Biochemical tests

Question 1

what are the 3 different carbohydrates that you need to know the tests for ?

Answer 1

• starch
• reducing sugars
• non-reducing sugars

Question 2

what are the 3 categories of biochemical tests that you need to know ?

Answer 2

• carbohydrates
• proteins
• lipids

Question 3

test for starch

Answer 3

• add the potassium iodide solution ( reagant)
• positive test = solution turns from orange to blue/black

Question 4

test for reducing sugars ?

Answer 4

• add benedict’s reagent and heat
• a positive test observation = turns from blue to green, yellow, orange or brick red

Question 5

what does it mean if the benedict’s reagent turns more red ?

Answer 5

the more red the solution is, the higher the concentration of the reducing sugar
green means it has a low conc of reducing sugars

Question 6

why does the colour change occur at the top of the solution first ?

Answer 6

because of convection current as the hottest particles will rise to the top and they will have the most kinetic energy and more successful collisions so the reaction will occur the fastest at the top

Question 7

test for non reducing sugars: what do you have to do before you peform the non reducing sugars test ?

Answer 7

firstly you have to confirm that the sample is not a reducing sugar by doing the benedict’s test
so a negative result will mean that the reagent remains blue

Question 8

test for non reducing sugars: ? (sucrose)

Answer 8

• add an acid and boil for 2 mins - to perform acid hydrolysis
• cool solution and add an alkali to neutralise it
• add benedict’s reagent and heat
• positive test observation = turns from blue to green, yellow, orange or brick red

Question 9

what will happen due to the fact you boil the non reducing sugar with acid ( acid hydrolysis ) ?

Answer 9

this will break the glycosidic bonds and cause sucrose to be hydrolysed and become glucose and fructose

Question 10

state all the reducing sugars
?

Answer 10

all of the monosacharrides :
- glucose
- fructose
- galactose
and 2 of the disacharrides :
- maltose
- lactose

Question 11

state the reducing sugar ?

Answer 11

sucrose

Question 12

what is meant by a reducing sugar ?

Answer 12

these sugars can reduce the copper sulphate (blue) in benedict’s reagent to copper oxide (brick red)

Question 13

what is meant by a non-reducing sugar ?

Answer 13

the reducing group is involved in the glycosidic bond in sucrose and therefore sucrose cannot reduce copper sulphate to copper oxide

Question 14

what happens when sucrose is hydrolysed (through boiling with an acid ) ?

Answer 14

the glycosidc bond is broken and therefore the reducing group becomes exposed
a positive test is then received with benedict’s reagent following hydrolysis

Question 15

test for proteins ?

Answer 15

• add biuret
• a positive test observation = solution turns from blue to purple / lilac

Question 16

test for lipids ?

Answer 16

• dissolve the sample in ethanol
• add distilled water and shake
• a positive test = a white cloudy emulsion forms