Testing for biological molecules Flashcards
(39 cards)
what is a qualitative test?
testing for the presence or absence of a particular biological molecule
(does not tell you the concentration of a particular biological molecule in the solution)
how do you test for starch?
iodine test
- add drops of iodine solution
- blue black = starch present
- yellow brown = starch absent
how do you test for protein?
biuret test
- add biuret solution
- shake thoroughly
- purple = protein present
- blue = protein absent
(NO HEAT)
how do you test for lipids?
alcohol emulsion test
- crush sample (if needed)
- add ethanol
- shake
- pour into water
- cloudy white emulsion = lipids present
how do you test for reducing sugars?
benedict’s test
- add benedict’s solution to sample
- shake mixture
- heat (not boil)
- red / orange / yellow / green precipitate = reducing sugar present
- blue precipitate = reducing sugar absent
how do you test for non-reducing sugars?
- carry out benedict’s test
- negative result
- boil with dilute HCl
- neutralise with sodium carbonate
- repeat benedict’s test
- red / orange / yellow precipitate = non-reducing sugar present
what is an example of a non-reducing sugar?
sucrose
what is an example of a reducing sugar?
maltose
why does the structure of enzyme molecules allow them to be detected in solution using the biuret test?
enzymes are globular proteins
what are quantitative tests?
used when you want to know the concentration or mass of a biological molecule in a solution
what is a colorimeter?
a device that shines a beam of light through a sample
a photoelectric cell picks up the light that has passed through
a sample of the solution is placed between the light and the photoelectric cell in a cuvette
advantages - not subjective, less effected by human error
how does a colorimeter work?
the more reducing sugar present in a sample = more copper ions used up to form a precipitate & then removed
less copper ions left in solution = less light absorbed by remaining solution & more light transmitted
gives a measure of the benedict’s reaction & the concentration of reducing sugar in the original solution
what is the relationship between a precipitate & sugar in benedict’s test?
larger amount of precipitate = larger amount of reducing sugar
how do you set the colorimeter to 0% absorbance?
use distilled water
how do you set the colorimeter to 0% transmission?
use unreacted benedict’s solution
why is a red filter used in the colorimeter?
it absorbs all parts of the visible spectrum (apart from red)
so only red light is transmitted to the blue solution in the cuvette
if only red light passes to the cuvette, it maximises absorption of light from the solution in the blue cuvette
this allows the intensity of blue colour to be measured
how is a colorimeter calibrated?
set to 0% absorbance using distilled water
why is it important to calibrate a colorimeter?
values are all measured to the same standard and are compared / valid
how is a calibration curve constructed?
conduct a benedict’s test on known concentrations of glucose
filter out precipitates
use a colorimeter with a red filter
plot the readings from the colorimeter on a graph
(y-axis = transmission / absorbance against x-axis = sugar concentration)
how can a calibration curve be used to estimate a glucose concentration?
test the solution with benedict’s
use a colorimeter to find the absorbance of the remaining solution after filtering
use the graph & curve to find the glucose concentration that corresponds with the absorbance
what other quantitative methods can determine glucose concentration?
use a biosensor
use clinistix / diastix
What are some control variables to make data valid?
same volumes
same concentrations
same length of time
calibrated colorimeter
same filter in colorimeter
what is a semi-quantitative method for determining glucose concentration?
observing the colour of the precipitate after a set amount of time and comparing
allows you to put them in order from most to least sugar
red = most
green = least
blue = none
How can you test the hypothesis that the higher the concentration of glucose in fruit juice, the sweeter it will be?
test for sweetness and make a judgement
however - tasting is subjective, would have to cleanse pallet each time, would take a long time
