30
UNIT 1
Organization of the Body
2
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.
Suspensions
Suspensions
are
heterogeneous
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
7.
What is the meaning of the term “molecule”?
8.
Why is sodium chloride (NaCl) considered a compound, but
oxygen gas is not?
9.
Blood contains a liquid component and living cells. Would it
be classified as a compound or a mixture? Why?
For answers, see Appendix H.
Chemical Bonds
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
6
H
12
O
6
, 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:
Atom
Number
of Atoms  
Atomic
Weight
 
Total
Atomic
Weight
C
6
3
12.011
5
72.066
H
12
3
1.008
5
12.096
O
6
3
15.999
5
95.994
 
 
 
 
 
180.156
Ten, to make a
one-molar
solution of glucose, you would
weigh out 180.156 grams (g), called a
gram molecular weight
, of
glucose and add enough water to make 1 liter (L) of solution. In
short, a one-molar solution (abbreviated 1.0
M
) 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
3
10
23
.
Tis number is called
Avogadro’s number
(av
0
o-gad
9
rōz). So
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
have 6.02
3
10
23
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
(m
M
; 1/1000 mole).
Colloids
Colloids
(kol
9
oidz), also called
emulsions
, are
heterogeneous
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
sol-gel transformations
, 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
M
solution of sodium chloride, 2
moles
of solute
particles are actually in solution.
previous page 64 Human Anatomy and Physiology (9th ed ) 2012 read online next page 66 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off