagine the water and sucrose molecules falling together into the final state, the sucrose solution. The internal energy increase involved as such solute molecules lose their association with each other is compensated by the formation of new intermolecular associations with water. Although the enthalpy change is a bit endothermic (we know it is endothermic because the solubility would increase with temperature), it is not so endothermic that sucrose is insoluble.

Conversely, imagine a similar solution process with a nonpolar substance such as napthalene. Nonpolar molecules cluster together in water. They do not dissolve. Thermodynamically, this is because the internal energy change of such a hypothetical solution would be determined by the separation of the hydrophobic molecules from each other as well as the separation of water molecules from each other. These positive internal energy changes would have no corresponding decreases through the formation of new associations between the polar water and the nonpolar molecules. Additionally, if the nonpolar solute is dispersed through the medium, water has less freedom, quantitatively fewer hydrogen bonding opportunities with itself, which represents a decrease in entropy. Due to positive internal energy changes and negative entropy change (positive enthalpy change, positive free energy change) the equilibrium constant for the solvation of a nonpolar solute by water is vanishingly small. The consequence of the thermodynamic facts leading to water's high affinity for itself is that nonpolar solute molecules are driven together in water, not dissolving.




Water

Proteins

Carbohydrates

Nucleic Acids

Lipids

Biological Membranes

The structure of biological molecules is strongly influenced by the aqueous solution environment in which they function. Water profoundly affects the structure of many proteins, in that the hydrophobic portions tend to be sequestered within the interior of the molecule.

As an interesting note, this hydrophobic zone within a protein is free of the dielectric properties of water. This allows electrostatic and polar interactions to be strong. In other words, it is a common motif in biochemistry for the active site of an enzyme to be isolated from the aqueous environment.

The significance of the solvent properties of water are also profound for carbohydrates, which were almost certainly selected in evolution for their roll in metabolism due to high solubility of simple sugars.

Lastly, the absence of water solubility is itself the defining property of lipids. An example of the roll of the solvent properties of water in lipid behavior is the structure of the phospholipid bilayer, where the differential solubilities of sections of the molecule lead to aggregation and formation of the bilayer structure in water.








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