Skip to content
MarkSchemeMarkScheme
Sign up — no card

9700 · 2.1

Testing for biological molecules — practice questions

Practice and worked examples for 9700 Testing for biological molecules. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A student tested an unknown food sample.

  1. A portion of the sample was heated with Benedict's solution and turned blue.
  2. Another portion was boiled with dilute HCl, neutralised, then heated with Benedict's solution, and turned brick-red.
  3. A third portion, shaken with ethanol and then added to water, formed a milky white emulsion.
  4. A fourth portion, treated with potassium hydroxide and then copper(II) sulfate solution, remained blue. What biological molecules are present in the food sample? Justify your answer with respect to each test.
Show solution outline

Solution:

  • Reducing sugars: Absent. Test 1 shows the Benedict's solution remained blue, indicating no reducing sugars were present.
  • Non-reducing sugars: Present. Test 2 shows that after hydrolysis and neutralisation, the Benedict's test was positive (brick-red precipitate). This means non-reducing sugars were broken down into reducing sugars, which then reacted.
  • Lipids: Present. Test 3 resulted in a milky white emulsion after adding ethanol and then water, which is a positive result for lipids.
  • Proteins: Absent. Test 4 showed no colour change (remained blue) in the Biuret test, indicating the absence of proteins.

Worked example 2

A biologist prepared a series of standard glucose solutions of known concentrations. They performed the Benedict's test on 2 cm³ of each solution and measured the absorbance of the resulting supernatant using a colorimeter with a red filter. They also tested an unknown fruit juice sample. The results are shown in the table below.

Glucose Concentration (mg cm⁻³)Absorbance (A.U.)
0.00.95
------
2.00.75
4.00.54
6.00.36
8.00.18
Fruit Juice0.45

Plot a calibration curve and determine the concentration of glucose in the fruit juice.

Show solution outline

Solution:

  1. Plot the calibration curve: Plot Absorbance (y-axis) against Glucose Concentration (x-axis) on graph paper. The points should form a downward-sloping line.
  2. Draw a line of best fit: Draw a smooth, straight line of best fit through the plotted points for the standard solutions. The line should pass as close as possible to all points.
  3. Find the unknown concentration: Locate the absorbance of the fruit juice (0.45) on the y-axis.
  4. Extrapolate to the x-axis: Draw a horizontal line from 0.45 on the y-axis across to the line of best fit. From the point of intersection, draw a vertical line down to the x-axis.
  5. Read the concentration: Read the value on the x-axis where the vertical line intersects. This value is the glucose concentration in the fruit juice.
  6. Final Answer: Following these steps on a plotted graph, an absorbance of 0.45 corresponds to a glucose concentration of approximately 5.0 mg cm⁻³.
Mark a 9700 question on this topicFull lessonOpen in course studio