Chapter 2
Chemistry Comes Alive
43
2
molecule is added to each bond, breaking the bonds and releas-
ing the simple sugar units.
Polysaccharides
Polysaccharides
(pol
0
e-sak
9
ah-rīdz) are polymers of sim-
ple sugars linked together by dehydration synthesis. Because
polysaccharides are large, fairly insoluble molecules, they are
ideal storage products. Another consequence of their large size
is that they lack the sweetness of the simple and double sugars.
Only two polysaccharides are of major importance to the
body: starch and glycogen. Both are polymers of glucose. Only
their degree of branching differs.
Starch
is the storage carbohydrate formed by plants. Te
number of glucose units composing a starch molecule is high
and variable. When we eat starchy foods such as grain products
and potatoes, the starch must be digested for its glucose units to
be absorbed. We are unable to digest
cellulose
, another polysac-
charide found in all plant products. However, it is important
in providing the
bulk
(one form of fiber) that helps move feces
through the colon.
Glycogen
(gli
9
ko-jen), the storage carbohydrate of animal tissues,
is stored primarily in skeletal muscle and liver cells. Like starch, it is
highly branched and is a very large molecule (Figure 2.15c). When
blood sugar levels drop sharply, liver cells break down glycogen and
release its glucose units to the blood. Since there are many branch
endings from which glucose can be released simultaneously, body
cells have almost instant access to glucose fuel.
Carbohydrate Functions
Te major function of carbohydrates in the body is to provide a
ready, easily used source of cellular fuel. Most cells can use only
a few types of simple sugars, and glucose is at the top of the “cel-
lular menu.” As described in our earlier discussion of oxidation-
reduction reactions (pp. 36–37), glucose is broken down and
oxidized within cells. During these chemical reactions, electrons
are transferred. Tis relocation of electrons releases the bond
energy stored in glucose, and this energy is used to synthesize A±P.
When A±P supplies are sufficient, dietary carbohydrates are con-
verted to glycogen or fat and stored. Tose of us who have gained
weight from eating too many carbohydrate-rich snacks have
personal experience with this conversion process!
Only small amounts of carbohydrates are used for structural
purposes. For example, some sugars are found in our genes.
Others are attached to the external surfaces of cells where they
act as “road signs” to guide cellular interactions.
Lipids
Lipids
are insoluble in water but dissolve readily in other lipids
and in organic solvents such as alcohol and ether. Like carbohy-
drates, all lipids contain carbon, hydrogen, and oxygen, but the
proportion of oxygen in lipids is much lower. In addition, phos-
phorus is found in some of the more complex lipids. Lipids include
triglycerides
,
phospholipids
(fos
0
fo-lip
9
idz),
steroids
(stĕ
9
roidz), and
a number of other lipoid substances.
Table 2.2
on p. 46 gives the
locations and functions of some lipids found in the body.
Carbohydrates
Carbohydrates
, a group of molecules that includes sugars and
starches, represent 1–2% of cell mass. Carbohydrates contain
carbon, hydrogen, and oxygen, and generally the hydrogen
and oxygen atoms occur in the same 2:1 ratio as in water.
Tis ratio is reflected in the word
carbohydrate
(“hydrated
carbon”).
A carbohydrate can be classified according to size and solu-
bility as a monosaccharide (“one sugar”), disaccharide (“two
sugars”), or polysaccharide (“many sugars”). Monosaccharides
are the monomers, or building blocks, of the other carbohy-
drates. In general, the larger the carbohydrate molecule, the less
soluble it is in water.
Monosaccharides
Monosaccharides
(mon
0
o-sak
9
ah-rīdz), or
simple sugars
, are
single-chain or single-ring structures containing from three to
seven carbon atoms
(Figure 2.15a)
. Usually the carbon, hydro-
gen, and oxygen atoms occur in the ratio 1:2:1, so a general for-
mula for a monosaccharide is (CH
2
O)
n
, where
n
is the number
of carbons in the sugar. Glucose, for example, has six carbon
atoms, and its molecular formula is C
6
H
12
O
6
. Ribose, with five
carbons, is C
5
H
10
O
5
.
Monosaccharides are named generically according to the
number of carbon atoms they contain. Most important in the
body are the pentose (five-carbon) and hexose (six-carbon)
sugars. For example, the pentose
deoxyribose
(de-ok
0
sĭ-ri
9
bo
¯
s)
is part of DNA, and
glucose
, a hexose, is blood sugar.
±wo other hexoses,
galactose
and
fructose
, are
isomers
9
so-mers) of glucose. Tat is, they have the same molecular
formula (C
6
H
12
O
6
), but as you can see in Figure 2.15a, their
atoms are arranged differently, giving them different chemi-
cal properties.
Disaccharides
A
disaccharide
(di-sak
9
ah-rīd), or
double sugar
, is formed when
two monosaccharides are joined by
dehydration synthesis
(Fig-
ure 2.14a, c). In this synthesis reaction, a water molecule is lost
as the bond is made, as illustrated by the synthesis of sucrose
(soo
9
krōs):
2C
6
H
12
O
6
S
C
12
H
22
O
11
1
H
2
O
glucose
1
fructose
sucrose
water
Notice that the molecular formula for sucrose contains two hy-
drogen atoms and one oxygen atom less than the total number
of hydrogen and oxygen atoms in glucose and fructose, because
a water molecule is released during bond formation.
Important disaccharides in the diet are
sucrose
(glucose
1
fructose), which is cane or table sugar;
lactose
(glucose
1
ga-
lactose), found in milk; and
maltose
(glucose
1
glucose), also
called malt sugar (Figure 2.15b). Disaccharides are too large to
pass through cell membranes, so they must be digested to their
simple sugar units to be absorbed from the digestive tract into
the blood. Tis decomposition process is
hydrolysis
, essentially
the reverse of dehydration synthesis (Figure 2.14a, b). A water
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