Essential Cell Biology 5th edition

Small Molecules in Cells
53
optical isomers. Isomers are widespread among organic molecules in
general, and they play a major part in generating the enormous variety
of sugars. A more complete outline of sugar structures and chemistry is
presented in Panel 2–4.
Monosaccharides can be linked by covalent bonds—called glycosidic
bonds—to form larger carbohydrates. Two monosaccharides linked
together make a disaccharide, such as sucrose, which is composed of
a glucose and a fructose unit. Larger sugar polymers range from the
oligo saccharides (trisaccharides, tetrasaccharides, and so on) up to
giant polysaccharides, which can contain thousands of monosaccharide
subunits (monomers). In most cases, the prefix oligo- is used to refer to
molecules made of a small number of monomers, typically 2 to 10 in the
case of oligosaccharides. Polymers, in contrast, can contain hundreds or
thousands of subunits.
The way sugars are linked together illustrates some common features of
biochemical bond formation. A bond is formed between an –OH group
on one sugar and an –OH group on another by a condensation reaction,
in which a molecule of water is expelled as the bond is formed (Figure
2–19). The sub units in other biological polymers, including nucleic acids
and proteins, are also linked by condensation reactions in which water
is expelled. The bonds created by all of these condensation reactions can
be broken by the reverse process of hydrolysis, in which a molecule of
water is consumed. Generally speaking, condensation reactions, which
synthesize larger molecules from smaller subunits, are energetically
unfavorable; hydrolysis reactions, which break down larger molecules
into smaller subunits, are energetically favorable (Figure 2−20).
Because each monosaccharide has several free hydroxyl groups that can
form a link to another monosaccharide (or to some other compound),
sugar polymers can be branched, and the number of possible polysaccharide
structures is extremely large. For this reason, it is much more
difficult to determine the arrangement of sugars in a complex polysaccharide
than it is to determine the nucleotide sequence of a DNA molecule or
the amino acid sequence of a protein, in which each unit is joined to the
next in exactly the same way.
The monosaccharide glucose has a central role as an energy source for
cells, as we explain in Chapter 13. It is broken down to smaller molecules
in a series of reactions, releasing energy that the cell can harness to do
useful work. Cells use simple polysaccharides composed only of glucose
units—principally glycogen in animals and starch in plants—as long-term
stores of glucose, held in reserve for energy production.
Sugars do not function exclusively in the production and storage of
energy. They are also used, for example, to make mechanical supports.
The most abundant organic molecule on Earth—the cellulose that forms
plant cell walls—is a polysaccharide of glucose. Another extraordinarily
abundant organic substance, the chitin of insect exoskeletons and fungal
cell walls, is also a polysaccharide—in this case, a linear polymer of a
sugar derivative called N-acetylglucosamine (see Panel 2–4, pp. 72–73).
Other polysaccharides, which tend to be slippery when wet, are the main
components of slime, mucus, and gristle.
H 2 O
A H + HO B A B
A H + HO B
CONDENSATION
HYDROLYSIS
energetically
unfavorable
H 2 O
energetically
favorable
H 2 O
O
monosaccharide
CONDENSATION
water expelled
O
OH
+
HO
O
glycosidic
bond
disaccharide
H 2 O
O
O
monosaccharide
HYDROLYSIS
water consumed
Figure 2–19 Two monosaccharides can
be linked by a covalent glycosidic bond
to form a disaccharide. This reaction
belongs to
ECB5
a general
E2.18/2.19
category of reactions
termed condensation reactions, in which two
molecules join together as a result of the loss
of a water molecule. The reverse reaction (in
which water is added) is termed hydrolysis.
Figure 2–20 Condensation and hydrolysis
are reverse reactions. The large polymeric
macromolecules of the cell are formed from
subunits (or monomers) by condensation
reactions, and they are broken down
by hydrolysis. Condensation reactions
are energetically unfavorable; thus
macromolecule formation requires an input
of energy, as we discuss in Chapter 3.