CARBOHYDRATESCarbohydrates are a group of organic compounds that contains

only carbon, hydrogen and oxygen.It usually have an empirical formula Cm(H2O)n

Common carbohydrates in biochemistry are glucose, fructose, ribose, glyceraldehyde, sucrose, maltose, starch, cellulose and glycogen.

Carbohydrates are termed sugar molecules, and are often classified as, monosaccharide, disaccharide, oligosaccharide and polysaccharide

MonosaccharideMonosaccharides are the basic sugar molecules.They are termed monomers, since they are the building block of

disaccharides and more complex carbohydrate.

Monosaccharide exists in the form of 3-C, 5-C and 6-C sugars.The sugars associated with these number of carbons are called triose,

pentose and hexose sugar respectively.

Examples include glyceraldehyde (3-C), ribose (5C) and glucose (6-C)

Monosaccharides have the general formula CnH2nOn

Glucose

Glucose formula is C6 H12 O6

It exist as both a straight chain molecule and a ring molecule.In solution, the ring molecule is the most abundant configuration.

Hexagonal ring structure: Straight Chain structure

Glucose Cont’dThe carbon atoms in organic molecules are usually numbered. In

the case of glucose, the numbering begins at the most electron deficient carbon.

That is, the carbon attached to two oxygen atom is number one, and the counting takes place in a clockwise direction.

Glucose Cont’d

Glucose, like other monosaccharides, exist in two forms, alpha and beta glucose.

In α-glucose, the hydroxyl group (OH-), attached to carbon-1 is pointed in the opposite direction as carbon-6 (downward).

In β-glucose, the hydroxyl group (OH-), attached to carbon-1, is pointed in the same direction as carbon-6 (upward).

FructoseFructose has the same molecular formula as glucose, but differs

in molecular structure.

In the ring form, fructose has a 5-membered ring. It also has the α and β forms.

In α-fructose, the hydroxyl group (OH-), attached to carbon-2 is drawn pointed upward.

In β-fructose, the hydroxyl group (OH-), attached to carbon-2, is pointed downward.

Alpha Fructose Beta Fructose

DisaccharideDisaccharides are made up of two monomers.These include sugars like maltose and sucrose.

Maltose is formed from the condensation reaction of two glucose molecules. In this process a water molecule is lost. Enzymatic hydrolysis reverses the process.

The glucose molecules are joined together by α 1-4 glycosidic bond.

Disaccharide Cont’dSucrose is formed from the condensation reaction between

α-glucose and β-fructose. In this process a water molecule is lost.

The sugar molecules are joined together by α1-β2 glycosidic bond.

PolysaccharideThe major polysaccharides of biological importance are starch,

cellulose and glycogen.

These are polymers of glucose; however, they differ in the type of glucose present and the and the bond which link the glucose monomers together.

Starch is commonly found in ……………………..Cellulose is commonly found in ……………………………Glycogen is commonly found in ……………………………

StarchStarch is made up of α-glucose, and It exist in two forms, amylose

and amylopectin.In amylose, the glucose monomers are linked by 1-4 glycosidic

bonds. This produces an unbranched chain of glucose which then folds up

to form a coil or helix.

In amylopectin there are two types of glycosidic bonds: 1,4 and 1,6. The 1,4 links are the same as in amylose. In addition some glucose molecules have a 1-6 glycosidic link to a new glucose molecule.

Amylopectin is therefore a branched polymer

Starch Cont’d

Amylose(common form of starch)

Starch Cont’d

Amylopectin

Starch Cont’d

Functions of Starch

• It stores energy in plants• Provide food for plants during drought• Its storage provides mass for tubers, thus helping to anchor

the plant • To facilitate homeostasis in plants• Facilitates the transfer of energy from plants to animals• Facilitate the timely delivery of glucose from the gut

(intestine) to the blood.

CelluloseCellulose is an unbranched polymer composed of beta glucose

molecules.The glucose monomers are linked by 1,4 glycosidic bonds. Hydrogen

bonds between adjacent cellulose molecules allow them to form strong fibres

GlycogenGlycogen is a α-glucose polysaccharide, that has a closely

resembled configuration to starch-amylopectin. It has α 1-4 and α 1-6 glycosidic linkages. That is, it forms more branch chains that amylopectin.

Glycogen Cont’d

Functions of glycogen

• It allow animals to store energy (in liver and muscles).• I supplies the blood and animal cells with α-glucose.• I helps to facilitate homeostasis in animals.• It enables muscle cells to receive energy in a timely

manner.

Tests CarbohydratesSugars are also classified as reducing and non-reducing sugars.

Reducing sugars are those that turns the blue Benedict’s reagent to a brick red precipitate, after heating. In this reaction, the Cu2+ ion, of the copper sulphate solution, is reduced to Cu+ ion (copper oxide).

Examples of reducing sugars are glucose, fructose and maltose.

Non-reducing sugars are those that will not reduce Cu2+ ion to Cu+

ion. However, these sugars, eg. sucrose, can be hydrolyzed by heating with dilute hydrogen acid, then upon further treatment with Benedict’s reagent, reduce Cu2+ ion to Cu+ ion.

In the hydrolysis reaction, the glycosidic bond in sucrose is broken to release the monomers (glucose and fructose), which are reducing sugars.