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Water and Its Importance in Plants and Animals

Detecting and measuring sugars and proteins

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on the Benedict's test for reducing sugars and making it quantitative, the Biuret test for protein, choosing a colorimeter filter, blanks and calibration curves, glucose test strips with glucose oxidase, and how a glucose biosensor works.

What you need to know

Answer each one before you reveal it.

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Key term

Describe how the Benedict's test can be made quantitative using a colorimeter.

Key term

How is a colorimeter used to measure the concentration of protein in a sample after the Biuret test?

Key term

How does a reagent test strip detect glucose in urine?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe how the Benedict's test for reducing sugars is carried out.

    Model answer

    An equal volume of Benedict's reagent is added to the sample in a test tube and heated in a boiling water bath for a few minutes; a coloured precipitate forms if a reducing sugar is present.

  2. 2

    What colour changes are seen in a positive Benedict's test, and what do they indicate?

    Model answer

    The blue reagent turns green, then yellow, then orange and finally brick red as the concentration of reducing sugar increases, because increasing amounts of the blue copper(II) ions are reduced to a red precipitate of copper(I) oxide.

  3. 3

    Name two reducing sugars that would give a positive Benedict's test.

    Model answer

    Glucose and lactose.

  4. 4

    Why is the Benedict's test on its own only semi-quantitative?

    Model answer

    It gives a colour that has to be judged by eye against a range, so it places the concentration in a broad band rather than giving a precise value.

  5. 5

    Describe how the Benedict's test can be made quantitative using a colorimeter.

    Model answer

    The tubes are heated for a fixed time, the copper(I) oxide precipitate is filtered or centrifuged out, and the absorbance of the remaining blue solution is measured with a red filter; the more reducing sugar there was, the less blue reagent remains, so absorbance is read against a calibration curve made from standard solutions.

  6. 6

    Describe how the Biuret test for protein is carried out and the result of a positive test.

    Model answer

    Sodium hydroxide solution is added to the sample followed by a few drops of dilute copper(II) sulfate solution, with no heating; the mixture turns purple or lilac if protein is present and stays blue if it is not.

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