Quantitative Tests for Biological Molecules

Question 1

Reducing sugars:

Answer 1

Benedict’s solution is used to test for reducing sugars, the more reducing sugar in the sample, the more precipitate will form and there will be less Cu2+ ions. The amount of precipitate is assessed with colorimetry.

Question 2

Colorimetry:

Answer 2

A centrifuge is used to separate the precipitate and any excess Benedict’s (the supernatant). The sample is placed in a cuvette using a pipette, which is placed into the colorimeter. Ensure there are no fingerprints in the cuvette as that will affect light transmission. The more precipitate the less light is transmitted through the colorimeter. Between readings the device is zeroed by placing an appropriate blank sample to reset the 100% transmission/absorption (i.e. with water).

Question 3

Calibration curves are used to find exact amounts of reducing sugar in the sample:

Answer 3

Calibration curves are used to find exact amounts of reducing sugar in the sample.
1. First take a series of known concentrations of sugar.
2. Use a sample from each, carry out the benedict’s test.
3. Use the colorimeter to record the percentage transmission of light through each sample.
4. Plot a graph of absorbance against concentration of sample and plot the point on it, this curve can
be used to calculate the conc of a sample when the absorbance is measured with the colorimeter.