section 2: biological molecules - topic 7: biochemical tests for molecules Flashcards
what test would you do for protein?
the biuret test.
describe the biuret test.
- the test solution needs to be alkaline, so first you add a few drops of sodium hydroxide solution.
- then add some copper(ll) sulphate solution.
what colour is the biuret solution if protein is present?
turns purple.
what colour is the biuret solution if no protein is present?
stays blue.
what test would you do for starch?
the iodine test.
describe the iodine test.
add iodine dissolved in potassium iodine solution to the test sample.
what colour is the iodine solution if starch is present?
changes from browny-orange to a dark, blue-black colour.
what colour is the iodine solution if no starch is present?
stays browny-orange.
what test would you do for lipids?
the emulsion test.
describe the emulsion test.
shake the test substance with ethanol for approx. a minute, then pour the solution into water.
what colour is the emulsion solution is lipids are present?
the solution will turn milky.
more lipid = more noticeable milkiness.
what colour is the emulsion solution if no lipids are present?
the solution will stay clear.
what test would you use for sugars?
the Benedict’s test.
what are the 2 types of sugars?
reducing.
non-reducing.
what do reducing sugars include?
all monosaccharides (i.e. glucose) and some disaccharides (i.e. maltose & lactose).
describe the Benedict’s test.
add Benedict’s reagent (blue) to a sample and heat it in a water bath that has been brought to the boil.
what happens if a reducing sugar is present?
a coloured precipitate will form.
the higher the concentration, the colour will form green, yellow, orange, then brick red.
how do you test for a non-reducing sugar?
first, break them down into monosaccharides.
this is done by getting a new sample of the test solution, adding dilute hydrochloric acid acid & carefully heating in a water bath that’s been brought to the boil.
then neutralise it with sodium hydrogencarbonate.
what happens if a non-reducing sugar is present?
sample forms green - yellow - orange - brick red precipitate.
how else can glucose be tested for?
by using test strips coated in a reagent.
what do the strips do if glucose is present?
they are dipped in a test solution and change colour if glucose is present.
how do you work out the concentration of glucose present from the test strips?
compare the colour change to a chart to give an indication of the concentration.
when may the test strips for glucose be useful?
for testing a persons urine for glucose - may indicate they have diabetes.
what do quantitative tests tell you?
the amount (i.e. concentration) of a substance that is present in a sample.
what can you use colorimetry and the Benedict’s test for?
to get a quantitative estimate of how much glucose (or other reducing sugar) there is in a solution.
what is a colorimeter?
a device that measures the strength of a coloured solution by seeing how much light passes through it.
what does a colorimeter measure?
absorbance (the amount of light absorbed by the solution).
how do colour concentration and absorbance relate in a colorimeter?
the more concentrated the colour of the solution, the higher the absorbance is.
how do you find out the glucose concentration of an unknown solution?
- first make up several solutions of known glucose concentrations.
- then measure the absorbance of these solutions.
- finally plot these absorbances on a graph to make a calibration curve.
- you can then use the calibration curve to estimate the concentration of glucose in the unknown solution.
what is the relation between glucose concentration and absorbance?
it’s easiest to measure the concentration of the blue Benedict’s solution that’s left after the test.
the paler the solution, the more glucose there was.
the higher the glucose concentration, the lower the absorbance of the solution.
