19 WJEC Biology AS Level - Marianne Izen - 2nd Edition Flashcards
What is the colour produced in the iodine-potassium iodide test at very low iodine or starch concentration?
At very low lodine or starch concentration, the colour produced is violet rather than blue-black.
Describe the structure of amylose
Amylose is a linear, unbranched molecule with a-1,4-glycosidic bonds forming between the first carbon atom (C1) on one glucose monomer and the fourth carbon atom (C4) on the adjacent one.
This is repeated, forming a chain, which coils into a helix.
In amylopectin and glycogen, what type of gycosidic bonds are seen
a-1,4 and a-1,6 glycosidic bonds are seen in amylopectin and glycogen
Describe the structure of amylopectin
Amylopectin also has chains of glucose monomers joined with a-1,4-glycosidic bonds.
They are cross-linked with a-1,6-glycosidic bonds and fit inside the amylose.
When a glycosidic bond forms between the C1 atom on one glucose molecule and the C6 atom on another, a side branch is seen.
These occur every 24-30 glucose molecules.
a-1,4- glycosidic bonds continue on from the start of the branch.
1 - B
2 - D
3 - A
4 - C
How do you test for the presence of starch?
We can test for the presence of starch using the iodine-potassium iodide test
lodine solution (iodine dissolved in aqueous potassium iodide) interacts with starch.
There is a colour change from the orange-brown of the iodine solution to blue-black.
The depth of blue-black colour gives an indication of relative concentration of starch.
An accurate concentration cannot be determined so, like the Benedict’s test, this test is qualitative.
What is the significance of temperature and ph in the iodine-potassium iodide test which is used to test for the presence of starch?
As temperature increases, the colour intensity decreases so above about 35°C, this test is unreliable.
It is also unreliable at very low pH, as the starch is hydrolysed.
What are the bonds present in amylose molecules and amylopectin molecules?
Amylose molecules have α-1,4- glycosidic bonds
Amylopectin molecules have both a-1,4- and a-1,6-glycosidic bonds.
