Page 42 - Biology_F5

Cytology

Monosaccharides (Single sugars) Tetroses: These are monosaccharides
These are the simplest carbohydrates, containing four carbon atoms
having only one sugar or saccharide such as erythrose and threose.
molecule, which cannot be hydrolysed Their empirical formula is
into small molecules. They are classified C H O .
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according to two different characteristics; Pentoses: These are monosaccharides
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the placement of its carbonyl group and containing five carbon atoms in
the number of carbon atoms it contains. As their molecules. Examples of
for the first characteristic, if the carbonyl pentoses are ribose, deoxyribose,
group is an aldehyde, the monosaccharide ribulose and arabinose. Their
is an aldose and if the carbonyl group is empirical formula is C H O .
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a ketone, the monosaccharide is a ketose. Hexoses: These are monosaccharides
Monosaccharides are further classified containing six carbon atoms
according to the number of carbon atoms in their molecules, such
they contain, therefore, monosaccharides monosaccharides include
with three carbon atoms are called trioses; glucose, fructose, and galactose.
those with four carbon atoms are called Their empirical formula is
tetroses; those with five carbon atoms C H O . They are the most
are called pentoses; and those with common monosaccharides.
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six carbon atoms are called hexoses.
These two systems of classification of Heptoses: These are monosaccharides
monosaccharides are often combined. containing seven carbon atoms
For example, glyceraldehyde an in their molecules. Their
aldotriose (a three-carbon aldehyde), empirical formula is C H O .
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ribose is an aldopentose (a five-carbon
aldehyde), glucose is an aldohexose (a Properties of monosaccharides
six-carbon aldehyde), dihydroxyacetone Most monosaccharides have a sweet taste.
is a ketotriose (a three-carbon ketone), They exist in a crystalline solid form at room
ribulose is a ketopentose (a five-carbon temperature and are extremely soluble in
ketone) and fructose is a ketohexose (a water; despite their high molecular weights.
six-carbon ketone). The presence of large number of OH groups
makes the monosaccharides much more
Trioses: These are the smallest molecules water soluble than most other molecules
of monosaccharides which of similar molecular weight. Moreover, all
contain three carbon atoms monosaccharides are reducing sugars; as
in their molecules. Examples they have free aldehyde or ketone group.
of trioses are glyceraldehyde They reduce mild oxidising agents, such
and dihydroxyacetone. Their as Tollens’ reagent, Fehling as well as
empirical formula is C H O . Benedict’s solution.
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