63
CHAPTER TWO
Chemical Basis of Life
The building blocks of fat molecules are
fatty acids
(fat
e
as
idz) and
glycerol
(glis
er-ol). Although the glycerol portion
of every fat molecule is the same, there are many types of
fatty acids and, therefore, many types of fats. All fatty acid
molecules include a carboxyl group (—COOH) at the end of
a chain of carbon atoms. Fatty acids differ in the lengths of
their carbon atom chains, which usually have an even num-
ber of carbon atoms. The fatty acid chains also may vary in
the ways the carbon atoms join. In some cases, single car-
bon-carbon bonds link all the carbon atoms. This type of
fatty acid is called a
saturated fatty acid;
that is, each car-
bon atom binds as many hydrogen atoms as possible and is
thus saturated with hydrogen atoms. Other fatty acid chains,
unsaturated fatty acids,
have one or more double bonds
between carbon atoms. Fatty acids with one double bond are
called
monounsaturated fatty acids,
and those with two or
more double bonds are
polyunsaturated fatty acids
(f g. 2.13)
.
A glycerol molecule combines with three fatty acid mole-
cules to form a single fat molecule, or
triglyceride
(f
g. 2.14)
.
The fatty acids of a triglyceride may have different lengths
and degrees of saturation making the fats very diverse. Fat
molecules that have only saturated fatty acids are called
sat-
urated fats
(sat
u-ra
ˉt
ed fatz), and those that have unsatu-
rated fatty acids are called
unsaturated fats
(unsat
u-ra
ˉted
fatz). Each type of fat molecule has distinct properties.
C
C
C
C
C
C
H
O
H
O
O
O
H
H
O
H
H
H
H
HO
H
H
H
H
C
H
O
O
H
H
O
H
H
O
H
H
HH
C
O
H
C
CC
O
O
C
H
(a)
Some glucose molecules
(C
6
H
12
O
6
) have a straight
chain of carbon atoms.
(b)
More commonly, glucose
molecules form a ring structure.
(c)
This shape symbolizes
the ring structure of a
glucose molecule.
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
CH
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
CH
2
O
O
O
O
(a)
Monosaccharide
(b)
Disaccharide
(c)
Polysaccharide
FIGURE 2.11
Structural formulas depict a
molecule of glucose.
FIGURE 2.12
Carbohydrate molecules vary in size. (
a
) A monosaccharide molecule consists of one 6-carbon atom building block. (
b
) A
disaccharide molecule consists of two of these building blocks. (
c
) A polysaccharide molecule consists of many building blocks.
A diet rich in saturated fat increases risk of atherosclerosis, which
obstructs blood vessels. The risk is even greater if the diet is also high
in reF
ned carbohydrates, such as white ±
our and rice, because these
raise triglyceride levels. Unsaturated, particularly monounsaturated,
fats are healthier to eat than saturated fats. Monounsaturated fats
include olive, canola, and macadamia nut oils.
Most saturated fats are solids at room temperature, such as but-
ter, lard, and most other animal fats. Most unsaturated fats are liquids
at room temperature, such as corn, sesame, peanut, sun±
ower, and
soybean oils. Coconut and palm kernel oils are exceptions—they are
relatively high in saturated fat.
A food-processing technique called hydrogenation adds
hydrogens to an unsaturated fat, making it more solid and there-
fore useful in prepared foods. Margarine is an example. However,
hydrogenation is an imperfect process. Some of the double bonds
are changed to single bonds when hydrogens are forced onto the
molecule, but some are not. Instead, the two hydrogens bonded to
the two carbons that share a partially hydrogenated bond assume a
“trans” conF
guration—that is, emanating in opposite directions from
the carbons with respect to each other. (In the natural “cis” conF
gura-
tion, the two hydrogens lie on the same side of the carbon backbone.)
Trans fats raise the risk of heart disease.
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