Download 9 Carbohydrates

9 Carbohydrates
Carbohydrates, also called saccharides, are compounds formed from carbon, hydrogen and oxygen. They are
classified according to their structures:
• Monosaccharides (monomers) e.g. glucose
• Disaccharides (dimers) e.g. table sugar
• Oligosaccharides (three to ten monomers)
• Polysaccharides (more than ten monomers)
Functions of carbohydrates include:
• Being raw materials for synthesis of other molecules
• Energy storage
• Providing structural support
• Indicating cell identity
Monosaccharides have the empirical chemical formula (CH2O)n, where n represents the number of carbon-hydrate
groups. While glucose is a monosaccharide with n=6, the simplest monosaccharide is one in which n=3. As the
simplest carbohydrates, functions of monosaccharides include providing chemical energy to cells and serving as
building blocks in the synthesis of larger compounds.
A monosaccharide molecule is made up of:
• a carbonyl group (C=O)
• a carbon skeleton of single bonds
• multiple hydroxyl group (-OH)
Monosaccarides are classified by:
1. the precise location of the carbonyl group: aldoses have the carbonyl group at the end of the carbon skeleton,
ketoses have the carbonyl group located within the carbon skeleton.
2. the length of the carbon skeleton: trioses have three carbons, tetroses have four carbons, etc until seven.
3. the arrangement of the hydroxyl groups around the asymmetric carbon: results in isomers.
Structural formulas commonly represent sugars as linear chain models, but this form rarely exists in nature.
Especially in solutions, sugars form a ring structure.
Compound
Glucose
Straight-chain structure
Ring structure
Fructose
Organisms convert monosaccharides that are not immediately needed into polysaccharides, to permit more efficient
long-term storage inside cells. This conversion occurs through the process of a dehydration (or condensation)
reaction. A glycosidic linkage joins two monomers. This covalent bond is formed when a hydroxyl group is lost from
one monomer and a hydrogen atom is lost from the other, resulting in the release of a water molecule.
Polysaccharides (such as starch, glycogen, cellulose, chitin and peptidoglycan) serve two primary functions in living
organisms: energy storage and structural support.
The primary energy storage polysaccharide in plants is starch, which is composed of glucose monomers. Two forms
of starch occur in nature: amylose and amylopectin. Amylose consists of α-glucose monomers with 1-4 glycosidic
linkages. Amylopectin contains 1-4 linkages and 1-6 linkages, which results in branching.
Animals store glucose as glycogen. Glycogen is made up of α-glucose chains in a structure similar to amylopectin,
but has more branches and 1-6 glycosidic linkages. Animals store glycogen primarily in liver and muscle cells. Both
starch and glycogen is hydrolysed to provide glucose for energy as needed.
Cellulose is one of the most important structural polysaccharides. Cellulose is a linear chain of β-glucose monomers,
and this branch-free structure allows hydrogen bonding to occur between adjacent cellulose molecules, resulting in
strong, parallel grouping of molecules called microfibrils. Cellulose is a major component of wood and cotton.
Chitin occurs in the cell walls of fungi and in the exoskeletons of arthropods. Chitin’s structure is similar to that of
cellulose, however its glycosidic linkages join a chain of nitrogen-containing monomers related to glucose. Similar
hydrogen bonds connect chitin’s monomer chains.
Peptidoglycan is a polysaccharide with a short chain of amino acids. It is abundant as structural support in the cell
wall of many bacterial species.
Besides energy storage and structure, sugars help cells recognise one another. All cells feature glycoproteins
(combined sugar-protein molecules) on their surface. Glycoproteins are composed of proteins covalently bonded to
an unique oligosaccharide. A cell’s glycoproteins conveys information about the cell type and species, which is
critical in cell-cell interactions such as an immune response.