Tests for biological molecules Flashcards
What chemical tests for starch and why and what colour does it turn?
When iodine dissolved in potassium iodide forms a triiodide ion I3-, which fits into the middle of the amylose helix causing the colour change from orange/brown to blue/black
How to test for starch? (experiment)
1)Place food sample to be tested in a boiling tube
2) Add iodine in potassium iodide solution
If it turns a blue/black colour appears there is starch
3) Repeat with a control
What chemical tests for reducing sugars and why and what colour does it turn?
If a reducing sugar is heated with Benedict’s solution (alkaline copper (II) sulphate) the Cu2+ ions are reduces to Cu+, forming a brick-red copper (I) oxide (Cu2O)
How to test for reducing sugars? (method)
1)Place food sample to be tested in a boiling tube
2) Add Benedict’s solution then heat in an 80 degrees water bath for 3 minutes till a colour change happens
If it turns a brick-red colour appears there is a reducing sugar
3) Repeat with a control
you can also weigh the precipitate to see what has more sugar relative to others
Benedicts
Boil
Brick-red
What is a reducing sugar?
All monosaccharides and some disaccharides are reducing sugars.
These can reduce or give electrons to other molecules.
Why is a control needed?
To make sure there are no external factors that affect the outcome
How to test for non reducing sugars
1)Do the test for reducing to check that it is none reducing.
2)Then use HCL to hydrolyse the bonds between the disaccharides to make monosaccharide
3)then do normal reducing test
if there are both types of sugars do a reducing test then weigh the precipitate and with a separate sample do a. non reducing test to see the difference in precipitate
How to test quantitatively for reducing sugars
1a) check with water as control
1) Do the test for reducing as normal
2) then use a centrifuge ot seperate the benedicts solution(supernatant)
3) place the supernatant in a curvette (small vial) and put it in the colorimeter to see the percentage transmission
4) the higher the transmission the less the cupper sulfate
5) create a calibration curve (another card is on that)
How does a colorimiter work
it works by seeing how much light is transmitted and how much is absorbed by seing how much light hits the photoelectric cell on the back sending electricity to quantify transmission. A filter is usually used to make it better. Eg a red filter would be used on the Blue benedicts sol. This is because the more red light passing onto the photoelectric cell on the back means that there is less CuS04 as it normaly absorbs red light .
How to calibrate the %age transmission
Test the percentage transmission on multiple known concentrations then plot it on a graph with the transmission on the y and the conc on the x. Then make a curve and use the line and the transmission to estimate the concentrion
Why calibrate?
Otherwise it is not qualitative as the transmission are only relative to other transmissions
How does a biosensor work?
it works by using biological molecluse to either show a result electronically or analogly like a canary in a mineshaft.
What is used in the protein test and why?
Biuret tests for protein beasue the CuSO4 in the biuret splits and the Cu2+ binds to the nitrogen in the peptide bonds
What is used in the lipid test?
ethanol
What is the positive result for proteins?
Blue to lilac
