SBI 4U
More About Carbohydrates
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Date: ___________________________
Monosaccharides
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Monosaccharides are the simplest form of carbohydrate, consisting of a single sugar unit and
acting as a building block for more complex carbohydrates.
Monosaccharides are used as sources of energy in most biological systems.
Monosaccharides contain hydroxyl and carbonyl functional groups (so they have – OH alcohol
bonds and the carbonyl group of either an aldehyde or a ketone). Most of the functional groups
found in biological molecules make the molecule they are in polar so monosaccharides tend to
be water soluble.
The most common monsaccharides contain 3 carbons (these are trioses), five carbons (these are
pentoses) or 6 carbons (these are hexoses). All of these occur in linear form but any
monosaccharide with 5 or more carbon atoms like the pentoses and hexoses will form ring
structures in water.
Example:
Isomers are molecules that have the same molecular formula (the same types and numbers of
atoms) but different arrangements of their atoms. The different arrangements of atoms mean
that the molecules have different physical and chemical properties. In the diagram above, alpha,
 , and beta,  , glucose have the same molecular formula but their atoms are arranged
differently. Glucose, fructose and galactose are all isomers of one another, too.
Disaccharides
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Disaccharides consist of 2 monosaccarides joined together by a dehydration synthesis reaction
in which water is a product.
Disaccharides are used as sources of energy in most biological systems.
The bonds linking the two monosaccharides together are called glycosidic bonds.
(Diagrams are from Nelson Biology 12, 2012.)
Examples:
A dehydration synthesis reaction
between two molecules of the
monosaccharide   glucose result in a
bond between carbon 1 of the molecule
on the left and carbon 4 of the molecule
on the right. This is called a 1  4
glycosidic linkage. This 1  4 linkage is
an  - linkage because the OH group
attached to carbon number 1 is oriented
downward.
Water is a by-product of all dehydration
synthesis reactions.
The resulting disaccharide formed when
2 glucose molecules undergo a
dehydration synthesis reaction to form a
1  4  -glycosidic linkage is called
maltose. Maltose is important in beer
making. It is also used as a food source.
To the left is a 1  5   glycosidic bond
between one molecule of glucose and
one molecule of fructose to make the
disaccharide, sucrose, or table sugar.
To the left is a 1  4   glycosidic
linkage between one molecule of
galactose and one molecule of glucose
to form one molecule of the disaccharide
lactose, found in milk.
Complex Carbohydrates: Polysaccharides
 A complex carbohydrate is formed when hundreds to thousands of monosaccharides link
together in a chain. There are two basic types of complex carbohydrates:
a) storage carbohydrates like starch and glycogen which store monosaccarides in plants and
animals respectively, until they are needed as fuels to release energy.
b) structural carbohydrates like the cellulose in plant cell walls that makes them hard and
protective and the chitin that forms the hard shells or exoskeletons of insects and
crustaceans.
 A polysaccharide molecule is a chain of monsaccharides with many subunits joined together by
glycosidic linkages. These are macromolecules – very large molecules.
 The dehydration synthesis reactions that form the glycosidic linkages between the identical or
non-identical monosaccharides (also referred to as “units” or “monomers”) are examples of
polymerization reactions, reactions that link smaller units to form much larger units or
polymers.
 Figure 11 page 32 compares the glycosidic linkages in cellulose and in starch. Humans can break
down the   linkages in starch and in glycogen but do not contain the digestive enzymes
required to break down the   linkages in cellulose. Instead, the cellulose from the cell walls of
plant material acts as indigestible fibre in the human digestive tract. Other organisms like sheep,
cows and rabbits contain bacteria and protists in their digestive tracts that produce the enzymes
required to break down the cellulose in grass and plants so that its monomers can be used as a
source of energy.
 Note that polysaccharides are often polar molecules and are hydrophilic. They will not dissolve
in water, though, because they are too large. The cellulose molecules that paper towels are
made from make them absorbent to water but do not let them dissolve away in it.