Biochemistry Study Guide

Study Guide: Biochemistry

A. Hydrophilic vs Hydrophobic. Since biological chemistry occurs largely in an aqueous environment, the interaction of a biological molecule with water is very important. That interaction is influenced by two primary causes: size and polarity (charge). The smaller a molecule is, the more likely it is to be willing to associate with water (dissolve). Also, the more polar and/or charged a molecule is, the more likely it is to be willing to associate with water. Since biological molecules are often very large, it is common for the different parts of the molecule to interact differently in water. For instance, a protein, which is composed of many different amino acids which have a large variety of characters, may be hydrophobic in part of its sequence and hydrophilic in other parts.

Hydrophilic (hydro=water; philios=love): Hydrophilic molecules or parts of molecules will dissolve in (interact with) water.

Hydrophobic (hydro=water; phobio=fear): Hydrophobic molecules or parts of molecules will refuse to interact with water. If sufficiently hydrophobic, a molecule or part of a molecule will actively repel or exclude water.

Hydrophilic/phobic characters are not an all-or-none phenomenon. Molecules fall along a scale, somewhere between extremely hydrophobic and extremely hydrophilic. Changing the parts of a molecule will often shift it more toward the hydrophobic or the hydrophilic end of the scale (depending upon the change).

B. Hydrocarbons. The basic skeletons of organic molecules are composed of hydrocarbon. Hydrocarbon is made only of the elements carbon and hydrogen. Since carbon atoms all have the same electronegativity, and the electronegativity of hydrogen is only slightly different than that of carbon, the bonds in hydrocarbons are all non-polar. Thus, hydrocarbons tend to be hydrophobic, especially if they are more than a few carbons in size.

The many, many different...