A level Biology 1.2-1.4 - Carbohydrates

Question 1

What are monosaccharides and what are the 3 common types?

Answer 1

• Monosaccharides also called simple sugars are the basic units from which all carbohydrates are made.
• Glucose, Galactose and fructose are common monosaccharides.
• They have the common formula C6H12O6.

Question 2

What are disaccharides ?

Answer 2

A condensation reaction between two monosaccharides forms a glycosidic bond. This is how disaccharides are formed.

Question 3

What are the two monosaccharides that make up Maltose ?

Answer 3

Maltose is a disaccharide formed by the condensation of two glucose molecules.

Question 4

What are the 2 monosaccharides that make up Lactose ?

Answer 4

Lactose is a disaccharide that is formed by the condensation of a glucose molecule and a galactose molecule.

Question 5

What are the 2 monosaccharides that make up sucrose ?

Answer 5

Sucrose is a disaccharide formed by the condensation of a glucose molecule and a fructose molecule.

Question 6

Describe the Benedict’s test for reducing sugars

Answer 6

1) Add an equal volume of Benedict’s reagent to a sample. 2) Heat the mixture in a water bath at 80 degrees Celsius for 5 minutes. 3) It should turn from Blue to orange or brick red.

Question 7

Describe the test for non-reducing sugars

Answer 7

1) if the sample is not already in liquid form it must first be ground up in water

2) Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil

3) Neutralise the solution with sodium hydrogen carbonate Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogen carbonate as the conditions need to be slightly alkaline for the Benedict’s test to work.

4) Then carry out Benedict’s test as normal; add Benedict’s reagent to the sample and heat in a water bath that has been boiled – if a colour change occurs (orange-red precipitate), a non-reducing sugar is present

Question 8

How would you make the Benedict test quantitative ?

Answer 8

To get exact values, one method we can use is colorimetry, a quantitative method that allows the concentration of compounds in a coloured solution to be determined as follows:

• A colorimeter is used to determine how much light a solution absorbs. Colorimetry involves the use of a colorimeter, a device that detects how much light is absorbed by the solution. In other words, how much of the light that is travelling through the solution is being stopped by the solution.

*The more concentrated the solution, the more light it will absorb. A calibration curve needs to first be produced. Prior to performing colorimetry on the test solution, we must first create a graph that demonstrates the degrees of light absorption at different concentrations of reducing sugars.

• To do this, we can measure the light absorbances of 5 glucose solutions at different known concentrations. Then plot these absorbances against the glucose concentrations and use this calibration curve to interpret the result of our test sample. Use the colorimeter on the test solution.
• Now that we have a calibration curve, we can use the colorimeter on the test sample to measure how much light is being absorbed by the solution.
• When you get a result, use the calibration curve to determine at what glucose concentration this degree of light would be absorbed. You now have the quantitative value of glucose concentration in your test solution.