Chemical Composition of the Body
25
Molecules that have a polar or ionized region at one
end and a nonpolar region at the opposite end are called
amphipathic
—consisting of two parts. When mixed with
water, amphipathic molecules form clusters, with their polar
(hydrophilic) regions at the surface of the cluster where they
are attracted to the surrounding water molecules. The nonpo-
lar (hydrophobic) ends are oriented toward the interior of the
cluster (
Figure 2–6
). Such an arrangement provides the max-
imal interaction between water molecules and the polar ends
of the amphipathic molecules. Nonpolar molecules can dis-
solve in the central nonpolar regions of these clusters and thus
exist in aqueous solutions in far higher amounts than would
otherwise be possible based on their low solubility in water.
As we will see, the orientation of amphipathic molecules plays
an important role in cell membrane structure and in both the
absorption of nonpolar molecules from the gastrointestinal
tract and their transport in the blood.
Concentration
Solute
concentration
is defi ned as the amount of the sol-
ute present in a unit volume of solution. One measure of the
amount of a substance is its mass expressed in grams. The unit
of volume in the metric system is a liter (L). (One liter equals
1.06 quarts; see the conversion table at the back of the book
for metric and English units.) Smaller units commonly used
in physiology are the deciliter (dL, or 0.1 liter), the milliliter
(ml, or 0.001 liter), and the microliter (
µ
l, or 0.001 ml). The
concentration of a solute in a solution can then be expressed
as the number of grams of the substance present in one liter
of solution (g/L).
A comparison of the concentrations of two different sub-
stances on the basis of the number of grams per liter of solu-
tion does not directly indicate how many molecules of each
substance are present. For example, if the molecules of com-
pound X are heavier than those of compound Y, 10 g of com-
pound X will contain fewer molecules than 10 g of compound Y.
Concentrations in units of grams per liter are most often
used when the chemical structure of the solute is unknown.
When the structure of a molecule is known, concentrations
Solid NaCl
CI
CI
CI
CI
CI
CI
CI
CI
CI
CI
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Solution of sodium and chloride ions
Water
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
δ
+
δ
+
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+
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+
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δ
+
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+
δ
δ
+
δ
+
δ
δ
+
δ
+
δ
Figure 2–5
The ability of water to dissolve sodium chloride crystals depends upon the electrical attraction between the polar water molecules and the
charged sodium and chloride ions.
δ
δ
+
δ
+
δ
δ
+
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+
δ
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+
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+
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+
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+
δ
δ
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+
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+
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+
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+
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+
δ
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+
δ
+
Water
molecule
(polar)
Amphipathic molecule
Nonpolar region
Polar region
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Figure 2–6
In water, amphipathic molecules aggregate into spherical clusters.
Their polar regions form hydrogen bonds with water molecules
at the surface of the cluster, while the nonpolar regions cluster
together away from water.
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