Chemistry Reference
In-Depth Information
5
Sugar and Fat
and All of That
5.1 CARBOHYDRATES
Carbohydrates (carbon hydrates, C
n
(H
2
O)
n
, hence their name), are the most abundant
class of organic compounds found in living organisms. They originate as products
of photosynthesis: condensation of carbon dioxide requiring light, energy, and the
pigment chlorophyll.
n CO
2
+ n H
2
O + energy → C
n
H
2n
O
n
+ n O
2
Carbohydrates, also known as polysaccharides or sugars, are a major source of
metabolic energy, both for plants and for animals that depend on plants for food.
They are a component of the energy transport compound ATP and are on recognition
sites on cell surfaces. Importantly, sugars are also one of the three essential compo-
nents of both DNA (e.g., ribose) and RNA (e.g., deoxyribose).
The simplest carbohydrate is a monomer (monosaccharide), represented either
by a six- or ive-membered ring structure, with the examples of glucose and ribose
respectively, shown in Figure 5.1.
The six-membered ring sugar can be represented by several other forms desig-
nated by the structures shown in
Figure 5.2. These forms of the monomer can be:
•
As ketose sugars having a ketone function or an acetal equivalent
•
As aldose sugars having an aldehyde function or an acetal equivalent
Examples of simple hexose and pyranose structures are shown in
Figure 5.3.
All
compounds are isomers of glucose because the configurations of the OH group can
vary on each of the carbons of the ring. The two rows represent common ways to
draw the structures of the monomers.
5.2 VARIETY AND COMPLEXITY OF SUGARS
When these
monosaccharides are linked they form polysaccharide chains through
glycosidic bond linkages.
Linking the sugars can be in a linear or branched manner,
or as a combination of both ways, and various carbohydrates (e.g., disaccharides,
oligosaccharides, and polysaccharides) can be generated. Polysaccharides can attain
very high molecular weight ranges of over 100,000 daltons.
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