Testing for Biological Molecules (CIE A Level Biology)

Students are presented with an unknown solution which they need to test for the presence of starch, reducing sugar and protein.

Outline the test for reducing sugar and describe the observation for a positive result.

[3]

(ii)

State how students could estimate the concentration of reducing sugar present in the solution.

[1]

Samples of the original solution are removed and iodine solution is added to one of the samples (sample A), while biuret reagent is added to the other sample (sample B). The results of these tests are found in Table 1.

Table 1

Discuss the results obtained for each sample.

Explain how the test for reducing sugar, outlined in part (a) (i), could be modified to test whether the solution contained sucrose.

After following the procedure outlined in part (c) the mass of precipitate that formed was heavier than when the test for reducing sugar was performed.

State the significance of these results.

The enzyme amylase breaks down starch to form the reducing sugar maltose. Iodine solution can be used to measure the progress of this enzyme-controlled reaction during a practical investigation.

Explain the role of iodine solution in measuring the rate of this enzyme-controlled reaction.

After all the starch was broken down into maltose by amylase, the enzyme maltase was added to the solution. A sample (sample 1) was taken from the solution immediately after the maltase was mixed with the solution. The remaining solution (sample 2) was incubated at 37 °C for an hour.

Describe a chemical test you could carry out to show that maltose is a reducing sugar.

The chemical test in part (b) was carried out on both samples 1 and 2. All experimental variables were kept the same when testing both of the samples. Both tubes were left for 15 minutes to allow the precipitate to settle.

State and explain how the result of sample 2 would differ from sample 1.

Explain why iodine solution would not be a suitable reagent to use when testing for biological molecules present in extracts from animal cells.

Triglycerides are the main form of dietary lipids ingested by humans. They are digested by the enzyme lipase in the stomach.

Describe a biochemical test that could be performed to determine whether a sample of food contains triglycerides.

Using your knowledge of the structure of triglycerides, explain why the solution goes cloudy when water is added during the biochemical test described at part (a).

Seeds are a good source of lipids. They also contain large amounts of starch that serve as a food source for the growing plant embryo during germination.

Outline the biological test that can be used to confirm the presence of starch and describe the observation for a positive result.

Scientists wanted to test the nutritional content of a new meal replacement shake under development. Table 1 shows some of the results that were gathered by the scientists.

Table 1

Complete the table by identifying the test reagents used in each case.

Describe the procedure the scientists would have followed to test for the presence of protein and state the meaning of the results that they obtained.

Discuss the results that were obtained by the scientists when they were testing for the presence of carbohydrates.

While testing for the presence of sucrose, the scientists accidentally left out the sodium hydroxide after adding the hydrochloric acid to the solution.

Explain how this could have affected their results.

The Benedict's test for reducing sugars gives a semi-quantitative result when used on samples of reducing sugar solution.

Explain the term semi-quantitative in this context. 

A solution with an unknown concentration of glucose is tested for reducing sugar.

Outline a method for determining the concentration of glucose in this solution. 

Table 1 below gives the experimental protocol for making a serial dilution of a glucose solution.

The concentration of the stock solution of glucose that is to be used is 20 mmol dm^-3.

Table 1

Calculate the missing values and complete Table 1 with your calculated values.

In a similar experiment to that outlined in part (c), a student wished to make up a solution of glucose of concentration 0.1 mmol dm ^-3 from a laboratory stock solution with a concentration of 100 mmol dm^-3.

The student evaluated two methods of producing this solution, methods A and B as shown below in Table 2.

Table 2

Select which method, A or B, would be the better method for the student to follow.

Give a reason for your choice.