Structure and Significance of Amylose Molecule

Structure and Property of Starch

• Composed of thousands of glucose linked by α(1→4) glycosidic bonds, storing large amounts of energy
• α(1→4) glycosidic bonds can be broken by amylase, hydrolysing starch into glucose for respiration
• Amylose chains are helical in shape, with six glucose per turn, maintained by intra-chain hydrogen bonding between hydroxyl groups of glucoses
• Amylopectin are highly branched due to branch points maintained by α(1→6) glycosidic bonds, compact and ideal for storage
• Debranching enzymes break α(1→6) glycosidic bonds, converting the branched structure of amylopectin into a more linear structure, increasing the accessibility of the remaining linear chains to amylase
• -OH groups are occupied in intra-chain hydrogen bonding, making starch insoluble and osmotically inactive, thus does not affect osmotic concentration in cells

Starch Test

• In the presence of starch, iodine forms a complex with starch molecules, resulting in a color change from brown to blue-black
• Iodine molecules fit into the helical structure of starch, changing the refractive index and causing the color change

Glycogen

• Branched chain polymer of α glucose, similar to amylopectin but larger and more highly branched
• Easily hydrolysed to α glucose by debranching enzymes and glycogen phosphorylase
• Functions as energy storage in animals, with a core protein of glycogenin surrounded by branches of glucose

Cellulose

• Comprises up to 50% of plant cell walls, providing strength, rigidity, and structural support to plant cells
• Protects plant cells and cytoplasm from damage and mechanical injuries
• Despite its strength, the cellulose cell wall is fully permeable to water and solutes, allowing for the functioning of plant cells
• Structure consists of unbranched polysaccharide of β glucose linked by β(1→4) glycosidic bonds, with each chain consisting of 1000 or more β glucose residues
• Formation of β(1→4) glycosidic bonds requires the 180º rotation of alternating glucose residues, resulting in straight chains of cellulose
• Hydroxyl groups (-OH) project outwards, allowing for the formation of hydrogen bonds between adjacent chains, establishing a rigid cross-linking between the chains
• Many unbranched linear chains run parallel to each other, forming microfibrils, macrofibrils, and fibers, contributing to the tensile strength of plant cell walls