Chemical Tests - Lipids, Proteins, Carbohydrates

Question 1

How do you carry out and interpret the Biuret test?

Answer 1

• Tests for peptide bond

1. Place sample in a clean test tube
2. Add Biuret A (NaOH)
3. Add few drops of Biuret B (CuSO₄ - copper II sulfate)
4. If proteins are present - BLUE TO PURPLE

Question 2

How do you carry out and interpret the emulsion test?

Answer 2

1. Place sample in a clean test tube
2. Add ethanol and mix to dissolve lipid
3. Add water and mix
4. If there are lipids present, a cloudy, white emulsion layer is on top
   
   • Occurs because lipids aren’t soluble in water so they come out as liver globules to form an emulsion

Question 3

How do you carry out and interpret the iodine test?

Answer 3

• To detect the presence of starch - iodine test

1. Add potassium iodide (KI) and shake
2. If starch is present colour change from yellow/brown to blue/black

Question 4

How do you test for reducing sugars using a reagent strip?

Answer 4

1. Dip the blue end of the reagent strip to test sample
2. Compare the test strip colour against a colour-coded chart

Used for urine test to see if one has diabetes

Question 5

How do you carry out and interpret the Benedict’s test?

Answer 5

• Tests for reducing sugars (sugars that lose electrons to other molecules)
• All monosaccharides and some disaccharides

1. Place in clean test tube
2. Add equal volume of Benedict’s Reagent (copper II sulfate)
3. Heat in water bath (100^oC) for 3-5 minutes
4. If there are reducing sugars, Cu²⁺ reduced to red Cu⁺ ions

Colour change from blue > green > yellow > red

>>>>> increasing reducing sugar conc.

Question 6

How do you test for non-reducing sugars?

Answer 6

Some disaccharides

1. • HCl and heat in water bath
   • Acid hydrolyses the disaccharides to reducing monosaccharides
2. Neutralise with NaOH
3. Continue with normal Benedict’s Test

Question 7

How can you quantitatively deterrmine the concentration of reducing sugars?

Answer 7

Calibrate Colorimeter

1. Add filter to colorimeter and calibrate with distilled water

Test

1. Perform the Benedict’s test on a range of known concentrations of glucose
2. Filter the resulting solutions to remove the precipitate
3. Test the solutions to find the % transmission
4. Plot a calibration curve with % transmission on the y-axis and [reducing sugar] on the x-axis
5. Repeat 1-4 for the unknown sample
6. Match the % transmission to the curve and find the concentration

Question 8

What is the function of a biosensor and how does it work?

Answer 8

• Biosensors convert chemical substances into electrical signals
• They use biological components to determine the presence of e.g. glucose and its conc.

1. Molecular recognition - biosensor dipped in analyte (sample being tested), an immoblised: enzyme, DNA strand or antibody interacts/binds with the molecules
2. Transduction - interaction causes a change in the transducer - the change is detected e.g. pH change, and a response is produced such as releasing a dye or electrical current
3. Display - this produces a quantitative or qualitative result