Starch and Glycogen

Question 1

Why glucose is not a good storage molecule

Answer 1

-Glucose is highly soluble in water as it contains a large amount of hydroxyl groups
-Hydroxyl groups are polar molecules due to the slightly negative charge on the oxygen atom and the slightly positive charge on the hydrogen atom
-This means that hydroxyl groups can form hydrogen bonds with water molecules
-This is an issue as if there is a large amount of dissolved glucose in a cell, then this will lower the water potential on the inside of the cell, causing water to move into the cell via osmosis
-This can lead to swelling of the cell or bursting (lysis) of the cell
-To solve this problem, plant cells store glucose as starch

Question 2

Starch

Answer 2

-Starch is an insoluble molecule that is found in starch grains inside a plant cell
-Starch is made up of two molecules: amylose and amylopectin

Question 3

Amylose

Answer 3

-Amylose is an unbranched polymer of alpha glucose joined by 1,4 glycosidic bonds
-Amylose forms a tight helix shape, with hydrogen bonds between glucose molecules along the chain
-When the cell needs glucose, water is added and the glycosidic bonds between the alpha glucose molecules are broken (hydrolysis reaction)

Question 4

Amylopectin

Answer 4

-Amylopectin is a polymer of alpha glucose joined by 1,4 glycosidic bonds
-Amylopectin has branches joined to the main chain by 1,6 glycosidic bonds every 25-30 glucose molecules

Question 5

How the structure of starch relates to it’s function

Answer 5

-Starch is insoluble in water, which means that it does not cause water to move into the cell via osmosis, preventing swelling or bursting of the cell (lysis)
-Since amylose and amylopectin are polysaccharides, they are too large to diffuse out of the cell
-Amylose forms a tight helix shape, which is compact; this means that a large amount of glucose can be stored in a cell for it’s size
-Enzymes work on the ends of the polysaccharide to break it down
-Since amylopectin has a large number of branches, it has more ends and can be hydrolysed more rapidly and broken down into glucose when it is needed in the cell

Question 6

Structure of glycogen

Answer 6

-Glycogen is the storage molecule of glucose in animals
-Glycogen is also a polymer of alpha glucose joined by 1,4 glycosidic bonds
-Glycogen also contains branches that are joined to the main chain by 1,6 glycosidic bonds
-Glycogen is more heavily branched than amylopectin

Question 7

How does the structure of glycogen relate to it’s function

Answer 7

-Glycogen is insoluble in water, so does not cause water to move into the cell via osmosis
-Glycogen is also a large molecule so does not diffuse out of the cell
-Glycogen is more heavily branched than amylopectin
-This means that glycogen is a more compact molecule
-Since enzymes work on the free ends of the molecule, and the glycogen molecule has a large number of branches, glycogen can be rapidly converted from glycogen to glucose, to be used in the cell
-This is important, as animals often have a high rate of respiration, and the energy needs of animals can change rapidly, for example having to move quickly to escape a predator
-This is why glycogen needs to be able to be quickly converted into glucose to be used for respiration