Cellular macromolecules

ISC 11>Content>Unit-4>Biomolecules : CARBOHYDRATES

Carbohydrates- general classification and functions of: monosaccharides (glucose, ribose and deoxyribose), disaccharides (maltose, lactose and sucrose), polysaccharides (glycogen, starch, cellulose, inulin, and chitin).

CLASS PRESENTATION

Carbohydrates:

CARBOHYDRATES ARE ALDEHYDE OR KETONE DERIVATIVES OF POLYHYDRIC ALCOHOLS.
Carbohydrates include sugars and the polymers of sugars.

Sources of carbohydrates

Types (Classification) of carbohydrate

Monosaccharides are those carbohydrates that cannot be hydrolyzed into simpler carbohydrates:
Disaccharides are condensation products of two monosaccharide units.
Oligosaccharides are condensation products of two to ten monosaccharides.
Polysaccharides are condensation products of more than ten monosaccharide units.

MONOSACCHARIDES

Monosaccharides are the simplest form of carbohydrates which cannot be broken down to simpler sugar by hydrolysis.
A) Chemical composition:

Composed of the elements C, H and O. (H & O in the ratio of 2:1)
General formula- Cn(H2O)n, n=number of c-atoms where n varies from 3 to 7

B) Properties:

Simple carbohydrate – one saccharide or sugar unit.
Do not hydrolyze further.
Sweet in taste.
Soluble in water.
Low molecular weight.

C) Classification of monosaccharides:

They may be classified on the basis of number of carbon as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms; and as aldoses or ketoses depending upon whether they have an aldehyde or ketone group.

Aldoses

Ketoses

Most common monosaccharides

Glucose (6C)
Fructose (6C)
Galactose (6C)
Ribose (5C)

1. Glucose

Occurs in fruits, like grapes, and honey.
It is white crystalline solid readily soluble in water and sweet in taste.

2. Fructose
• Most common sugar in fruits, honey.
• Sweetest among all the natural sugar.

3. Galactose

Occurs in combined form as constituent of lactose(is not found free in nature).
It is rarely found in plants.

4. Ribose and deoxyribose
In DNA : deoxyribose sugar
In RNA: Ribose sugar

Drawing the Cyclic Structure : Glucose

D and L Notations

Alpha and beta forms

Reference link :

Basic sugar chemistry

Functions of monosaccharides

Have important role in metabolism.
Trioses, pentoses, heptosse etc are intermediates in the pathway of photosynthesis.
Glucose is broken to release energy during respiration.
Mammary glands synthesise lactose from glucose.
Ribose and deoxy ribose are components of nucleic acids.
Used in synthesis of fats and amino acids.

DISACCHARIDES

These are carbohydrates that yield two molecules of same or different types of monosaccharides on hydrolysis.
The general formulais Cn(H2O)n-1

LACTOSE

Lactose is called as milk sugar.
It is present in milk and is made up of monosaccharides - glucose and galactose.

Glucose + Galactose= Lactose

MALTOSE

Maltose is also known as ‘malt sugar’ and is present in germinating cereals, malt etc.
It is the intermediate product in the hydrolysis of starch by amylase in the alimentary canal.
It is made up of 2 molecules of glucose.

Glucose + Glucose= Maltose

SUCROSE

Sucrose is called as ‘table sugar’ or ‘cane sugar’.
It is the common sugar and is widely distributed in all photosynthetic plants.
It does not exist in the body but occurs in sugarcane, pineapple, sweet potato and honey.
It is made up of glucose and fructose.

Glucose + Fructose= Sucrose

Glycosidic bond

A glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate.

The subunits of disaccharides and polysaccharides are linked by glycosidic bonds.
During this bond formation water is formed and thus is called as Condensation.

POLYSACCHARIDES

Polysaccharides are long carbohydrates molecules of repeated monomer units joined together by glycosidic bonds.

These are polymers of monosaccharides joined together by glycosidic linkage, between 1st and 4th C-atom of two units.
Number of monomers- 10 to thousands.
High molecular weight and colloidal size.
Branched or unbranched linear chains
Sparingly soluble in water.

Polysaccharides :Classification

1) Based on Composition
A. Homopolysaccharides

Formed by one type of monosaccharide.
Eg. Glycogen, starch and cellulose.

B. Heteropolysaccharides

Formed by more than one type of monosaccharide.
Eg. Mucopolysaccharides, glycoproteins, peptidoglycans.

2) Based on functions
A) Food storage polysaccharides : Starch, Glycogen, Inulin
B) Structural polysaccharide: Cellulose, Chitin
C) Mucopolysaccharides :

Common polysaccharides

1. STARCH

Starch is a homopolymer of glucose forming an α- glycosidic chain, called a glucosan or glucan.
It is the most abundant dietary carbohydrate in cereals, potatoes, legumes, and other vegetables.
The two main constituents are amylose (15–20%), which has a nonbranching helical structure and amylopectin (80–85%), which consists of branched chains composed of 24–30 glucose residues united by 1 → 4 linkages in the chains and by 1 → 6 linkages at the branch points

a) AMYLOSE

b) AMYLOPECTIN

c) the starch-iodine complex.

2. GLYCOGEN

Glycogen is the storage homopolysaccharide in animals.
It is a more highly branched structure than amylopectin, with chains of 12–14 α -D-glucose residues (in α[1 → 4]-glycosidic linkage), with branching by means of α(1 → 6)-glycosidic bonds.
It is mainly stored in the muscles and liver of mammals.
Fungi also store food in the form of glycogen.
The glycogen is converted to glucose as and when required.Hydrolysis

3. INULIN

Inulin is a polysaccharide of fructose (and hence a fructosan) found in tubers and roots of dahlias, artichokes, and dandelions.
Fructose units are joined by β-2,1 linkage.
It is readily soluble in water and is used to determine the glomerular filtration rate.

Dandelion

4. CELLULOSE

Fibrous, tough, water insoluble.
Wood (25-50%) and cotton (90%) contains cellulose.
Form long tensile fibers that form bundles called microfibrils.
They are held by hydrogen bonds.
Animals digest, humans cannot – form roughage.

USES OF CELLULOSE

Cellulose-containing material are used for shelter, fuel, tools etc.
Fibres of cotton, jute and linen are used for making textile, ropes and bags.
In synthesis of artificial rayon.
Cellulose acetate is used to make plastics and shatter-proof glass.
Cellulose nitrate is used in propellant explosives.
Carboxymethyl cellulose is added to ice-creams, cosmetics and medicine to emulsify and to give smooth texture.
Cellulose rich wood is used in building furniture, tools, sports article etc.
Cellulose rich plant material are used in making paper.
Major component of roughage.

5. CHITIN

Chitin is a structural polysaccharide found in the exoskeleton of insects,crabs, lobsters and cell wall of fungi.
Composed of units of N-acetyl--D-glucosamine joined by 1,4-glycosidic bonds.
Being insoluble in water, chitin helps organisms in retaining the form and strengthening the structure.

Exoskeleton of insects

6. MUCOPOLYSACCHARIDES

Peptidoglycan :Occur in cell wall of bacteria.
Mucilage: Slimy substance produced by plants. (mucilage), Isabgol seeds,
Hyaluronic acid: matrix of connective tissues and as a synovial fluid.
Heparin: Anticoagulant
Pectin : Component of cell wall and middle lamella
Agar- agar: Obtained from sea weeds, polymer of galactose.

Functions of Polysaccharides

Storage food: Starch and glycogen
Energy :Hydrolysis of starch and glycogen give glucose units.
Structural compounds:Cellulose, chitin and peptidoglycan
Protective coat: Mucilage forms protective coat around aquatic plants, algae and bacteria.
Anticoagulant: Heparin prevents blood clotting in blood vessels.
Lubricant :Hyaluronic acid acts as a lubricant.
Medicinal importance: Some mucopolysaccharides like husk of Plantago ovata (Isabgol), mucilage of aloe, agar, carageen obtained from marine algae source of medicinal and commercial value.
Immunological reactions: Glycoprotein and glycolipids present at cell surface recognise stimuli reaching the cells.

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