CHAPTER
Simple Carbohydrates
Carbohydrates are major functional constituents of living
systems. The primary source of energy in animal cells,
carbohydrates are synthesized in green plants from carbon
dioxide, water, and solar energy. They provide the skele-
tal framework for tissues and organs of the human body
and serve as lubricants and support elements of connec-
tive tissue. Major energy requirements of the human body
are met by dietary carbohydrates. They confer biological
specificity and provide recognition elements on cell mem-
branes. In addition, they are components of nucleic acids
and are covalently linked with lipids and proteins.
9.1
Classification
Carbohydrates consist of polyhydroxyketone or polyhy-
droxyaldehyde compounds and their condensation prod-
ucts. The term “carbohydrate” literally means hydrate of
carbon, a compound with an empirical formula (CH
2
0)„.
This formula applies to many carbohydrates, such as glu-
cose, which is CöH^Og, or (CH
2
0)6. However, a large
number of compounds are classified as carbohydrates even
though they do not have this empirical formula; these
compounds are derivatives of simple sugars (e.g., de-
oxyribose, C
5
H
1 0
O
4
). Carbohydrates may be classified as
monosaccharides, oligosaccharides, and polysaccharides;
the term
saccharide
is derived from the Greek word for
sugar. Monosaccharides are single polyhydroxyaldehyde
(e.g., glucose) or polyhydroxyketone units (e.g., fructose),
whereas oligosaccharides consist of two to ten monosac-
charide units joined together by glycosidic linkages.
Sucrose and lactose are disaccharides, since they are each
made up of two monosaccharide units. Names of common
monosaccharides and disaccharides take the suffix
-ose
(e.g., glucose, sucrose). Polysaccharides, also known as
glycans,
are polymers that may contain many hundreds
of monosaccharide units. They are further divided into
homopolysaccharides and heteropolysaccharides. The for-
mer contain only a single type of polysaccharide unit (e.g.,
starch and cellulose, both of which are polymers of glu-
cose), whereas the latter contain two or more different
monosaccharide units.
Monosaccharides
Monosaccharides are identified by their carbonyl func-
tional group (aldehyde or ketone) and by the number of
carbon atoms they contain. The simplest monosaccha-
rides are the two trioses: glyceraldehyde (an aldotriose);
and dihydroxyacetone (a ketotriose). Four-, five-, six-, and
seven-carbon-containing monosaccharides are called tet-
roses, pentoses, hexoses, and heptoses, respectively. Struc-
tures of some monosaccharides are shown in Figure 9-1.
All monosaccharides, with the exception of dihydroxy-
acetone, contain at least one asymmetrical or chiral car-
bon atom, and therefore two or more stereoisomers are
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