Biochemical tests and starch

Question 1

How are carbohydrates used by cells?

Answer 1

They are used as respiratory substrates. They also form structural components in plasma membranes and cell walls

Question 2

All monosaccharides and some disaccharides are what type of sugar?

Answer 2

reducing sugars

Question 3

What is reduction?

Answer 3

a chemical reaction involving the gain of electrons or hydrogen

Question 4

What is reducing sugars?

Answer 4

a sugar that an donate electrons to another chemical

Question 5

How do you test for reducing sugars?

Answer 5

• Add 2cm^3 of the food sample to e tested to a test tube. If the sample is not already in liquid form, first grind it up in water
• Add an equal volume of Benedict’s reagent
• Heat the mixture in a gently boiling water bath for 5 minutes

a coloured precipitate will form if it is a reducing sugar

Question 6

Describe the different colours formed in the Benedict’s test and there meaning?

Answer 6

• Blue = None
• Green = very low
• Yellow = low
• Orange = medium
• Brick red = high

Question 7

What type of test is the Benedict test?

Answer 7

qualitative test

Question 8

What is a quantitative test for reducing sugars? (there are two)

Answer 8

use a colorimeter to measure the absorbance/ radiaation of light : If you were measuring absorption, as the concentration increases, the number should increase since there is more precipitate causing more absorption of the light. If you were measuring radiation, the number would go down since less light is able to pass through the sample!

filter the precipitate and measure the mass

Question 9

Give an example of a dissacharide that is a reducing sugar

Answer 9

maltose

Question 10

Give an example of a disaccharide that is a non-reducing sugar

Answer 10

sucrose

Question 11

What are non-reducing sugars?

Answer 11

Sugars that do not change the colour of Benedict solution

Question 12

How do you test for Non-reducing sugars?

Answer 12

1) if the sample is not already in liquid form, it must first be ground up in water
2) Add 2cm^3 of the food sample being tested to 2cm^3 of Benedicts reagent in a test tube and filter
3) Place the test tube in a gently boiling water bath for 5 minutes. If the Benedict’s reagent does not change colour, then a reducing sugar is present.
4) Add another 2cm^3 of the food sample to 2cm^3 of dilute HCl in a test tube and place the test tube in a gently boiling water bath for 5 minutes. The dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides
5) Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid as Benedict reagent will not work in acidic conditions. Test with pH paper to check that the solution is alkaline.
6) re-test the resulting solution by heating it with 2cm^3 of Benedict’s reagent in a gently boiling water bath for 5 minutes
7) if non-reducing sugar was present in the original sample the Benedict’s reagent will now turn orange brown.

Question 13

Why, after adding HCl and sodium hydrocarbonate, do non-reducing sugars turn an orange brown colour?

Answer 13

Because reducing sugars are produced from the hydrolysis of the non-reducing sugars.

Question 14

Why are polysaccharides insoluble and what does this make them suitable for?

Answer 14

Because they are very large, making them suitable for storgae

Question 15

How is starch formed?

Answer 15

by the joining of between 200 and 100,000 alpha glucose molecules by glycosidic bonds in a series of condensation reactions

Question 16

How do you test for starch?

Answer 16

• at room temp, place 2cm^3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile
• Add two drops of iodine solution and shake or stir
• the presence of starch is indicated by a blue-black colouration

Question 17

What two different polysaccharides is starch made from and what are these polysaccharides a polymer of?

Answer 17

Amylose and amylopectin, which are polymers of glucose

Question 18

Describe the structure of amylose

Answer 18

polymer of alpha glucose molecules

compact cylindrical polysaccharide

Glycosidic bond linking the α-glucose molecules is an α-1,4 glycosidic bond.

Twisted into a complex helix with hydrogen bonds forming between glucose molecules along the chain

Question 19

Describe the properties of the amylose component of starch (mention something about its coiled structure

Answer 19

It is insoluble in cold water

It is more resistant to digestion due to packed helical structure

It limits the infiltration of water in starch

It gives dark blue/black color with iodine solution

Angles of the glycosidic bonds = coiled structure → makes it compact = can fit more into a small space (= good for storage)

Question 20

Amylose constitutes to approximately what percentage of starch?

Answer 20

20%

Question 21

Describe the structure of amylopectin

Answer 21

polymer of alpha glycose joined by 1-4 glycosidic bonds

has a branch every 25-30 alpha glucose molecule. This branch is connected to the main chain by a glycosidic bond but between carbon 1 of an alpha molecule and carbon 6 of another one (1 6 glycosidic bond)

Question 22

Describe the properties of amylopectin

Answer 22

It is soluble in both hot and cold water

It does not resist digestion

It shows extensive branching

It gives reddish-brown color with iodine solution

Question 23

Amylopectin constitutes to approximately what percentage of starch?

Answer 23

80%

Question 24

Draw an amylopectin

Answer 24

Question 25

Draw amylose

Answer 25

Question 26

Draw the simplified version of amylose and amylopectin

Answer 26

Question 27

What properties of starch molecules make it ideal for storing glucose?

Answer 27

They are insoluble and compact

Question 28

What type of molecule is starch and where is it only found and in what form?

Answer 28

It is a polysaccharide only found in plants in small grains - intracellular grains in organelles called plastids

Question 29

What do plastids contain?

Answer 29

green chloroplasts and amyloplasts

Question 30

What makes starch molecules very compact?

Answer 30

the unbranched chain in the structure, which is wound into a tight coil that makes the molecule very compact.

Question 31

How is starch adapted for its main role, energy storage?

Answer 31

• its insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis
• being large and insoluble, it does not diffuse out of cells
• its compact, so a lot of it can be stored in a small space
• when hydrolysed it forms alpha glucose, which is both easily transported and readily used in respiration
• the branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly

Question 32

Starch is a store of glucose. What happens when glucose is needed by the plant cell? Why is the process to obtain glucose from the starch quick?

Answer 32

water is used to break the glycosidic bonds (hydrolysis reaction). This is done quickly because the enzymes that break down starch act at the ends of the molecules. Since amylopectin has a lot of branches, it has a lot of ends, allowing the enzymes to break down starch rapidly