3.4- Testing For Carbohydrates

Question 1

What is meant by reducing sugars?

Answer 1

All monosaccharides and some disaccharides are reducing sugars. This means that they can donate electrons to (or reduce) another molecule/chemical.

Question 2

Benedict’s test for reducing sugars.

Answer 2

1. Place sample in a boiling tube in liquid form.
2. Add an equal volume of Benedict’s reagent.
3. Heat the mixture gently in a water bath for 5mins.

• reducing sugars react with copper ions in Benedict’s reagent. Results in the addition of electrons to the blue Cu2+ ions, reducing them to brick red Cu+ ions.
• brick red precipitate represents positive result.
• the more reducing sugar present, the more precipitate formed so colour depends on conc of reducing sugars present = qualitative test.

Question 3

Test for starch.

Answer 3

Iodine test:

1. A few drops of iodine dissolved in potassium iodide solution are mixed with the sample.
2. Yellow/brown to purple/black indicates presence of starch.

Question 4

What is the use if reagent strips?

Answer 4

Can be used to test for reducing sugars, most commonly glucose.

advantage: with the use of a colour coded chart, concentration of the sugar can be determined.

Question 5

What are the 2 quantitative methods to find concentration?

Answer 5

1. Colorimetry

2. Biosensors

Question 6

Explain colorimetry.

Answer 6

Allows you to estimate glucose present in a solution.
It quantitatively measures the absorbance/transmission of light by a coloured solution.
More concentrated solution= higher absorbance & less transmission.

1. Place filter in colorimeter.
2. Calibrate using distilled water.
3. Benedict’s test done on a range if conc.
4. Solutions filtered to remove precipitate.
5. % transmission of solution measured.
6. Calibration curve plotted then used to calculate concentration in unknown sample.

Question 7

Explain biosensors.

Answer 7

They use biological components to find the presence and concentrations of molecules.
Analyte= compound under investigation.j

1. Molecular recognition= a protein/ single DNA strand is immobilised to a surface which will interact/ bind to the specific molecule under investigation.
2. Transduction = transducer detects changes (eg. pH) and produces a response (such as a dye or electric current being released).
3. Display= produces visible (quantitative or qualitative) signal.