Organization of the Body
largely because of its dispersed proteins. Its sol-gel transforma-
tions underlie many important cell activities, such as cell divi-
sion and changes in cell shape.
mixtures with large, oFen visible
solutes that tend to settle out. An example of a suspension is a
mixture of sand and water. So is blood, in which the living blood
cells are suspended in the ﬂuid portion of blood (blood plasma).
If leF to stand, the suspended cells will settle out unless some
means—mixing, shaking, or circulation in the body—keeps
them in suspension.
As you can see, all three types of mixtures are found in both
living and nonliving systems. In fact, living material is the most
complex mixture of all, since it contains all three kinds of mix-
tures interacting with one another.
Distinguishing Mixtures from Compounds
Now let’s zero in on how to distinguish mixtures and com-
pounds from one another. Mixtures diﬀer from compounds in
several important ways:
Te chief diﬀerence between mixtures and compounds is
that no chemical bonding occurs between the components
of a mixture. Te properties of atoms and molecules are not
changed when they become part of a mixture. Remember
they are only physically intermixed.
Depending on the mixture, its components can be separated
by physical means—straining, ﬁltering, evaporation, and
so on. Compounds, by contrast, can be separated into their
constituent atoms only by chemical means (breaking bonds).
Some mixtures are homogeneous, whereas others are het-
erogeneous. A bar of 100% pure (elemental) iron is ho-
mogeneous, as are all compounds. As already mentioned,
heterogeneous substances vary in their makeup from place
to place. ±or example, iron ore is a heterogeneous mixture
that contains iron and many other elements.
Check Your Understanding
What is the meaning of the term “molecule”?
Why is sodium chloride (NaCl) considered a compound, but
oxygen gas is not?
Blood contains a liquid component and living cells. Would it
be classiﬁed as a compound or a mixture? Why?
For answers, see Appendix H.
Explain the role of electrons in chemical bonding and in
relation to the octet rule.
Differentiate among ionic, covalent, and hydrogen bonds.
Compare and contrast polar and nonpolar compounds.
Glucose is C
, which indicates that it has 6 carbon
atoms, 12 hydrogen atoms, and 6 oxygen atoms. ²o compute
the molecular weight of glucose, you would look up the atomic
weight of each of its atoms in the periodic table (see Appendix E)
and compute its molecular weight as follows:
Ten, to make a
solution of glucose, you would
weigh out 180.156 grams (g), called a
gram molecular weight
glucose and add enough water to make 1 liter (L) of solution. In
short, a one-molar solution (abbreviated 1.0
) of a chemical
substance is one gram molecular weight of the substance (or
one gram atomic weight in the case of elemental substances) in
1 L (1000 milliliters) of solution.
Te beauty of using the mole as the basis of preparing solu-
tions is its precision. One mole of any substance always contains
exactly the same number of solute particles, that is, 6.02
Tis number is called
whether you weigh out 1 mole of glucose (180 g) or 1 mole of
water (18 g) or 1 mole of methane (16 g), in each case you will
molecules of that substance.* Tis allows al-
most mind-boggling precision to be achieved.
Because solute concentrations in body ﬂuids tend to be quite
low, those values are usually reported in terms of millimoles
; 1/1000 mole).
oidz), also called
mixtures, which means that their composition is dissimilar in
diﬀerent areas of the mixture. Colloids oFen appear translucent
or milky and although the solute particles are larger than those
in true solutions, they still do not settle out. However, they do
scatter light, so the path of a light beam shining through a col-
loidal mixture is visible.
Colloids have many unique properties, including the ability
of some to undergo
, that is, to change
reversibly from a ﬂuid (sol) state to a more solid (gel) state. Jell-O,
or any gelatin product (±igure 2.4), is a familiar example of
a nonliving colloid that changes from a sol to a gel when re-
frigerated (and that gel will liquefy again if placed in the sun).
Cytosol, the semiﬂuid material in living cells, is also a colloid,
*Te important exception to this rule concerns molecules that ionize and break up
into charged particles (ions) in water, such as salts, acids, and bases (see p. 39). ±or
example, simple table salt (sodium chloride) breaks up into two types of charged
particles. Terefore, in a 1.0
solution of sodium chloride, 2
particles are actually in solution.