Carbohydrates (3.1.2)

Question 1

what are monosaccharides?

Answer 1

the monomers from which larger carbohydrates are made

Question 2

what are examples of monosaccharides?

Answer 2

glucose, galactose and fructose

Question 3

what does a condensation reaction between two monosaccharides do?

Answer 3

form a glycosidic bond

Question 4

how are disaccharides formed?

Answer 4

from the condensation of two monosaccharides

Question 5

what is maltose?

Answer 5

a disaccharide formed from the condensation of two glucose molecules

Question 6

what is sucrose?

Answer 6

a disaccharide formed from the condensation of a glucose molecule and a fructose molecule

Question 7

what is lactose?

Answer 7

a disaccharide formed from the condensation of a glucose molecule and a galactose molecule

Question 8

what are isomers?

Answer 8

two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties

Question 9

what are the isomers of glucose?

Answer 9

alpha glucose and beta glucose

Question 10

what is the key difference between alpha and beta glucose?

Answer 10

in alpha glucose, the carbon 1 hydroxyl points below the plane but in beta glucose it points above

Question 11

how are polysaccharides formed?

Answer 11

from the condensation of many glucose units

Question 12

what does the condensation of alpha glucose form?

Answer 12

glycogen and starch

Question 13

what does the condensation of beta glucose form?

Answer 13

cellulose

Question 14

what is a reducing sugar?

Answer 14

a sugar which can donate an electron to another molecule

Question 15

what are examples of reducing sugars?

Answer 15

all monosaccharides and some disaccharides (maltose and lactose)

Question 16

give an example of a non-reducing sugar?

Answer 16

sucrose

Question 17

what do reducing sugars do when they have Benedict’s solution added to them?

Answer 17

they reduce the copper sulphate in the reagent (by adding an electron so it goes from a Cu2+ ion to a Cu+ ion) to produce a red precipitate of copper oxide - the amount of precipitate produced depends on the concentration of reducing sugars present

Question 18

how do you carry out a test for reducing sugars?

Answer 18

add 1cm^3 of your sample to a boiling tube
add 1cm^3 of Benedict’s solution (blue) to the boiling tube
place the boiling tube into a water bath and leave for three minutes
remove the tube and observe the colour

Question 19

how do you identify if there is a reducing sugar present?

Answer 19

there will be a colour change depending on the amount of reducing sugar present so the solution is no longer blue:
small amount - green
more - yellow
higher level - orange
lots - brick-red

Question 20

why is the Benedict’s test semi-quantitative?

Answer 20

it only gives an approximate idea of the amount of reducing sugar present because it only shows a narrow range of colour changes and all humans perceive colour differently

Question 21

how do you test for a non-reducing sugar?

Answer 21

first you need to see if the solution also contains some reducing sugar by carrying out a Benedict’s test on it and recording any colour change that occurs
take 1cm3 of the solution and add 1cm3 of dilute HCl then boil in a water bath for two minutes
remove from the water bath and leave to cool slightly
add 1cm3 of NaOH to neutralise (Benedict’s test doesn’t work in acidic conditions)
carry out another Benedict’s test and record any colour change

Question 22

what happens when HCl is added to a solution if a non-reducing sugar is present?

Answer 22

the acid hydrolyses the glycosidic bonds which releases the monosaccharides

Question 23

how do you interpret results of a non-reducing sugar test?

Answer 23

if there is a colour change in the first Benedict’s test then that tells you that there is a reducing sugar present in some amount
if there is a colour change in the second Benedict’s test then that tells you that there is also a non-reducing sugar present in some amount
if the colour change in the first Benedict’s test is red that tells you that there is a large amount of reducing sugar present - in this case you can’t test for a non-reducing sugar as you can’t see a colour change beyond red

Question 24

when quantifying the Benedict’s test, what is the relationship between glucose concentration and mean light transmission and why?

Answer 24

As concentration increases, transmission increases, but at higher concentrations it levels off
This is because as concentration increases, more copper sulphate in the Benedict’s solution is reduced to produce copper oxide, meaning a greater amount of red precipitate forms. Therefore the solution contains less copper sulphate when filtered so it loses more of its blue colour, and this means that more light can be transmitted through

Question 25

what is glycogen?

Answer 25

a glucose storage molecule found in animals

Question 26

how would you describe the structure of glycogen?

Answer 26

it is a polymer of alpha glucose and each glucose molecule is joined by 1,4 glycosidic bonds
it is a branched molecule and at the branch points there are 1,6 glycosidic bonds

Question 27

how is glycogen suitable for its function?

Answer 27

because it has many branches it has lots of free ends, which means there is a large surface area for enzymes to rapidly convert glycogen back to glucose, allowing them to accommodate the rapid changes in energy needs in animals
it is insoluble in water so it doesn’t dissolve into glucose and cause water to move into the animal cell via osmosis
it is a large molecule so cannot move out of the cell

Question 28

what is cellulose?

Answer 28

a polysaccharide which is found in the cell wall of plant cells

Question 29

how would you describe the structure of cellulose?

Answer 29

it is a polymer of beta glucose, so in a molecule every second beta glucose molecule flips to allow 1,4 glycosidic bonds to be created
large numbers of hydrogen bonds form between cellulose chains making it very strong
cellulose chains group together to form microfibrils, which then group together to form macrofibrils, which group together to form cellulose fibres

Question 30

how is cellulose suitable for its function?

Answer 30

it creates a cellulose cell wall which is permeable to water
cellulose is very strong so when water moves into the cell and causes it to become turgid, the cell wall can withstand the high pressure caused by this
this allows plant cells to have a rigid structure which keeps the plant upright

Question 31

why do plant cells store glucose as starch?

Answer 31

glucose has many hydroxyl groups which makes it very soluble in water, and if cells had lots of glucose this could cause too much water to move into the cell by osmosis

Question 32

what are the two molecules that make up starch?

Answer 32

amylose and amylopectin

Question 33

how would you describe the structure of amylose?

Answer 33

it is a polymer of alpha glucose with 1,4 glycosidic bonds
the molecule twists into a compact helix and hydrogen bonds form between glucose molecules along the chain

Question 34

how would you describe the structure of amylopectin?

Answer 34

it is a polymer of alpha glucose with 1,4 glycosidic bonds
it has a branch every 25-30 glucose molecules and the branches are connected to the main chain by 1,6 glycosidic bonds

Question 35

how is starch suitable for its function?

Answer 35

it is a very compact molecule
it is insoluble in water so doesn’t cause water to enter the cell
amylose and amylopectin are large so can’t diffuse out of the cell
because amylopectin is branched there are many ends for enzymes to hydrolyse the glycosidic bonds when the cell needs glucose