Testing For Biological Molecules Flashcards
What can a reducing sugar do?
It can donate electrons or reduce another chemical
Benedict’s test for reducing sugars
1) Place the sample in a test tube if liquid (if solid, crush and add to water)
2) Add an equal volume of Benedict’s reagent
3) Heat the mixture gently in a water bath for 5 minutes
What happens in Benedict’s test?
The reducing sugars will react with Copper ions in Benedict’s reagent
Electrons will be added to the blue Cu2+ ions
They are reduced to brick red Cu+ ions
Colours for Benedict’s test
Green - low concentration of reducing sugar
Yellow/Amber/Orange - medium concentration of reducing sugar
Red - high concentration of reducing sugar
Example of a non reducing sugar
Sucrose
How to test for non reducing sugars
1) Place sample in a test tube
2) Add Hydrochloric acid and heat in a water bath
3) Neutralise the solution with Sodium Hydrogencarbonate
4) Carry out Benedict’s test as usual
Why does Benedict’s reagent work for non reducing sugars after adding HCl?
HCl will hydrolyse (break down) glycosidic bonds to form monosaccharides
The resulting monosaccharides will have an aldehyde or ketone group that can donate electrons to Copper (II) Sulfate, forming a precipitate
What is Benedict’s reagent?
An alkaline solution of Cooper (II) Sulfate
How does the iodine test for starch work?
A few drops of iodine dissolved in potassium iodide solution is added to the sample
If the colour changes from orange to blue/black, starch is present
Why is iodine used in a potassium iodide solution?
Because iodine is insoluble in water
How do reagent strips work?
The strips contain chemicals in them and when the sample reacts with these chemicals, there is a colour change
The result is compared to a chart
Advantage of reagent strips
The concentration can be determined using a colour coded chart
How to use colorimetry to determine concentration of sugars
1) Filter was placed in the colorimeter
2) The colorimeter was calibrated using distilled water
3) Benedict’s test was carried out on samples
4) Solutions were filtered to remove precipitate
5) The % transmission of each glucose solution was measured using the colorimeter
6) These values were used to plot a calibration curve to determine unknown concentrations
How does a colorimeter work?
Measures transmission of light by a coloured solution
More concentrated solution = more light absorbed = less light transmitted
How do biosensors work?
Molecular recognition - a protein enzyme or antibody or single strand of DNA is immobilised to a surface, such as a glucose strip. This will bind to the specific molecule being investigated
Transduction - this interaction causes a change in a transducer, which will produce a response, such as a dye being released on a test strip
Display - this produces a visible, qualitative or quantitative result, like a colour on a test strip or reading on a test machine
Test to identify lipids
Sample is mixed with ethanol
Solution is mixed with water and shaken
If white emulsion is formed, a lipid is present
Separating amino acids using thin layer chromatography
Stationary phase - silica gel
Mobile phase - organic solvent
1) Draw a line on the plate to place the amino acids
2) Dot the amino acids onto the line using a capillary tube
3) The plate was placed in a jar with the solvent and closed
4) Plate was removed from the solvent and a pencil line was drawn along the solvent front
5) Plate was sprayed with ninhydrin spray so the colours would show up
Identifying proteins
1) Add Biuret’s reagent to the sample (this makes the solution blue)
2) If colour changes from blue to mauve/purple, then proteins are present
If colour remains the same, no proteins are present
Conditions for Biuret’s test to work
There must be at least 2 peptide bonds in any protein molecule, so with amino acids and dipeptides, the test will come up negative
What is Buiret’s reagent?
Copper (II) Sulfate solution and an alkali
Limitations of the Biuret’s test
It’s qualitative and does not show how much protein is in a sample
If the sample contains amino acids or dipeptides, the result will be negative
