3.4 Testing for Carbohydrates Flashcards
Chapter 3 - Biological Molecules
What are reducing sugars?
Saccharides (sugars) that donate electrons resulting in the reduction (gain of electrons) of another molecule.
All monosaccharides and some disaccharides are examples of what type of sugars?
Reducing sugars.
What is used to test for a reducing sugar?
Benedict’s reagent, an alkaline solution of copper(II)sulfate.
What is the first step when testing for a reducing sugar?
Place the sample to be tested in a boiling tube. If it is not in a liquid form, grind it up or blend it in water.
What is the second step when testing for a reducing sugar?
Add an equal volume of benedict’s reagent.
What is the third step when testing for a reducing sugar?
Heat the mixture gently in a boiling water bath for five minutes.
What is a positive test for reducing sugars?
Brick-red precipitate.
Why does the brick-red colour appear in a positive test for reducing sugars?
Reducing sugars will react with the copper ions in Benedict’s reagent, resulting in the addition of electrons to the blue Cu2+ ions, reducing them to brick-red Cu+ ions.
How is the test for reducing sugars qualitative?
Concentration of the reducing sugars present in the solution depends on the amount of brick-red precipitate and blue unchanged copper ions left in the boiling tube.
What does a green colour indicate in a benedict’s test?
Low concentration of reducing sugars.
What does a yellow/amber/orange colour indicate in a benedict’s test?
Medium concentration of reducing sugars.
What does a red colour indicate in a benedict’s test?
High concentration of reducing sugars.
What is the most common non-reducing sugar?
Sucrose.
How could you test for non-reducing sugars?
Boil the sample with hydrochloric acid before testing for reducing sugars.
Why is the non-reducing sugar sample boiled with hydrochloric acid first?
The acid hydrolyses the non-reducing sugar to reducing sugars to be able to carry out the Benedict’s test on a non-reducing sugar.
What is used to test for starch?
Iodine test.
How is an Iodine test for starch carried out?
Mix the sample with a few drops of iodine dissolved in potassium iodide solution.
What is a positive test for starch?
Solution changes colour from yellow/brown to purple/black.
What is a negative test result for starch?
Solution remains yellow/brown.
What are reagent strips used to test?
Reducing sugars, most commonly glucose.
What is the advantage of using reagent test strips?
The use of colour-coded chart, where the concentration of the sugar can be determined.
What is a colorimeter?
A piece of equipment used to quantitatively measure the absorbance, or transmission, of light by a coloured solution.
Using a colorimeter, what is seen in a more concentrated solution?
More light will be absorbed and less light will be transmitted.
What are biosensors?
A device that uses biological components to determine the presence and concentration of molecules such as glucose.
