Biological molecules - Tests for Carbohydrates Flashcards
describe the chemical test for starch, including what a positive and negative result looks like
add a couple of drops of iodine solution to the test sample
if starch is present - sample will change to a dark blue/black colour
if starch is not present - iodine solution will stay brown
what are reducing sugars, giving examples
can reduce, or give electrons to, other molecules
e.g all monosaccharides, some disaccharides e.g maltose
describe the test for reducing sugars
to your sample add an equal volume of Benedict’s solution (blue)
heat at 80 degrees C in a water bath for 3 minutes
a brick red precipitate forms - the Cu2+ ions in the Benedict’s solution are reduced to Cu+ ions and a red copper oxide precipitate forms
what happens if Benedict’s solution is used in excess
the intensity of the red colour is proportional to the sugar concentration ie if only a little ppt is formed, the reaction mix will appear green/yellow/orange
what does using Benedict’s solution in excess allow
allows qualitative comparisons of glucose concentrations
what are non-reducing sugars, giving an example
unable to reduce, or give electrons to, other molecules
e.g sucrose
when is the test for non-reducing sugars carried out
when a substance gives a negative Benedict’s test (stays blue)
describe the test for non-reducing sugars
add hydrochloric acid to the test solution and boil in a water bath (the acid catalyses the hydrolysis of glycosidic bonds)
cool the tube and neutralise the acid by adding an alkali
test with UI to check the solution is neutral or alkaline
repeat the Benedict’s test again, if sucrose was present, it will have hydrolysed to glucose and fructose and give a positive results (brick red)
how does a colorimeter work
works by shining light through a sample and can measure how much light is transmitted through it (or absorbed)
what is the relationship between concentration of colour and absorbance and transmission
the more concentrated the colour pf the solution, the greater the absorbance of light by the solution and the less transmission of light through the sample
how must the cuvettes be held and why
by the ridges sides to prevent greasy fingerprints impacting results
how is the colorimeter reset between readings
the colorimeter is zeroed between readings by placing distilled water in to reset
briefly describe the calibration curve method
use serial dilution to make different known concentrations of glucose from stock solution
carry out test for reducing sugars
use a centrifuge to separate the solutions from their ppts
select the red filter on the colorimeter and set it to zero
read the absorbance of light of each solution, zeroing between each reading
plot curve of absorbance against sugar concentration
what is serial dilution
a succession of step dilutions to create known concentrations from a stock solution
describe the process of serial dilution
some of the stock solution is taken and diluted
some of the new solution is then diluted to make the next solution
each time you dilute the previous solution by the same factor as the step before e.g dilute a stock solution by half each time (dilution factor of 2)
