STPM Biology - chapter 1: Revision essay question 13

13. Describe the structure of starch and cellulose . Explain how the structures of these molecules are related to their funtions.


Answer:

Starch is made up of 2 polymers :
(i) Amylose (20%) and

(ii) Amylopectin (80%)

Amylose:

Amylose is  a long , straight unbranched chain.It is made up of a glucose molucules linked together by glycosidic α-1,4 bonds. Each amylose molecule contains 200-1500 glucose units .In aqeous solution,the amylose coils to form a helix , each helical coil contains 6 glucose units .

Amylopectin:

Amylopectin is a shorter and branched chain. It is made up of a glucose molecules linked together by glycosidic α-1,4 bonds. Branches occur at intervals where α-1,6 glycosidic bonds. Each amylopectin molecule contains 2000-200000 glucose units . The components of starch , amylose helices are entangled in the branches of amylopectinto form a complex ,compact three dimensinal molecule.

Structure of starch related to its functions :
The hydroxyl group(-OH) in carbon atom 2 point to the same direction .Hydrogen bonds are formed between the hydroxyl groups.This causes the polysaccharides chains to coil into a helix,with the hydroxyl groups pointing inwards. Starch has a compact structure .It therefore can be stored in large cells .Starch is insoluble in water .It does not diffuse out from the storage cells.There is little effect on the water potential of plant cells and does not effect the cellular metabolic activities.When required , starch can be hydrolysed to maltose then to glucose .Glucose is oxidised during respiration to produce energy .Starch is leucoplasts, usually found in cereals e.g maize , wheat and potato tuber .

Cellulose

Cellulose is composed long ,straight unbranched polysaccharides chains made up of about 10000 β-glucose units .The glucose units are linked together by β-glucose bonds .Each β-glucose unit is linked to the next glucose unit by a rotation of 180 degree.This orientation causes the hydroxyl groups to project outwards on wither side of the chain .Hydrogen bonds are formed between the hydroxyl groups of adjacent polysaccharides chairs.The parallel chains are grouped together to form microfibrils and then rearranged to form macrofibrils .

Structures of cellulose related to its functions .

This orientation causes the hydroxyl groups (-OH) to project outwards on either side of each chain.Many hydrogen bonds can be formed between the protruding group .These collectively produce a more stable structure.The parallel chains grouped together to form microfibrils which then are held together to form thicker and stronger bundles called macrofibrils or fibers.Cellulose is the main component of plant cell wall.  The fibers gives the cell its high tensile strength and rigidity . The binding of cellulose fibers with hemicellulose and pectages in the matrix further increases the strength of cell wall .It protects the plant cells and prevent its from bursting when enters by osmosis.Cellulose are loosely arranged in the primary plants cell wall. This produces spaces between fibers and the cell wall is therefore freely permeable to water and solutes .Cellulose is an energy source for herbivores . The β-1,4 glcosidic bonds can be hydrolysed to release glucose molecules which can be oxidized to release energy.

Assignment submitted by C.Y., Tan, B.X., Loh, J.E., Tai, W.L., Kwan, G.H., Tan  2011/2012 (IBM)