Molecule tests

Question 1

Starch

Answer 1

-Iodine solution (potassium iodide) turns yellow-brown to blue-black.
-Iodine forms a triiodide ion which enters the middle of the amylose helix.

Question 2

Reducing sugars

Answer 2

-Monosaccharides and some disaccharides.
-Heat (80C) with Benedict’s solution (copper2 sulphate).
-Blue to brick red (yellow or green in lower conc).
-Cu2+ ions reduced to Cu+ ions, forming red copper oxide precipitate suspended in the mixture.

Question 3

Non-reducing sugars

Answer 3

-No free bonds, so require hydrolysis.
-Boil sample with hydrochloric acid to hydrolyse into monosaccharides.
-Cool solution and use sodium hydrogencarbonate to neutralise.
-Test for reducing sugars. A positive result means a non-reducing sugar was present in the original sample.

Question 4

Lipids (emulsion)

Answer 4

-Mix sample with ethanol. Any lipid will go into solution in the ethanol (but no water)
-Filter and pour solution into water.
-A cloudy white emulsion (lipid droplets) indicates the presence of lipids.

Question 5

Proteins

Answer 5

-Add Biuret solution to sample on spotting tile.
-Changes from light blue to lilac.
-Biuret A (NaOH) then Biuret B (copper sulphate).
-Complex forms between nitrogen atoms on a peptide chain and Cu2+ ions.

Question 6

Colorimetry

Answer 6

-Quantifies conc of reducing sugar in Benedict’s by measuring amount of precipitate (increases with more sugar, as Cu2 ions decrease).
-Centrifuge used to separate precipitate and supernatant (excess) Benedict’s.
-Supernatant is placed in a cuvette in the colorimeter.

Question 7

Colour filters

Answer 7

-In colorimeter a filter is used.
-With a red filter, red light is shone through the solution and the amount passed through is detected.
-Since Benedict’s is blue when unreacted, a high amount of blue in the solution, and high absorption of red light,
-If absorption of red light is low (and % transmission is high) then there is a high amount of reducing sugars.

Question 8

Stationary phase

Answer 8

-Chromatography paper or TLC plate.
-Free -OH groups pointing outwards in contact with the mobile phase.

Question 9

Mobile phase

Answer 9

-Solvent for biological molecules.
-Usually water (polar) or ethanol (non-polar).

Question 10

Chromatography

Answer 10

-Solvent travels up stationary phase, and components of solution travel with it at different speeds due to their polarity (more polar=slower) and solubility.
-Where solvent reaches top of the plate, the pigments are separated.
-Colourless molecules can be seen by shining UV light on TPCs (spots of pigment will not glow) or adding iodine.