Page 37 - Color Atlas of Biochemistry

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28 Basics

Hydrophobic interactions is therefore positive (the disorder in the water
increases), and the negative term –T ∆S
Water is an excellent solvent for ions and for makes the separation process exergonic
substances that contain polarized bonds (see (∆G < 0),so thatitproceeds spontaneously.
p. 20). Substances of this type are referred to
as polar or hydrophilic (“water-loving”). In C. Arrangements of amphipathic substances
contrast, substances that consist mainly of
in water
hydrocarbon structures dissolve only poorly
in water. Such substances are said to be apolar Molecules that contain both polar and apolar
or hydrophobic. groups are called amphipathic or amphiphilic.
This group includes soaps (see p. 48), phos-
pholipids (see p. 50), and bile acids (see p. 56).
A. Solubility of methane
As a result of the “oil drop effect” amphi-
To understand the reasons for the poor water pathic substances in water tend to arrange
solubility of hydrocarbons, it is useful first to themselves in such a way as to minimize the
examine the energetics (see p.16) of the pro- area of surface contact between the apolar
cesses involved. In (1), the individual terms of regions of the molecule and water. On water
the Gibbs–Helmholtz equation (see p. 20) for surfaces, they usually form single-layer films
the simplest compound of this type, methane, (top) in which the polar “head groups” face
are shown (see p. 4). As can be seen, the tran- toward the water. Soap bubbles (right) consist
sition from gaseous methane to water is ac- of double films, with a thin layer of water
0
tually exothermic (∆H < 0). Nevertheless, the enclosed between them. In water, depending
0
change in thefreeenthalpy ∆G is positive on their concentration, amphipathic com-
(the process is endergonic), because the en- pounds form micelles—i. e., spherical aggre-
tropy term T ∆S 0 has a strongly positive gates with their head groups facing toward
value. The entropy change in the process the outside, or extended bilayered double
0
(∆S ) is evidently negative—i. e., a solution of membranes. Most biological membranes are
methane in water has a higher degree of order assembled according to this principle (see
than either water or gaseous methane. One p. 214). Closed hollow membrane sacs are
reason for this is that the methane molecules known as vesicles. This type of structure
areless mobilewhen surroundedbywater. serves to transport substances within cells
More importantly, however, the water around and in the blood (see p. 278).
the apolar molecules forms cage-like “clath- The separation of oil and water (B)can be
rate” structures, which—as in ice—are stabi- prevented by adding a strongly amphipathic
lized by H bonds. This strongly increases the substance.Duringshaking,a more or less
degreeoforder in the water—and themoreso stable emulsion then forms, in which the sur-
the larger the area of surface contact between face of the oil drops is occupied by amphi-
the water and the apolar phase. pathic molecules that provide it with polar
properties externally. The emulsification of
fats in food by bile acids and phospholipids
B. The “oil drop effect”
is a vital precondition for the digestion of fats
The spontaneous separation of oil and water, (see p. 314).
a familiar observation in everyday life, is due
to the energetically unfavorable formation of
clathrate structures. When a mixture of water
and oil is firmly shaken, lots of tiny oil drops
form to begin with, but these quickly coalesce
spontaneously to form larger drops—the two
phases separate. A larger drop has a smaller
surface area than several small drops with the
same volume. Separation therefore reduces
the area of surface contact between the water
and the oil, and consequently also the extent
of clathrate formation. The ∆S for this process

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