Biochemical tests for molecules

Question 1

Biuret test tests for?

Answer 1

Proteins

Question 2

Biuret test procedure -

Answer 2

Two stages -
Test solution needs to be added to alkaline with a few drops of sodium hydroxide solutions

Then add some copper sulphate solutions.

These peptide bonds form violet (purple) coloured complexes which react with copper ions in alkaline.

Purple indicates protein where as blue is remained if not present.

Question 3

Emulsion test tests for -

Answer 3

Lipids

Question 4

Emulsion test procedure -

Answer 4

Sample is mixed with ethanol, resulting solution is mixed with water and shaken, a white emulsion forms a layer on top of the solution indicating there is a lipid if it remains clear there is no lipid.

Question 5

Iodine test tests for?

Answer 5

Starch

Question 6

Iodine test procedure -

Answer 6

Add a few drops iodine dissolved in potassium iodide solution and mix with the sample being tested, changes from yellow/brown to purple/black if starch is present, no starch is present if it remains the original colour.

Question 7

Benedict’s test for sugar (reducing) -

Answer 7

1. Place the sample to be tested in a boiling tube, if not liquid grind it up.
2. Add an equal volume of benedicts reagent.
3. Heat the mixture for 5 minutes gently.

A reducing sugar will react with the copper ions, a brick-red precipitate is formed indicating a positive result, the more reducing sugar the more precipitate forms and the less blue cu2+ are left in the solution.

Question 8

Benedict’s test for sugar (non-reducing) -

Answer 8

Non-reducing sugars do not react with benedicts solution and the solution will remain blue after warming.

If sucrose is first boiled with dilute hydrochloric acid it will then give a positive result when warmed with benedicts solution, as the sucrose has become hydrolysed by the acid.

Question 9

Reducing sugars -

Answer 9

glucose, galactose, fructose, and lactose

Question 10

Non-reducing sugars -

Answer 10

Sucrose

Question 11

Colorimeter -

Answer 11

Useful for the measurement and the absorbance rates of such benedicts tests.