BIOLOGICAL MOLECULES: Polysaccharides

Polysaccharides are polymers of monosaccharides. They're good energy stores and sources.

Glucose is source of energy as it is a reactant in respiration. The energy released is used to make ATP which is the energy currency of the cell. 

Glucose + Oxygen -> Carbon Dioxide + Water

Joining glucose molecules together to create a polysaccharide will create an energy store. Examples include amylose, amylopectin and glycogen. Plants store energy as starch in chloroplasts and in membrane bround starch grains whereas humans store energy as glycogen in cells of the muscles and liver. These form good forms of monosaccharides because:

- Glycogen and starch are compact meaning they don't occupy a large amount of space. They both occur in dense granules within the cell.

- Polysaccharides hold glucose molecules in chains so can be easily 'snipped off' the end of the chain when required for respiration by hydrolysis. Hydrolysis reactions are catalysed by enzymes.

- Some chains are unbranched (amylose) and others are branched (amylopectin and glycogen). Branched chains tend to be more compact, but give chance for glucose molecules to be chopped off quickly when required for respiration. Amylase is the enzyme responsible for hydrolysing 1-4 glycosidic linkages and glucosidase is responsible for hydrolysing the 1-6 glycosidic linkages.

- Polysaccharides are less soluble in water than monosaccharides. If many glucose molecules dissolved in the cytoplasm, the water potential would be reduced and excess water would have to diffuse into the cell which would disrupt the functions of the cell. Polysaccharides are less soluble because of their size but also because regions which could hydrogen-bond with water molecules are hidden away on the inside of the polysaccharide. Sometimes the amylose polysaccharide, as it is unbranched, will form a double helix presenting a hydrophobic external surface in contact with the surrounding solution.

Amylose

Amylose (found in plants) is a long chain of a-glucose molecules. It has 1-4 glycosidic bonds hydrolysed by amylase, like maltose does. Amylose coils into a spiral shape as it is unbranched - hydrogen bonds hold this spiral in place. Hydroxyl groups on carbon 2 are situated on the inside of the coil which means the molecule is less soluble as it is less able to bond with water & this allows hydrogen bonds to form to maintain the molecules structure.

Amylopectin

Amylopectin (found in plants) is like amylose but is highly branched with both 1-4 and…