3.4 Testing for carbohydrates

Question 1

What solution is used to test for carbohydrates (sugars)

Answer 1

Benedict’s solution (copper sulphate)

Question 2

Benedict’s test steps

Answer 2

Add 2cm^3 of food sample (liquid form)
Add equal amount of Benedict’s reagent
Heat mixture gently in water bath for 5 mins

Question 3

Results in benedict’s test for reducing sugars

Answer 3

Reducing sugars react with blue Cu2+ ions in benedict’s
form a red precipitate (brick red)

Question 4

Method for benedict’s test for non-reducing sugars

Answer 4

Must first be hydrolysed into its monosaccharide components
- first test with benedicts

Question 5

If there is no colour change with non-reducing sugars…

Answer 5

Add HCl
Heat in water bath for 5 mins
Add alkali sodium hydrogen carbonate and indicator until neutralised
Then reconduct benedict’s test

Question 6

Example of reducing sugar

Answer 6

Glucose

Question 7

Example of non-reducing sugar

Answer 7

Sucrose

Question 8

Why does a red precipitate form with reducing sugars?

Answer 8

The reducing sugar reacts with the blue Cu2+ ions in the reagent which reduces them to Cu + ions which forms a red precipitate

Question 9

Why must non-reducing sugars need to be hydrolysed before re-conducting the benedict’s test?

Answer 9

It must be broken down into its monosaccharide components
e.g. sucrose breaks down into glucose and fructose (both reducing sugars)

Question 10

Test for starch and results

Answer 10

Iodine mixed with potassium iodide
If solution turns black = positive result
If solution remains yellow/brown = negative result

Question 11

What are reagent strips?

Answer 11

Manufactured reagent strips can be used to test for the presence of reducing sugars using a coloured chart to determine the concentration of sugar

Question 12

What is a colorimeter?

Answer 12

Measures the absorbance/transmission of light by a coloured solution
The more concentrated a solution is, the less light it will transmit which can be used to calculate its concentration from a plotted graph

Question 13

Steps of colorimetry

Answer 13

1) Filter is placed in colorimeter
2) Calibrated using distilled water
3) Benedict’s test performed on a range of known glucose concentration
4) Solutions filtered to remove precipitate/ centrifuged
5) % transmission calculated by colorimeter
6) Calibration curve plotted
Stages 3-6 repeated with unknown glucose concentration

Question 14

Why is it important to use a filter in colorimeters?

Answer 14

To maximise absorption

Question 15

What is the name of the solution after centrifuging/ filtering?

Answer 15

Supernatant

Question 16

What are biosensors used for?

Answer 16

Use biological molecules to determine the presence and concentration of molecules such as glucose

Question 17

What happens during molecular recognition in biosensing?

Answer 17

a protein or single strand of DNA is immobilised to test strip which will interact/bind to the molecule being investigated

Question 18

What happens during transduction in biosensing?

Answer 18

interaction causes a change in the transducer which detects this change and produces a response
e.g. release of dye or electric current

Question 19

What happens during the display stage of biosensing?

Answer 19

produces visual signal e.g. particular colour or a reading on the test machine

Question 20

Why are enzymes specialised for use in biosensors?

Answer 20

They have a specialised active site that binds to specific substrates and so can identify only desired molecules

Question 21

How can iodine be used to prove that photosynthesis requires light?

Answer 21

Starch is a product of photosynthesis and the test shows the starch is produced when light is available but not in darkness

Question 22

Why are reagent strips good for those with diabetes?

Answer 22

Quantitative
Estimates reducing sugar levels in the blood easily and simply
Simple method