Biochemical tests for molecules

Question 1

Describe the test for Reducing sugars

Answer 1

1. Place a sample in a boiling tube and add equal volumes of Benedict’s reagent
2. Heat the mixture gently
3. Blue → green → yellow→ orange→ brick red precipitates depending on their concentration
4. A qualitative test

Question 2

What is in Benedict’s reagent

Answer 2

An alkaline solution of Copper (II) sulfate

Question 3

What causes the change in colour in Benedict’s reagent when mixed with reducing sugars

Answer 3

1. Reducing sugars can donate electrons and reduce other molecules
2. Blue Cu2+ are reduced to brick red Cu+

Question 4

Give examples of reducing sugars

Answer 4

1. All monosaccharides

2. Some disaccharides- maltose and lactose

Question 5

Describe the test for non-reducing sugars

Answer 5

1. Do not react with Benedict’s solution so the solution will remain blue after warming
2. If you then add dilute HCl and heat in a water bath
3. Then neutralise it with sodium hydrogen carbonate
4. This hydrolyses the sugar into monosaccharides so gives a positive result

Question 6

Give an example of a non-reducing sugar

Answer 6

Sucrose

Question 7

Describe the test for starch

Answer 7

1. Add a few drops of iodine dissolved in potassium iodide solution to the sample
2. If solution turns from yellow/brown to blue/black starch is present

Question 8

Describe the test for lipids

Answer 8

1. Shake the test substance with ethanol for about a minute
2. Then pour into water.
3. If lipid is present a white emulsion forms as a layer on top of the solution.
4. If no lipid present the solution will remain clear

Question 9

Describe the test for proteins

Answer 9

1. Add sodium hydroxide to make the solution alkaline
2. Add copper (II) sulfate solution
3. If protein is present the solution is purple
4. If there’s no protein, the solution will stay blue

Question 10

Describe what reagent test strips are with an example of how they are used.

Answer 10

1. Glucose can also be tested for using test strops covered in reagent
2. They change colour if glucose is present
3. The colour change can be compared with a colour chart to give an indication of concentration of glucose present

Question 11

Define qualitative test

Answer 11

1. The process of determining whether or not a particular chemical is present in a sample.

Question 12

Define quantitative test

Answer 12

1. Tell you the amount that is present in the sample

Question 13

Describe what a calorimeter does

Answer 13

1. It measures the absorbance- the more concentrated the colour of the solution, the higher the absorbance is

Question 14

Describe how to use a calorimeter in a quantitative test for reducing sugars

Answer 14

1. First calibrate the colorimeter using distilled water.
2. Carry out a serial dilution to get a range of known glucose concentrations
3. Carry out the benedict’s test on these solutions- use the same amount of Benedict’s solution in each case
4. Remove any precipitate
5. % transmission of each solutions of glucose is measured using the colorimeter
6. Plot a calibration curve of absorbance over concentration of glucose.

Question 15

Describe, in principle, how biosensors work.

Answer 15

1. Molecular recognition- The biological molecule produced a signal
2. Transducer- THis signal is converted to an electrical signal by a transducer
3. Display- The electrical signal then produced a visible, qualitative or quantitative signal such as a particular colour on a test strip or reading on a test machine