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Carbohydrates contain 3 elements:
Carbohydrates are found in one of three forms:
Disaccharides (both sugars)
(CH2O)n where n is a number between 3 and 9. They are classified according to the number
of carbon atoms. The monosaccharides you will have to know fall into these categories:
C = 3 = triose
C = 4 = tetrose
C = 5 = pentose
C = 6 = hexose
Trioses: (e.g. glyceraldehydes), intermediates in respiration and photosynthesis.
Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids (RNA and DNA),
co-enzymes (NAD, NADP, FAD) and ATP.
Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration and as building
blocks for larger molecules.
All but one carbon atom have an alcohol (OH) group attached. The remaining carbon atom
has an aldehyde or ketone group attached.
Due to the bond angles between the carbon atoms, it
is possible for pentoses and hexoses to form stable
ring structures. The carbon atoms are numbered 1 to
5 in pentoses and 1 to 6 in hexoses.
Depending on the orientation of the OH group on carbon 1, the monosaccharide can have
either or configurations.
These are formed when two monosaccharides are condensed together. One
monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH
group from carbon 4 to form the bond.
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The reaction, which is called a condensation reaction, involves the loss of water (H2O) and
the formation of an 1,4-glycosidic bond. Depending on the monosaccharides used, this can
be an -1,4-glycosidic bond or a -1,4-glycosidic bond.
The reverse of this reaction, the formation
of two monosaccharides from one
disaccharide, is called a hydrolysis reaction
and requires one water molecule to supply the H and OH to the sugars formed.
Examples of Disaccharides
Sucrose: glucose + fructose,
Lactose: glucose + galactose,
Maltose: glucose + glucose.…read more
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Functions of carbohydrates
Substrate for respiration (glucose is essential for cardiac tissues).
Intermediate in respiration (e.g. glyceraldehydes).
Energy stores (e.g. starch, glycogen).
Structural (e.g. cellulose, chitin in arthropod exoskeletons and fungal walls).
Transport (e.g. sucrose is transported in the phloem of a plant).
Recognition of molecules outside a cell (e.g. attached to proteins or lipids on cell
Iodine solution or potassium iodide solution can be used to test for the presence of starch.…read more