Carbohydrates

Question 1

what is a condensation reaction and give an example

Answer 1

a reaction which joins together monomers to create a polymer, which forms a new chemical bond and a water molecule

eg. glucose -> starch + water
nucleotides -> DNA + water

Question 2

what is a hydrolysis reaction and give an example

Answer 2

a reaction which breaks the chemical bonds between a polymer to form individual monomers, which requires water

eg. protein + water -> amino acids
starch + water -> glucose

Question 3

what is the bond called which holds together carbohydrate monosaccharides?

Answer 3

glycosidic bond

Question 4

how is maltose formed?

Answer 4

through the condensation reaction between 2 molecules of glucose

Question 5

how is sucrose formed?

Answer 5

through the condensation reaction between a glucose molecule and a fructose molecule

Question 6

how is lactose formed?

Answer 6

through the condensation reaction between a glucose molecule and a galactose molecule

Question 7

what are the two isomers of glucose called?

Answer 7

alpha and beta glucose

Question 8

what is the structure of alpha glucose?

Answer 8

OH below ring

Question 9

what is the structure of beta glucose?

Answer 9

OH above ring

Question 10

describe the function and structure of starch

Answer 10

function is for storage

structure:
-is insoluble which doesn’t affect cells water potential
-amylopectin is branched due to 1-6 glycosidic bonds and amylose is coiled due to 1-4 glycosidic bonds, so is compact, can be easily stored and easily hydrolysed
-large meaning it cannot leave the cell through diffusion
-condensation reaction between alpha glucose molecules

Question 11

describe the function and structure of glycogen

Answer 11

function is for storage in animal cells

structure:
-branched due to 1-6 glycosidic bonds so is compact, easily stored and can be more easily hydrolysed
-insoluble so doesn’t affect cells water potential
-large so cannot easily leave cell
-many glucose molecules provide glucose for respiration
-condensation reaction between alpha glucose molecules

Question 12

describe the function and structure of cellulose

Answer 12

function is to provide strength to cell walls

structure:
-linear chains made from the condensation of beta glucose molecules
- alternate position of hydrogen and hydroxyl group inverted
-many hydrogen bonds between chains collectively provide strength to cell wall
-chains form microfibrils
-1-4 glycosidic bonds so is coiled and compact

Question 13

how do you test for reducing sugars? eg. glucose, fructose

Answer 13

-add an equal volume of the sample and Benedict’s reagent
-heat the mixture in a gently boiling water bath for 5 mins
-brick-red precipitate indicates the presence of a reducing sugar
-negative result is no change (stays blue)

Question 14

how do you test for non-reducing sugars?

Answer 14

-add an equal volume of the sample and hydrochloric acid (to break glycosidic bonds) and place in water bath for 5 mins
-slowly add sodium hydrogencarbonate (to neutralise the solution)
-retest using Benedict’s solution and a brick-red precipitate indicates the presence of a non-reducing sugar
-negative result is no change (stays blue)

Question 15

how do you test for starch?

Answer 15

-place 2cm^3 of the sample into a test tube
-add 2 drops of potassium iodide solution and shake
-a blue/black colour indicates the presence of starch
-negative result is no change (stays orange)

Question 16

How can you measure the quantity of a reducing sugar?

Answer 16

• use a colorimeter
• filter and dry the solution, then weigh the mass