Page 63 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Carbohydrates 371

figures are often inaccurate or unavailable, the percent-
age for the world as a whole has been estimated to be
more than 80%. The exothermic reactions that produce
energy in the cell are the outcome of a number of com-
plex enzyme cycles that originate with hexoses and end
up with one-, two-, or three-carbon units. The energy re-
leased from these reactions is stored in the cell in the
form of a key sugar intermediate, adenosine triphosphate
(ATP), and released when needed by its conversion to the
di- or monophosphate. Finally, carbohydrates are the most
abundant organic components of plants (>50% of the dry
weight) and therefore constitute the major part of our re-
newable fuels and the starting material from which most
of our fossil fuels were made.

FIGURE 1 Reproduction of a seventh-century Chinese drawing
C. Classification
representing the manufacture of sugar.
Carbohydrates are classified according to their degree of
polymerization into monomeric carbohydrates, which in-
refining were developed, including treatment of the syrup clude monosaccharides and their derivatives, and poly-
with lime to precipitate the calcium complex, regeneration meric carbohydrates, which comprise oligosaccharides,
of the sugar with sulfur dioxide, and decolorization with polysaccharides,DNA,andRNA.Thepolymericcarbohy-
animal charcoal. drates differ in the type of bridge that links their monosac-
charide units. Thus, oligosaccharides and polysaccha-
rides are polyacetals, linked by acetal oxygen bridges,
B. Importance of Carbohydrates
whereas DNA and RNA are poly(phosphoric) esters,
In addition to the manufacture of sucrose and paper, which linked by phosphate bridges. In addition to these well-
are today important industries, carbohydrates play a major defined groups of carbohydrates, there exist a number of
role in a number of other industries. These include (1) the derivatives, for example, antibiotics, that are best studied
food industry, which uses huge amounts of starch of var- as a separate group, because some of their members may
ious degrees of purity in the manufacture of baked goods be monomeric, whereas others are oligomeric.
and pastas, of gums in food processing, and of mono-
and oligosaccharides as sweeteners and employs carbo-
1. Monosaccharides
hydrates in fermentation to make beer and wine; (2) the
textile industry, which, despite the advent of synthetics, is Monosaccharides are chiral polyhydroxyalkanals or poly-
still dependent to a large extent on cellulose; (3) the phar- hydroxyalkanones that often exist in cyclic hemiacetal
maceutical industry, particularly in the areas of antibiotics, forms. Monosaccharides are divided into two major
intravenous solutions, and vitamin C; and (4) the chemical groups according to whether their acyclic forms possess
industry, which produces and markets several pure sugars an aldehyde group or a keto group, that is, into aldoses or
and their derivatives. ketoses(glyculoses).Eachofthese,inturn,isclassifiedac-
Carbohydrates also play a key role in the process of life; cording to the number of carbon atoms in the monosaccha-
the master molecule DNA is a polymer made up of re- ride chain (usually 3–10) into trioses, tetroses, pentoses,
peating units composed of four nucleotides of 2-deoxy-D- hexoses, and so on. By prefixing “aldo” to these names,
erythro-pento-furanose (2-deoxy-D-ribose), the sequence one can define more closely a group of aldoses, for exam-
of which constitutes the coded template responsible for ple, aldopentoses, whereas for ketoses, it is customary to
replication and transcription. Saccharide derivatives also use the ending “ulose,” as in hexulose. Finally monosac-
form part of many vital enzyme systems, specifically as charides can be grouped according to the size of their
coenzymes. rings into five-membered furanoses and six-membered
The best-known use of carbohydrates is undoubtedly in pyranoses. It should be noted that, in order to form a
nutrition, as members of a major food type that are metab- furanose ring, four carbon atoms and one oxygen atom
olized to produce energy. Although the average percent- are needed, so that only aldotetroses and higher aldoses,
age of carbohydrates consumed by humans in comparison and 2-pentuloses and higher ketoses, can cyclize in this
with other food types differs from country to country, and ring form. Similarly, in order to form a pyranose ring five