Biochemical tests Flashcards
Test for starch?
- Add iodine In potassium iodide to test solution
- Shake or stir
- A positive test observation = solution turns from yellow/orange to blue/black
Test for reducing sugars?
- 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)
Why does the colour change occur at the top of the solution first?
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
Test for a non reducing sugars?
(- 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
Reducing sugars v non reducing sugars examples?
Reducing: All monosaccharides, some disaccharides: Maltose and lactose
Non-reducing: sucrose
Reducing sugars v non-reducing sugars?
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
Test for lipids?
- 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
Test for proteins?
- 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