3.4 Testing for Carbohydrates

Question 1

What does reducing sugars mean?

Answer 1

They can donate electrons, or reduce another molecule or chemical

Question 2

Give examples of reducing sugars

Answer 2

All monosaccharides and some dissacharides (maltose and lactose)

Question 3

What is the chemical test for reducing sugars?

Answer 3

Benedicts reagent (alkaline solution of copper(II) sulfate)

Question 4

Describe the reducing sugars test (Benedict’s reagent)

Answer 4

1. Place the sample to be tested in a boiling tube. If it is not in liquid form, grind it up or blend it in water.
2. Add an equal volume of Benedict’s reagent
3. Heat the mixture gently in a boiling water bath for 5 minutes

Question 5

How does the Benedict’s reagent work?

Answer 5

Reducing sugars will react with the copper ions in Benedict’s reagent, results in the addition of electrons to the blue Cu2+ ions, reducing them to brick red Cu+ ions

Question 6

Explain colour results of the reducing sugars test

Answer 6

green (low concentration of reducing sugar)
yellow/amber/orange (medium concentration of reducing sugar)
red (high concentration of reducing sugar)

Question 7

What is the most common non-reducing sugar?

Answer 7

sucrose

Question 8

What colour would be a negative result of reducing sugar?

Answer 8

Blue

Question 9

What colour would be a positive result of reducing sugar?

Answer 9

Red.

Question 10

What is the iodine test?

Answer 10

detect the presence of starch

Question 11

What are the steps of the iodine test?

Answer 11

A few drops of iodine dissolved in potassium iodide solution are mixed with a sample.

Question 12

What are the results of the iodine test?

Answer 12

If it changes from yellow/brown to purple/black starch is present.
If the solution remains yellow/brown it is a negative result

Question 13

Why does sucrose give a positive result when warmed with Benedict’s reagent when it is boiled with dilute hydrochloric acid?

Answer 13

Because the sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars

Question 14

What are regeant strips?

Answer 14

Used to test for the presence of reducing sugars, mainly glucose. The use of a colour-coded chart, the concentration of the sugar can be determined.

Question 15

How would you calculate % absorbance from a % transmission reading?

Answer 15

100%-transmission % = absorbance %

Question 16

Why is it important to use the correct filter in colorimetry?

Answer 16

To maximise absorption and so its a complementary colour for Benedict’s solution

Question 17

How do you calibrate a colorimeter?

Answer 17

Use distilled water and set colorimeter to 100%

Question 18

How would use colorimetry to determine concentration?

Answer 18

1. A filter was placed in the colorimeter
2. The colorimeter was calibrated using distilled water.
3. Benedict’s test was performed on a range of known concentrations of glucose.
4. The resulting solutions were filtered to remove the precipitate.
5. The % transmission of each of the solutions of glucose was measured using the colorimetry.
6. Using this information a calibration curve was plotted. Steps 3-6 were repeated using the solution with the unknown concentration of glucose.

Question 19

What are biosensors?

Answer 19

Detects the presence of chemicals

Question 20

What do biosensors use?

Answer 20

Biological components to determine the presence and concentration of molecules such as glucose.

Question 21

What is molecular recognition?

Answer 21

A protein (enzyme or antibody) or single strand of DNA is immobilised to a surface, for example a glucose test strip. This will interact with, or bind to, the specific molecule under investigation.

Question 22

What is transduction?

Answer 22

This interaction will cause a change in a transducer. A transducer detects changes, for examples in pH, and produces a response such as the release of an immobilised dye on a test strip or an electric current in a glucose-testing machine.

Question 23

What is a display?

Answer 23

This then produces a visible, qualitative nor quantative signal such as a particular colour on a test strip or reading on a test machine.

Question 24

Describe the features of enzymes essential to their role as components in a biosensor

Answer 24

Enzymes have an active site that is specific to the substance or molecule it is testing for

Question 25

Explain why an iodine test is used in experiments to show that plants require light for photosynthesis

Answer 25

In an iodine test a purple/black colour indicates the presence of starch. Starch is a product of photosyntheis. The test shows that starch is produce when light is available to the plant, but not when the plant is kept in the dark.