2.11.4 Structure, Properties, and Roles of Chitin

Definition: Chitin is the second most abundant organic molecule on earth. It is also a homopolysaccharide.

Key Points:

• It is a structural carbohydrate found in the cell walls of fungi and in the exoskeleton of arthropods.
• Due to its occurrence in fungal cell walls, it is also known as fungal cellulose.
• Chitin is a derivative of N-acetylglucosamine monomers, which is a modified form of glucose.
• It has an unbranched structure.
• Its monomers are linked together by β-1,4-glycosidic linkages.

STEAM Activity 2.1 (from textbook)

Title: The presence or absence of starch, glycogen, cellulose, and glucose in a given sample can be confirmed by biochemical tests.

Intermediate Level

Properties of Polysaccharides

Comparison of Common Polysaccharides

Glycosidic Linkages in Polysaccharides

Why Are Polysaccharides Insoluble?

• Polysaccharides are large molecules.
• They have many hydroxyl groups that form hydrogen bonds with each other.
• This makes it hard for water to dissolve them.
• Therefore, they are generally insoluble.

Why Are Polysaccharides Tasteless?

• Sweetness depends on the structure of the sugar molecule.
• Polysaccharides are too large to fit into the sweet taste receptors.
• Therefore, they are tasteless.

Why Are Polysaccharides Good Storage Molecules?

• Insoluble – They do not affect the osmotic balance of the cell.
• Compact – They fold into compact shapes, storing a lot of energy in a small space.
• Easily hydrolyzed – They can be broken down into glucose when energy is needed.

Important Points

• 1. Polysaccharides: Those carbohydrates which upon hydrolysis yield more than ten monosaccharide units are called polysaccharides.
• 2. This is the largest group of carbohydrates.
• 3. Polysaccharides which are composed by the condensation of only one kind of monosaccharides are called homopolysaccharides (e.g., starch, glycogen, cellulose, chitin).
• 4. Polysaccharides which are composed by the condensation of different kind of monosaccharides are called heteropolysaccharides (e.g., agar, pectin, peptidoglycan).
• 5. Polysaccharides function chiefly as food and energy stores (e.g., starch, glycogen) and structural material (e.g., cellulose and chitin).
• 6. They are convenient storage molecules for several reasons:
• - Their large size makes them more or less insoluble in water, so they exert no osmotic or chemical influence in the cell.
• - They fold into compact shapes.
• - They are easily converted to sugars by hydrolysis when required.
• 7. Starch:
• - Homopolysaccharide formed by the condensation of hundreds of α-glucose monomers.
• - Storage carbohydrate of plants.
• - Mainly stored in root, stem, and seeds.
• - Cereal grains and potato tubers are rich sources.
• - Digested in oral cavity and small intestine by amylase.
• - Upon hydrolysis yields maltose first, then maltose is further digested by maltase enzyme and yields glucose.
• - Presence confirmed by iodine test (blue colour).
• - Two types: Amylose (unbranched, soluble in hot water) and Amylopectin (branched, insoluble in water).
• 8. Glycogen:
• - Homopolysaccharide composed of multiple α-glucose monomers.
• - Storage carbohydrate of animals.
• - Mainly stored in liver and muscles.
• - Also known as animal's starch.
• - Digestion similar to starch.
• - Much more branching than amylopectin.
• 9. Cellulose:
• - Most abundant carbohydrate on earth.
• - Homopolysaccharide formed by the condensation of hundreds of β-glucose.
• - Structural carbohydrate of plants.
• - Major constituent of plant cell wall.
• - Cotton and paper are pure forms of cellulose.
• - Shows no colour with iodine solution.
• - Unbranched structure.
• - β-1,4-glycosidic linkages between glucose residues.
• - Cannot be digested by human body but works as roughage or fibre.
• - Herbivore animals have symbiotic bacteria that secrete cellulase enzyme for its digestion.
• - Upon hydrolysis yields cellobiose, then cellobiose is further digested into glucose.
• 10. Chitin:
• - Second most abundant organic molecule on earth.
• - Homopolysaccharide.
• - Structural carbohydrate found in cell walls of fungi and exoskeleton of arthropods.
• - Also known as fungal cellulose.
• - Derivative of N-acetylglucosamine monomers.
• - Unbranched structure.
• - β-1,4-glycosidic linkages.

Remember

• - Polysaccharides = more than 10 monosaccharide units
• - Largest group of carbohydrates
• - Tasteless and generally insoluble
• - Types:
• - Homopolysaccharides = one kind of monosaccharide (starch, glycogen, cellulose, chitin)
• - Heteropolysaccharides = different kinds of monosaccharides (agar, pectin, peptidoglycan)
• - Starch = storage in plants (amylose + amylopectin)
• - Glycogen = storage in animals (liver and muscles)
• - Cellulose = structural in plants (plant cell wall)
• - Chitin = structural in fungi and arthropods
• - Iodine test = blue-black for starch, red for glycogen, no colour for cellulose
• - Glycosidic bonds: α-1,4 and α-1,6 (starch, glycogen); β-1,4 (cellulose, chitin)

Common Mistakes

• 1. Thinking all polysaccharides are the same – Starch, glycogen, cellulose, and chitin have different compositions, structures, and functions.
• 2. Confusing starch and glycogen – Starch is the storage carbohydrate of plants; glycogen is the storage carbohydrate of animals.
• 3. Thinking cellulose is a storage carbohydrate – Cellulose is a structural carbohydrate; starch and glycogen are storage carbohydrates.
• 4. Thinking chitin is made of glucose – Chitin is made of N-acetylglucosamine, a modified form of glucose.
• 5. Forgetting that cellulose cannot be digested by humans – Humans lack the enzyme cellulase.
• 6. Thinking all polysaccharides are branched – Amylose and cellulose are unbranched; amylopectin and glycogen are branched.
• 7. Confusing homopolysaccharides and heteropolysaccharides – Homopolysaccharides = one kind of monosaccharide; Heteropolysaccharides = different kinds of monosaccharides.
• 8. Forgetting the iodine test results – Starch = blue-black; Glycogen = red; Cellulose = no colour.
• 9. Thinking polysaccharides are sweet – Polysaccharides are tasteless.
• 10. Thinking polysaccharides are soluble in water – Polysaccharides are generally insoluble in water.
• 11. Forgetting the functions of polysaccharides – Energy storage (starch, glycogen) and structural support (cellulose, chitin).
• 12. Confusing the glycosidic bonds in different polysaccharides – Starch and glycogen have α-1,4 and α-1,6 bonds; cellulose and chitin have β-1,4 bonds.
• 13. Thinking all polysaccharides are found in plants – Glycogen is found in animals; chitin is found in fungi and arthropods.
• 14. Forgetting the importance of branching in glycogen – More branches mean more ends for enzymes to act on, allowing rapid breakdown.

Quick Revision

• Polysaccharides: Those carbohydrates which upon hydrolysis yield more than ten monosaccharide units are called polysaccharides.
• This is the largest group of carbohydrates.
• The polysaccharides which are composed by the condensation of only one kind of monosaccharides are called homopolysaccharides e.g., starch, glycogen, cellulose, chitin; whereas the polysaccharide which are composed by the condensation of different kind of monosaccharides are called heteropolysaccharides e.g., agar, pectin, peptidoglycan.
• Polysaccharides function chiefly as food and energy stores, e.g., starch, glycogen, and structural material, e.g., cellulose and chitin.
• They are convenient storage molecule for several reasons:
• Their large size makes them more or less insoluble in water, so they exert no osmotic or chemical influence in the cell.
• They fold into compact shapes.
• They are easily converted to sugars by hydrolysis when required.
• 2.11.1 Structure properties and roles of starch:
• Starch is a homopolysaccharide which is formed by the condensation of hundreds of α-glucose monomers.
• It is storage carbohydrate of plants.
• It is mainly stored in root, stem, and seeds.
• Cereal grains and potato tubers are rich sources of starch in human diet.
• Starch is digested in oral cavity and in small intestine by the enzyme amylase.
• Upon hydrolysis it yields maltose first and then maltose is further digested by maltase enzyme and yields glucoses.
• The presence of starch in a given sample can be confirmed by iodine test as it gives blue colour with iodine solution.
• There are two types of starches i.e., amylose and amylopectin.
• Amylose is un-branched i.e., a linear chain of glucoses in which glucoses are attached together by α-1,4-glycosidic linkages. It is soluble in hot water only.
• On the other hand, amylopectin has branched structure i.e., a linear chain of glucoses but more chains of glucoses in the form of branches are also attached by α-1,6-glycosidic linkages. It is completely insoluble in water.
• 2.11.2 Structure properties and roles of Glycogen:
• Like starch, glycogen is also a homopolysaccharide composed of multiple α-glucose monomers.
• It is storage carbohydrate of animals.
• It is mainly stored in liver and muscles.
• Therefore, it is also known as animal's starch.
• The digestion of glycogen is also quite similar to that of starch.
• Glycogen has much more branching than amylopectin.
• 2.11.3 Structure properties and roles of Cellulose:
• Cellulose is most abundant carbohydrate on earth.
• It is also a homopolysaccharides but unlike starch and glycogen it is formed by the condensation of hundreds of β-glucose.
• It is structural carbohydrate of plants as it is major constituent of plant cell wall.
• Cotton and paper are the pure forms of cellulose.
• Cellulose shows no colour with iodine solution.
• Structure of cellulose resembles with amylose starch in such a way that it has un-branched structure but it has β-1,4-glycosidic linkages between glucose residues.
• Cellulose cannot be digested by human body but it has to be taken into diet because it works as roughage or fibre so it prevents abnormal absorption of food in intestine. However, herbivore animals have some symbiotic bacteria that secrete cellulase enzyme for its digestion. Upon hydrolysis it first yields a disaccharide, the cellobiose and then cellobiose is further digested into glucoses.
• 2.11.4 Structure properties and roles of Chitin:
• Chitin is the second most abundant organic molecule on earth.
• It is also a homopolysaccharides.
• It is a structural carbohydrate found in the cell walls of fungi and in the exoskeleton of arthropods.
• Due to the occurrence of chitin in fungal cell wall, it is also known as fungal cellulose.
• Chitin is the derivative of N-acetyl glucosamine monomers which is a modified form of glucose.
• It has an un-branched structure and its monomers are linked together by β-1,4-glycosidic linkages.
• STEAM Activity 2.1: The presence or absence of starch, glycogen, cellulose, and glucose in a given sample can be confirmed by biochemical tests.
• Iodine test for Starch in solution or food material:
• A. Take 5 ml water in a test tube. Add a small amount of starch in water in test tube and then boil to prepare starch solution. Add a few drops of iodine solution into the clear starch solution in the test tube. → Dark blue-black coloration is produced in test tube. → Starch is confirmed in solution.
• B. Take food material like potato, then cut a small slice of potato and add a few drops of iodine solution on the potato slice. → Dark blue-black coloration is produced on potato slice. → Starch is confirmed in food.
• C. Take glycogen solution in test tube and add few drops of iodine solution. → Red coloration is produced in test tube. → Glycogen is present in solution.

60-Second Revision

• Polysaccharides = more than 10 monosaccharide units
• Largest group of carbohydrates
• Tasteless and generally insoluble
• Types:
• Homopolysaccharides = one kind of monosaccharide (starch, glycogen, cellulose, chitin)
• Heteropolysaccharides = different kinds of monosaccharides (agar, pectin, peptidoglycan)
• Starch = storage in plants (amylose + amylopectin)
• Glycogen = storage in animals (liver and muscles)
• Cellulose = structural in plants (plant cell wall)
• Chitin = structural in fungi and arthropods
• Iodine test = blue-black for starch, red for glycogen, no colour for cellulose
• Glycosidic bonds: α-1,4 and α-1,6 (starch, glycogen); β-1,4 (cellulose, chitin)
• Warning: Polysaccharides are not all the same. Starch and glycogen are storage; cellulose and chitin are structural. Cellulose cannot be digested by humans. Chitin is made of N-acetylglucosamine, not glucose. Amylose is unbranched; amylopectin and glycogen are branched. The iodine test gives different colours for starch and glycogen.