Polysaccharides (or simply glycans) consist of repeat units of monosaccharides or their derivatives, held together by
glycosidic bonds. They are primarily concerned with two important functions-structural, and storage of energy.
Polysaccharides are linear as well as branched polymers. This is in contrast to structure of proteins and nucleic acids
which are only linear polymers. The occurrence of branches in polysaccharides is due to the fact that glycosidic linkages
can be formed at any one of the hydroxyl groups of a monosaccharide. Polysaccharides are of two types
1. Homopolysaccharides which on hydrolysis yield only a single type of monosaccharide. They are named based
on the nature of the monosaccharide unit. Thus, glucans are polymers of glucose whereas fructosans are polymers
of fructose.
2. Heteropolysaccharides on hydrolysis yield a mixture of a few monosaccharides or their derivatives.
Homopolysaccharides
Starch
A. Structure of Starch
i. It is the reserve carbohydrate of plant kingdom
which is the most important dietary source for
higher animals, including man.
ii. Sources: Potatoes, tapioca, cereals (rice, wheat)
and other food grains.
iii. Starch is a homopolymer composed of D-
glucose units held by α-glycosidic bonds. It is
known as glucosan or glucan. Starch consists of
two polysaccharide components-water soluble
amylose (15-20%) and a water insoluble
amylopectin (80-85%). Amylose is made up of
200-1,000 D-glucose units joined with α-1,4
glycosidic linkages to form an unbranched long chain with a molecular weight 400,000 D or more.
iv. Amylopectin molecule containing a few thousand glucose units that looks like a branched tree (20-
30 glucose units per branch) with molecular weight more than 1 million. The branching points are
made by α-1,6 linkage (similar to isomaltose)..
B. Hydrolysis of Starch
i. Starch will form a blue colored complex with iodine; this color disappears on heating and reappears
when cooled. This is a sensitive test for starch. Starch is non reducing because the free sugar groups
are negligible in number
ii. When starch is hydrolysed by mild acid, smaller and smaller fragments are produced.
iii. Thus hydrolysis for a short time produces amylodextrin which gives violet color with iodine and is
nonreducing. Further hydrolysis produces erythrodextrin which gives red color with iodine and mild
reduction of Benedict’s. solution. Later achrodextrins (no color with iodine, but reducing) and
further on, maltose (no color with iodine, but powerfully reducing) are formed on continued
hydrolysis.
C. Action of Amylases on Starch
i. Salivary amylase and pancreatic amylase are alpha-amylases, which act at random on alpha- 1,4
glycosidic bonds to split starch into smaller units (dextrins), and finally to maltose.
ii. Beta-amylases are of plant origin (almond, Germinating seeds, etc) which split starch to form beta-
maltose. They act on amylose to split maltose units consecutively. Thus the enzyme starts its action
from one end.
iii. When beta-amylase acts on amylopectin, maltose units are liberated from the ends of the branches
of amylopectin, until the action of enzyme is blocked at the 1,6-glycosidic linkage. The action of
beta-amylase stops at branching points, leaving a large molecule, called limit dextrin or residual
dextrin.
glycosidic bonds. They are primarily concerned with two important functions-structural, and storage of energy.
Polysaccharides are linear as well as branched polymers. This is in contrast to structure of proteins and nucleic acids
which are only linear polymers. The occurrence of branches in polysaccharides is due to the fact that glycosidic linkages
can be formed at any one of the hydroxyl groups of a monosaccharide. Polysaccharides are of two types
1. Homopolysaccharides which on hydrolysis yield only a single type of monosaccharide. They are named based
on the nature of the monosaccharide unit. Thus, glucans are polymers of glucose whereas fructosans are polymers
of fructose.
2. Heteropolysaccharides on hydrolysis yield a mixture of a few monosaccharides or their derivatives.
Homopolysaccharides
Starch
A. Structure of Starch
i. It is the reserve carbohydrate of plant kingdom
which is the most important dietary source for
higher animals, including man.
ii. Sources: Potatoes, tapioca, cereals (rice, wheat)
and other food grains.
iii. Starch is a homopolymer composed of D-
glucose units held by α-glycosidic bonds. It is
known as glucosan or glucan. Starch consists of
two polysaccharide components-water soluble
amylose (15-20%) and a water insoluble
amylopectin (80-85%). Amylose is made up of
200-1,000 D-glucose units joined with α-1,4
glycosidic linkages to form an unbranched long chain with a molecular weight 400,000 D or more.
iv. Amylopectin molecule containing a few thousand glucose units that looks like a branched tree (20-
30 glucose units per branch) with molecular weight more than 1 million. The branching points are
made by α-1,6 linkage (similar to isomaltose)..
B. Hydrolysis of Starch
i. Starch will form a blue colored complex with iodine; this color disappears on heating and reappears
when cooled. This is a sensitive test for starch. Starch is non reducing because the free sugar groups
are negligible in number
ii. When starch is hydrolysed by mild acid, smaller and smaller fragments are produced.
iii. Thus hydrolysis for a short time produces amylodextrin which gives violet color with iodine and is
nonreducing. Further hydrolysis produces erythrodextrin which gives red color with iodine and mild
reduction of Benedict’s. solution. Later achrodextrins (no color with iodine, but reducing) and
further on, maltose (no color with iodine, but powerfully reducing) are formed on continued
hydrolysis.
C. Action of Amylases on Starch
i. Salivary amylase and pancreatic amylase are alpha-amylases, which act at random on alpha- 1,4
glycosidic bonds to split starch into smaller units (dextrins), and finally to maltose.
ii. Beta-amylases are of plant origin (almond, Germinating seeds, etc) which split starch to form beta-
maltose. They act on amylose to split maltose units consecutively. Thus the enzyme starts its action
from one end.
iii. When beta-amylase acts on amylopectin, maltose units are liberated from the ends of the branches
of amylopectin, until the action of enzyme is blocked at the 1,6-glycosidic linkage. The action of
beta-amylase stops at branching points, leaving a large molecule, called limit dextrin or residual
dextrin.
