Biochemical tests Flashcards

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1
Q

Test for starch?

A
  • Add iodine In potassium iodide to test solution
  • Shake or stir
  • A positive test observation = solution turns from yellow/orange to blue/black
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2
Q

Test for reducing sugars?

A
  • 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)
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3
Q

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

A

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

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4
Q

Test for a non reducing sugars?

A

(- 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
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5
Q

Reducing sugars v non reducing sugars examples?

A

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

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6
Q

Reducing sugars v non-reducing sugars?

A

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

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7
Q

Test for lipids?

A
  • 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
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8
Q

Test for proteins?

A
  • 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
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