Topic 1- biochemical tests

Question 1

What is the test for reducing sugars?

Answer 1

Benedicts

Question 2

what is the method for benedicts?

Answer 2

-Add Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube
-Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes

Question 3

What is the result if a reducing sugar is present?

Answer 3

-A coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
-A colour change somewhere along a colour scale from blue (no reducing sugar), through green, yellow and orange (low to medium concentration of reducing sugar) to brown/brick-red (a high concentration of reducing sugar)

Question 4

Give examples of the reducing sugars

Answer 4

-Galactose
-Glucose
-Fructose
-Maltose

Question 5

What is the non reducing sugar?

Answer 5

Sucrose

Question 6

What do you change to test for non-reducing sugars? Explain why

Answer 6

-Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to a boil-addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules

-Neutralise the solution with sodium hydrogen carbonate-monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

Question 7

What is the test for starch?

Answer 7

Iodine

Question 8

What is the method to test for starch?

Answer 8

-Add a few drops of iodine to the sample

Question 9

What is the positive result for starch?

Answer 9

• iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour

Question 10

1.____________________relates to the concentration of reducing sugar present in the sample

Answer 10

1. The intensity of any colour change

Question 11

How do you find concentrations of a reducing sugar?

Answer 11

-Add the same volume of Benedict’s solution to each sample and heat in a water bath that has been boiled (ideally at the same temperature each time) for a set time (5 minutes or so) to allow colour changes to occur
-important that an excess of Benedict’s solution is used so that there is more than enough copper (II) sulfate present to react with any sugar present
-The same procedure is carried out on a sample with an unknown concentration of reducing sugar which is then compared to the stock solution colours to estimate the concentration of reducing sugar present

Question 12

To avoid issues with human interpretation of colour,1._______to measure the absorbance or transmission of light through the sugar solutions of known concentration to establish a range of values that an unknown sample can be compared against a calibration curve

Answer 12

1. Colourimeter