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Practical Biochemistry Notes
Practical biochemistry 1: Qualitative tests for biological molecules
- Each test determines simply the presence or absence of a particular biological molecule
- Reply on the biological molecules in the sample passing into solution so the samples need to be squashed
and mixed with water (or alcohol for lipids)
- Eye protection is needed for all tests
Testing for carbohydrates
Starch
- Add iodine solution (in potassium iodide) to the sample
- If starch is present it turns from yellow-brown to blue-black
- When dissolved in potassium iodide, the iodine (I2) forms a triiodide ion I3- which slips into the middle of
the amylose helix
- As a result, it changes colour
Reducing sugars
- Include all monosaccharides and some disaccharides
- Called reducing sugars because they can reduce (give electrons to) other molecules
- Heat reducing sugar with Benedict’s solution: colour change blue to green to yellow to orange-red
- Benedict’s solution (alkaline copper II sulphate) contains Cu2+ ions which are reduced to Cu+ ions, forming
orange-red copper I oxide (Cu2O) – precipitate comes out of solution, forms a solid in the reaction mixture
- If Benedict’s solution is in excess, intensity of the red colour is proportional to the concentration of sugar
- The reaction mix will appear green if one a little precipitate is formed, and fully orange-red if a lot of
precipitate is formed
- Commercially manufactured test strips: strip is dipped in the test solution and the colour is compared to
the calibration card which tells you whether reducing sugar is present or absent from your solution ->
used to test for glucose in the urine of diabetic patients
Non-reducing sugars
- To test for a non-reducing sugar we need to hydrolyse the bond first to ‘free up’ these ‘reducing groups’
and then test for reducing sugars as normal:
1. Test a sample for reducing sugars to check there aren’t any
2. Take separate sample and boil with hydrochloric acid to hydrolyse sucrose into glucose and fructose
3. Cool solution and use sodium hydrogencarbonate solution to neutralise it
4. Test for reducing sugars again
- Positive result: green-yellow-orange-red indicates a non-reducing sugar was present in original sample
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