Biochemical tests

Question 1

Test for starch?

Answer 1

• Add iodine In potassium iodide to test solution
• Shake or stir
• A positive test observation = solution turns from yellow/orange to blue/black

Question 2

Test for reducing sugars?

Answer 2

• Add Benedict’s reagent to an equal volume of sample
• heat in a water bath
• Our positive test observation = solution turns from blue to brick red (Green, yellow, orange) (the more red the higher the concentration of Reducing sugar)

Question 3

Why does the colour change occur at the top of the solution first?

Answer 3

Hotter particles in solution rising so - hottest point in solution is at the top - most kinetic energy atp - More successful collisions - faster reaction - colour change happens first at the top

Question 4

Test for a non reducing sugars?

Answer 4

(- Add Benedict’s reagent
- Heat in a water bath
Observe the result - negative (remains blue))

• In a new test tube, add the unknown sugar (that got the negative result) & boil with HCl in a water bath (acid hydrolysis)
• Add an alkali (e.g. sodium hydrogen carbonate) to neutralise
• Add benedict’s reagent and heat
• Should see a colour change to brick red (or green/orange/brown) if a non-reducing sugar was present

Question 5

Reducing sugars v non reducing sugars examples?

Answer 5

Reducing: All monosaccharides, some disaccharides: Maltose and lactose
Non-reducing: sucrose

Question 6

Reducing sugars v non-reducing sugars?

Answer 6

Reducing: can reduce the copper sulphate (Cu2+ ions)(blue) in Benedict’s reagent to copper oxide (Cu+)(Brick red precipitate)

Non-reducing: the reducing group is involved in the glycosidic bond in sucrose
- When hydrolysed with acid - glycosidic bond is broken - reducing group becomes exposed - positive result with Benedict’s

Question 7

Test for lipids?

Answer 7

• Mix/ dissolve sample with/ in ethanol in a test tube
• Shake the tube thoroughly
• Add cold/ distilled water and shake gently
• Positive test observation = milky-white emulsion formed

Question 8

Test for proteins?

Answer 8

• Add few drops of biuret (NaOH + Cu (II) SO4) to sample
• Positive test observation = solution blue to purple (purple indicates presence of peptide bond so protein - N in bonds forms purple complex w Cu 2+ ions)
• the more conc. the protein, the darker the purple so - a qualitative test
• semi quantitative: comparing colour of two identically treated samples
• colorimeter would make more accurate