These are single sugar units which can not be hydrolysed furthur into smaller carbohydrates. General formula is , e.g. Triose-3C, glyceraldehyde, dihydroxyacetone, etc., tetrose, pentose, hexose, etc. About 70 monosaccharides are known, out of which only 20 are present in plants and animals.

(i) Important Hexoses
(a) Glucose : . Grape sugar is dextrose. Grape is sour due to presence of tartaric acid. Fructose is called fruit sugar (sweetest among natural sugars) and glucose is called " sugar of body". Normal level of blood glucose is 80-120mg/100ml. If it exceeds then condition is called "glucosuria".
(b) Fructose : Occurs naturally in fruit juices and honey. Hydrolysis of cane sugar in body also yields fructose.
(c) Galactose : It is called as brain sugar. It's an important constituent of glycolipids and glycoproteins.
(d) Mannose : It is obtained on hydrolysis of plant mannans and gums. It is constituent of albumins, globulins and mucoproteins.

(a) Monosaccharides are colourless, sweet tasting, solids.
(b) Due to asymmetric carbon, they exist in different isomeric forms. They can rotate polarized light hence they are dextrorotatory and leavorotatory.
(c) D-glucose after reduction gives rise to a mixture of polyhydroxy alcohol, sorbitol or mannitol.
(d) The sugars with a free aldehyde or ketone group reduce to (cupric to cuprous)
(e) Sugars show oxidation, esterification and fermentation.
(f) The aldehyde or ketone group of a simple sugar can join an alcoholic group of another organic compound bond C-O-C the process involves loss of water and is called condensation (H-O-H).

(a) Glucose is the ultimate source of ATP in the cell respiration.
(b) It is used in formation of vitamin C.
(c) The intermediate compounds for the formation of glucose in photosynthesis are triose, tetrose, pentose and heptose, etc.
(d) Galactose is a constituent of agar-agar.
(e) Glucose is a blood sugar and xylose is a non nutritive sweetner.
(f) Polymerisation of these molecules forms macromolecules.
(g) Ribose and deoxyribose are constituent of nucleic acids and nucleotides
(h) Glyceraldehyde and dihydroxyacetone are trioses.
(i) Sugars have free aldehyde or ketone group which can reduce Cu++ to Cu+ and are called reducing sugars. Benedicts or fehling's test are used to confirm the presence of reducing sugars.

Oligosaccharides : Formed due to condensation of 2-10 monosaccharide units, the Oxygen bridge is known as "glycoside linkage" and water molecule is eliminated. The bond may be alpha and beta.

# (i) Disaccharides : Composed of two molecules of same or different monosaccharide units. Also called "double sugars". Molecular formula is .
(a) Maltose : Also called "malt sugar" stored in germinating seeds of barley, oat, etc. It is formed by enzymatic (enzyme amylase) action on starch. It is a reducing sugar.
(b) Sucrose : "Cane sugar" or " table-sugar". Obtained from sugarcane and beet root and on hydrolysis splits into glucose and fructose.
(c) Lactose : Milk sugar or 5% in mammalian milk. On hydrolysis yields glucose and galactose. Streptococus lacti converts lactose in to lactic acid and causes souring of milk.

# (ii) Trisaccharides : Composed of three molecules of sugars. Molecular formula is .
(a) Raffinose : Found in sugar beet, cotton and in some fungi. It is made up of glucose, fructose and galactose.
(b) Gentianose : Found in rhizomes of gentian species, made up of glucose and fructose.

# (iii) Tetrasaccharides : Composed of four molecules of same or different sugars. Stachyose is found in Stachys tubefera. It is made up of two unit of galactose, one unit of glucose and one unit of fructose.

#(v) Reducing and Non-reducing carbohydrates : Those which reduce Tollen's reagent or fehling solution are called reducing sugars and those do not reduce are called non-reducing sugars. All monosaccharides and disaccharides except sucrose are reducing. While all polysaccharides are non-reducing sugars.

They are branched or unbranched polymers of monosaccharides jointed by glycosidic bond. Their general formula is (C6H10O5)n They are also called glycans polysaccharides are amorphous, tasteless and insoluble or only slightly soluble in water and can be easily hydrolysed to monosaccharide units.

# (1) Types of polysaccharides
On the basis of structure
(i) Homopolysaccharides : These are made by polymerisation of single kind of monosaccharides. e.g. starch, cellulose, glycogen, etc.
(ii) Heteropolysaccharide : These are made by condensation of two or more kinds of monosaccharides. e.g. chitin, pectin, etc.

# On the basis of functions
(i) Food storage polysaccharides : They serve as reserve food. e.g. starch and glycogen.
(ii) Structural polysaccharides : These take part in structural framework of cell wall e.g. chitin and cellulose.

# (2) Description of some polysaccharides
- (i) Glycogen : It is a branched polymer of glucose and contain 30,000 glucose units. It is also called animal starch. Their general formula is (C6H10O5)n .It is also found as storage product in blue green algae, slime moulds, fungi and bacteria. It is a non-reducing sugar and gives red colour with iodine. In glycogen, glucose molecule are linked by 1 – 4 glycosidic linkage in straight part and 1 – 6 linkage in the branching part glycogen has branch points about every 8-10 glucose units.

- (ii) Starch : Starch is formed in photosynthesis and function as energy storing substance. Generally found in the form of grains, which contain 20% water. It is found abundantly in rice, wheat, legumes, potato (oval and ecentric shaped), banana, etc. Starch is of two types. Straight chain polysaccharides known as amylose and branched chain as amylopectin. Both composed of D – glucose units jointed by linkage and linkage. It is insoluble in water and gives blue colour when treated with iodine. Amylose consists of 200 – 500 glucose units. It is stored inside chloroplast or spherical leucoplast and known as amyloplasts.

- (iii) Inulin : Also called “dahlia starch”(found in roots). It has unbranched chain of 30 – 35 fructose units linked by – 1 glycosidic linkage between 1 and 2 of carbon atom of D – fructose unit.
(iv) Cellulose : An important constituent of cell wall (20 – 40%), made up of unbranched chain of 6000 –D glucose units linked by 1 – 4 glycosidic linkage. It is fibrous, rigid and insoluble in water. Wood (20 – 50%) and cotton (90%) contain large amount of it. Rayon (artificial fibre) cellulose, nitrate (used as explosive) and carboxyl methyl cellulose (used as cosmetics and ice cream) are obtained by activity of “cellulase” enzyme. It doesn’t give any colour when treated with iodine.

- (v) Chitin : It is a polyglycol consisting of N-acetyl–D–glucosamine units connected with glycosidic linkage. Mostly it is found in hard exoskeleton of insects and crustaceans and some times in fungal cell wall. Second most abundant carbohydrate.

- (vi) Agar-Agar : It is a galactan, consisting of both D and L galactose and it is used to prepare bacterial cultures. It is also used as luxative and obtained from cell wall of red algae e.g. Gracilaria, Gelidium, etc.

- (vii) Pectin : It is a cell wall material in collenchyma tissue may also be found in fruit pulps, rind of citrus fruits etc. It is water soluble and can undergo sol - gel transformation. It contain arabinose, galactose and galacturonic acid.

- (viii) Neutral sugars : It is found associated with cellulose in cell wall. The common sugars in hemicellulose are D-xylose, L–arabinose, D-galactose, D-mannose and D-glucusonic acid. e.g. hemicellulose.

- (ix) Gum : It secreted by higher plants after injury or pathogenic attacks. It is viscous and seals the wound. It involves sugars like L-arabinose, D-galactose, D-glucusonic acid. e.g. gum arabic.

- (x) Mucopolysaccharides : These are gelatinous substance, containing amino sugars, uronic acid, etc. All slimy substances of plant are mucopolysaccharide. e.g. hyaluronic acid, vitreous humour, chondridine sulphate, heparin, husk of isabgul and mucilage of also.

- (xi) Glycoproteins : They include some plasmaprotein and blood group substances. They doesn’t contain uronic acid.

- (xii) Murein : It is a peptidoglycan, linked to short chains of peptides. It is constituent of cell wall of bacteria and blue green algae.

# (3) Properties of polysaccharides
(i) They are tasteless and colourless solids.
(ii) Insoluble in water, soluble in alcohol and more soluble in ether.
(iii) Can be easily hydrolyzed into their monosaccharide.
(iv) Their molecular weight is high.
(v) They do not diffuse through plasma membrane.

# (4) Functions
(i) Cellulose pectin and chitin are constituents in cell wall of higher plants but peptidoglycan in the cell wall of prokaryotes.
(ii) They are reserve food material.
(iii) They form protective covering.
(iv) They can be used as culture medium.
(v) Being insoluble they do no exert osmotic or chemical influence in the cell.
(vi) Fibres are obtained used in making cloth and rope.
(vii) Nitrocellulose and trinitrate cellulose (gun-cotton) used as explosive.