LGS A-Level OCR Biology - Unit 2 - Testing for Biological Molecules

Tests for the presence of starch

Add iodine dissolved in potassium iodide solution to test sample

If starch is present the colour will change from brown/orange to dark blue/black

If no starch is present, the colour stays brown/orange

Test for the presence of protein

The test solution needs to be alkaline so first step is to add few drops of sodium hydroxide solution

Next add the copper (ii) sulphate solution

If protein is present (positive result) the solution turns pale purple

If no protein is present (negative result) then the solution stays pale blue

Iodine

When dissolved in potassium iodide the iodine forms a tri-iodide ion, which slips into the middle of the amylose helix.

Peptide bonds in proteins

Colour is formed by a complex between the nitrogen atoms in a peptide chain and Cu2+ ions

Tests for the presence of lipids

Shake test sample with ethanol for about 1 minute then pour solution into water

If lipid is present, solution turns milky

The more lipid present, the more noticeable the milky colour

If there is no lipid present the solution stays clear

Benedict’s solution

Add Benedicts reagent (blue) to sample

Heat it in water bath that’s been brought to boil

Watch for colour change

Colour of precipitate changes from: blue - green - yellow - orange - brick red

If positive result, forms coloured precipitate

The higher the concentration of reducing sugar, further the colour change goes (used to compare amount of reducing sugars in different solutions)

A sugar which can give electrons (reduce) to other molecules

Reducing sugars include all monosaccharides and some disaccharides

The Cu2+ in benidict solution reduces into Cu+ making copper oxide giving that orange colour

Commercially manufactured test strips

Glucose strips like for testing for diabetes etc

Biosensors use biological components to determine the presence and concentration of molecules

Glucose blood strips are an example

Blood is the alayte which is placed onto the test strip,

Molecular recognition uses glucose oxidase immobilised upon the surface of the strip

Glucose in the blood reacts with a glucose oxidase producing gluconic acid causing a change in the tranducer (a change in the current between the terminals of a glucose monitor)

Data is processed by the glucose monitor to give a reading dependent upon the current

Useful to test a person’s urine for reducing sugars which could indicate if person has diabete

Also can help a diabetic person to manage their bloods

Glucose tested using test strips coated in reagent (Benedict’s solution)

Strips dipped in test solution and change colour if glucose (reducing sugar) is present

Colour change is compared to chart to give indication of the concentration of glucose present

N.B this test only shows presence of a reducing sugar, which could be glucose, this is why a blood test is then done to determine if this is glucose

Filter the solution and weigh the precipitate

Benedict’s test

Boiled with hydrochloric acid to hydrolyse the bonds and free up the reducing sugar group e.g. Sucrose split into glucose and fructose

Sodium hydrogen carbonate

Positive result, forms coloured precipitate.

Colour changes:

Blue - Green - Yellow - Orange - Red

Negative result, stays blue so no sugar present

A colorimeter is a piece of equipement that quantitatively measures the absorbance, or transmission, of light be a coloured solution.

The more concentrated a solution is the more light it will absorb and the less light it will transmit.