CARBOHYDRATES
What are Carbohydrates?
The carbohydrates
are widely distributed in both animals and plant tissues. In plants, carbohydrates are produced by photosynthesis while in the animal’s cells, carbohydrates are
produced in the form of glucose and glycogen serves as an important source of
energy for vital activities. Some carbohydrates have highly specific functions, e, g, Ribose in the nucleoproteins of the cells, glucose in certain lipids and lactose in milk.
Definition
Carbohydrates
can be defined as polyhydroxy alcohols
with aldehydes or ketones and their derivatives.
Classification of carbohydrates
Types of CarbohydratesCarbohydrates
can be divided into 4 major groups, which are as follows
- Monosaccharide
- Disaccharides
- Oligosaccharides
- Polysaccharides
Monosaccharides
Monosaccharides are
also called simple sugars, which cannot be hydrolyzed into a simpler form.
The general formula is Cn(H2O)n, these can be further subdivided as follows
Carbon atoms |
Aldoses |
Ketoses |
trioses |
Glyceraldehydes or glycerol |
Dihydroxy-acetone |
tetroses |
Erythrose |
Erythrulose |
pentoses |
Ribose, Xylose,
Arabinose |
Ribulose |
hexoses |
Glucose,
Galactose, Mannose |
Fructose |
heptoses |
Glucoheptose,
glactoheptose |
sedoheptulose |
Simplest form of aldose – glyceraldehydes
Simplest form of ketose – dihydroxyacetone
Commonest aldose – glucose
Commonest ketose – fructose
These carbohydrates produce two molecules of the same
or of different monosaccharide’s on hydrolysis. The general formula is Cn(H2O)n-1.
Examples are
Maltose = 1 MOL. Glucose
+ 1 MOL. Glucose
Lactose = 1 MOL. Glucose
+ 1 MOL. Galactose
Sucrose = 1 MOL.
Glucose+ 1 MOL fructose,
Maltose
it does not occur in the body
The sources of it
are germinating cereals and malt, and it is the intermediary product of the breakdown of starch by amylase in the alimentary canal.
It is hydrolyzed to glucose by enzyme maltose and the product is absorbed.
Lactose (milk sugar)
It is present in
milk and formed in the lactating mammary gland
It is hydrolyzed to glucose and galactose by the enzyme lactase in the alimentary canal and the
product are absorbed
Sucrose
It does not exist
in the body, but occurs in cane sugar, pineapple, carrot roots, sweet potatoes
and honey
It is hydrolysed
to glucose and fructose, the enzymes' invertase in the alimentary canal. The product
of hydrolysis is then absorbed in the body
Polysaccharides
These carbohydrates yield more than 10 molecules of monosaccharide on hydrolysis.
These can be further classified into homo polysaccharides and hetero polysaccharides
depending on the presence of either the same monosaccharide or more than simple sugar
in alternating repeating sequence
Cellulose
It is the main
constituent of the supporting tissue of plants and forms a considerable part of
our vegetable food, it does not occur in the human body.
It is of
considerable human dietetic value only because it adds bulk to the intestinal
contents, thereby stimulating peristalsis and elimination of food residues.
Glycogen
It is the reserve
carbohydrate found in the liver and muscles of animals and human beings.
The glycogen
content of the liver is more than that of muscle.
It is also found
in plants, which have no chlorophyll system e.g., fungi and yeasts, but not in
green plants.
Starch
It is the stored carbohydrates of chlorophyll containing plants. In plants, the starch is laid
down in the cell granules.
It is the most
important source of carbohydrates in our food and is found in cereals, potatoes
in high concentration
Oligosaccharide
These carbohydrates
yield two to ten monosaccharide’s units on hydrolysis for ex materials
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