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Carbohydrates

Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen atoms in the proportion of 1:2:1 (CH₂O)_n

Some carbohydrates serve as a source of
energy. Other carbohydrates are used as structural materials.

Carbohydrates are grouped as

• monosaccharides,
• disaccharides, or
• polysaccharides.

MONOSACCHARIDES

A monomer of a carbohydrate is called a monosaccharide. A monosaccharide—or simple sugar—
contains carbon, hydrogen, and oxygen in a ratio of 1:2:1. The general
formula for a monosaccharide is written as (CH₂O)_n, where n
is any whole number from 3 to 8.

Monosaccharides are also grouped according to the number of carbon atoms in their structure:

Trioses: (3C) Glyceraldehydes

Pentoses: (5C) Ribose and deoxyribose are common
pentoses found in DNA and RNA

Hexoses: (6C) Glucose, fructose, galactose, and other six-carbon sugars.

(Below you can see two-dimensional chain structures of different monosachharides. You are not supposed know these structures!)

Glucose is a main source of
energy for cells. Fructose is found in fruits and is the sweetest of
the monosaccharides. Galactose is found in milk.

Glucose, fructose, and galactose have
the same molecular formula, C₆H₁₂O₆, but differing structures. Compounds like these sugars, with a single
chemical formula but different structural forms, are called isomer.

When glucose forms a ring, two isomeric forms are possible: &alpha-glucose and &beta-glucose

DISACCHARIDES

Two monosaccharides joined by a covalent bond called disaccharides. This covalent bond is named as glycosidic bond.Here are some common disaccharides:

The glycosidic linkage of a disaccharide generally forms between carbon 1 of one molecule and carbon 4 of the other molecule. That's why it is called as 1,4 glycosidic bond.

POLYSACCHARIDES

The most abundant carbohydrates are the polysaccharides, a group that includes

• starches,
• glycogen, and
• cellulose.

A polysaccharide is a macromolecule consisting of repeating units of simple sugars, usually glucose. The polysaccharide may be a single long chain or a branched chain.

Starch

Starch, the typical form of carbohydrate used for energy storage in plants, is a polymer consisting of α-glucose subunits.Starch occurs in two forms;

1. amylose: the simpler form, is unbranched
2. amylopectin: consists of about 1000 units in a branched chain

It is the 1,6 glycosidic bond that makes the branches.

Plant cells store starch mainly as granules within specialized organelles called amyloplasts.

Glycogen

Glycogen (sometimes referred to as animal starch) is the form in which glucose is stored as an energy source in animal tissues. It is similar in structure to plant starch, but more extensively branched and more water-soluble. Glycogen is stored mainly in liver and muscle cells.

Cellulose

Cellulose is the most abundant carbohydrate.Wood is about half cellulose, and cotton is at least 90% cellulose.Cellulose is a structural carbohydrate

When a chain of glucose molecules consists of all beta-glucose subunits, a polysaccharide with very different properties results.

This structural polysaccharide is cellulose, the chief component of plant cell walls.

Humans, like most organisms, do not have enzymes that can digest cellulose and therefore cannot use it as a nutrient.cellulose-digesting bacteria live in the digestive systems of cows and sheep, enabling these grass-eating animals to obtain nourishment from cellulose.

Cellulose molecules have characteristics that make them well suited for a structural role. The beta-glucose subunits are joined in a way that allows extensive hydrogen bonding among different cellulose molecules.

Chitin

The structural building material in insects, many fungi, and certain other organisms is called chitin. Chitin is a modified form of cellulose with a nitrogen group added to the glucose units. Chitin forms very tough structures because, as in cellulose, its molecules interact through multiple hydrogen bonds.

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