Biochemical tests Flashcards
What reagent is used to test for sugars?
Benedict’s reagent
What colour is Benedict’s reagent?
Pale blue
Describe the test for reducing sugars and the results.
- Add excess Benedict’s reagent to the sample
- Heat in a water bath above 80°C (boiling)
- If a reducing sugar is present the sample will turn from blue to green to yellow to orange to brick red, depending on how many reducing sugars there are
What are reducing sugars?
All monosaccharides and some disaccharides including maltose and lactose
Describe the test for non-reducing sugars and the results.
- Following a negative result of reducing sugars, the sample will remain blue
- Add dilute hydrochloric acid and heat in a water bath (to break the bonds and produce monosaccharides)
- Cool the solution then add an alkali to neutralise
- Add excess benedict’s reagent to the sample and heat in a water bath above 80°C
- If a non-reducing sugar is present the sample will turn from blue to green to yellow to orange to brick red, depending on how many non-reducing sugars there are
Describe how we can obtain quantitative data from a positive result of reducing sugars
- filter the solution and weigh the precipitate
- remove the precipitate
- use a colorimeter to measure the absorbance of the remaining Benedict’s solution
What are non-reducing sugars?
Disaccharides and some polysaccharides like sucrose
What are the hazards and precautions when testing for sugars?
Benedict’s reagent is an irritant
- wear goggles
- wash hands on contact with skin
Hot water can cause burns
- use caution when poring / carrying hot water
What reagent is used to test for starch?
Iodine
What colour is iodine?
Yellow / brown
Describe the test for starch and the results.
- Add iodine to the sample
- If starch is present the sample will turn from brown/yellow to blue/black
What reagent is used to test for lipids?
Ethanol and water
What colour is ethanol and water?
Clear
Explain the emulsion test for lipids.
- Add ethanol to the sample
- Then add water and shake for a minute
- If a white milky emulsion appears, there is a lipid present
What are the hazards and precautions when testing for lipids?
Ethanol is flammable
- do not conduct this test near a naked flame
What reagent is used to test for amino acids?
Biuret’s reagent
What colour is Biuret’s reagent?
Pale blue
Describe the test for amino acids and the results.
- Add excess Biuret’s reagent to the sample
- Shake and leave at room temperature
- If amino acids are present the sample will turn from a pale blue to purple
Describe how we can obtain quantitative data from a positive result of amino acids
Chromatography can be used to identify amino acids in a mixture, separating them by mass
What are the hazards and precautions when testing for amino acids?
Biuret’s reagent is an irritant
- wear goggles
- wash hands on contact with skin
Why do we add Benedict’s or Biuret’s reagent in excess?
To ensure all reducing sugars or amino acids have reacted fully to produce accurate results
