2.2.12 practical biochem 1: qualitative tests for bio molecules

Question 1

name the qualitative tests for bio molecules

Answer 1

(carbohydrates:)
- starch
- reducing sugars
- non-reducing sugars
(other:)
- testing for lipids
- testing for proteins

Question 2

how do you test for starch

Answer 2

1. add iodine solution (in potassium iodide) to sample

Question 3

results for starch test

Answer 3

positive
= colour change from yellow-brown to blue-black

Question 4

reasoning behind colour change when starch is present

Answer 4

• when dissolved in potassium iodide, iodine (I2) forms triiodide ions I3-
• slips into middle of amylose helix

Question 5

what does reducing sugars include

Answer 5

all monosaccharides & some disaccharides

Question 6

why are reducing sugars named this

Answer 6

can reduce/give electrons to other molecules

Question 7

how do you test for reducing sugars

Answer 7

• heat reducing sugar w/ benedicts solution
• colour change from blue -> green -> yellow -> orange-red

Question 8

what is benedicts solution

Answer 8

alkaline copper (II) sulfate

Question 9

reasoning behind colour change

Answer 9

• benedicts solution contains Cu2+ ions
• reduced to Cu+
• forms orange-red copper (I) oxide (Cu2O) precipitate

Question 10

how is a commercially manufactured test strip used to test for reducing sugars

Answer 10

• dip strip into test solution
• compare colour with calibration card provided

Question 11

what do we have to do when testing for a non-reducing sugar

Answer 11

hydrolyse bond first = ‘free up’ reducing groups

Question 12

procedure for testing for non-reducing sugars

Answer 12

1. test sample for reducing sugars (check there’s none)
2. take separate sample & boil w/ HCl to hydrolyse sucrose into fructose & glucose
3. cool & use sodium hydrogencarbonate to neutralise it
4. test for reducing sugars again

Question 13

positive result for non-reducing sugars

Answer 13

green-yellow-orange-red colour change indicates non-reducing sugar (eg. sucrose) present in original sample

Question 14

can a sample contain non-reducing & reducing sugars

Answer 14

yes
- may have positive test in 1. but test for non-reducing in equal-sized second sample in 2. –> 4.
- if present, precipitate in second sample will have bigger mass
- extract precipitate via filtration

Question 15

test used for lipids

Answer 15

emulsion test

Question 16

procedure for emulsion test for lipids

Answer 16

1. take sample & mix thoroughly w/ ethanol
2. filter
3. pour solution into water in clean test tube
4. cloudy white emulsion indicates presence of lipids

Question 17

test for proteins

Answer 17

biuret test

Question 18

colour change in biuret test

Answer 18

if present = light blue to lilac
- sample in spotting tile

Question 19

separate components of biuret

Answer 19

A = sodium hydroxide
B = copper sulfate

Question 20

how is colour change formed in biuret test

Answer 20

complex between N atoms in peptide chain & Cu2+ ions