Testing for Biological Molecules — Structured Questions (Cambridge International A-Level Biology): Exam Questions

A student wants to determine whether a sample of food contains lipids.

Describe the test the student should carry out and include the positive result.

Extract:

A student wants to determine whether a sample of food contains lipids.

The test for lipids is:

• Add ethanol to the sample
• Shake to mix the sample and the ethanol
• Add to a test tube of water
• A positive result is a milky emulsion

Describe how the student could determine the concentration of the lipid in the sample.

Extract:

A student wants to determine whether a sample of food contains lipids. Describe the test the student should carry out and include the positive result.

The test for lipids is:

• Add ethanol to the sample
• Shake to mix the sample and the ethanol
• Add to a test tube of water
• A positive result is a milky emulsion

State one health and safety precaution the student should take when using the reagent to test for a lipid.

Fig. 1 shows the structure of a lipid, also known as a triglyceride.

Fig. 1

Lipids cannot cross the cell-surface membranes of cells lining the gut during the absorption of nutrients.

Suggest why it cannot cross cell-surface membranes.

Iodine in potassium iodide is used to test the presence of starch.

Describe what you would expect to observe in a positive result.

Starch is a carbohydrate often stored in plant cells.

Fig. 1 shows a starch grain in a section of plant tissue at a magnification of ×500.

Fig. 1

The size of the diameter of the starch grain on the image is 18 mm.

Calculate the actual diameter of the starch grain between points A and B. Give your answer in micrometers.

Extract:

Starch is a carbohydrate often stored in plant cells.

Fig. 1 shows a starch grain in a section of plant tissue at a magnification of ×500.

State the type of microscope that was used to obtain the image shown in Fig. 1 and explain the evidence for this.

Extract:

Iodine in potassium iodide is used to test the presence of starch.

A different reagent is used to test for the presence of proteins.

Give the name of this reagent.

Describe the test for non-reducing sugars.

Give the name of a non-reducing sugar.

The presence of other molecules can also be tested for with a range of biochemical reagents.

Complete Table 1 to show which reagents are used for which molecule.

Table 1

Some of the biological molecules identified in Table 1 can be described as polymers.

Define the term polymer.

Complete the following sentences.

The presence of protein may be tested for using __________ reagent. In the presence of proteins the reagent will turn from __________ to __________.

The reagent reacts with _________ bonds present in the protein.

Fig. 1 shows two amino acids.

Fig. 1

Draw the chemical bond that forms when these two amino acids are joined during a condensation reaction. You need only draw the parts of the molecules shown in the box.

The chemical test for a protein is described as a qualitative test.

Explain why this is the case.

Extract:

The presence of protein may be tested for using biuret reagent. The reagent reacts with peptide bonds present in the protein.

A student was using the reagent, named in part (a), to test for the presence of an amylase enzyme. The student accidentally heated the reaction solution, containing amylase, as part of their experiment and then tested the reaction solution using the reagent for proteins.

Suggest why the error in their method may cause an inaccurate result.

Benedict's reagent can be used to test for the presence of reducing and non-reducing sugars.

Outline the method for test for a reducing sugar.

The Benedict's test for reducing sugar is often referred to as a semi-quantitative test.

(i) Suggest what is meant by a semi-quantitative test.

(ii) Describe the colour change in Benedict's solution you would expect to see as the concentration of the test substance increases.

Extract:

Benedict's reagent can be used to test for the presence of reducing and non-reducing sugars.

The qualitative test for reducing sugar is:

• Add Benedict's reagent to the sample (to be tested) in a test tube
• Heat/boil the sample
• A positive result will show an orange/brown/red coloured precipitate form

The Benedict's test for reducing sugar is often referred to as a semi-quantitative test.

(i) A semi-quantitative Benedict's test means the results do not give a precise value BUT they show a relative concentration / indicate low and high concentration / allow concentration to be estimated.

(ii) The colour change you can expect to see is from blue to green to yellow to orange/brown to (brick) red.

Fig. 1 shows the colour change scale of five known concentrations of reducing sugar solutions.

Fig. 1

Estimate the concentration of the unknown sample.

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

(i) Outline the test for reducing sugar.

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

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

Table 1

Discuss the results obtained for each sample.

Extract:

(i) The test for reducing sugar involves the following steps...

• Add Benedict's solution/reagent; [1 mark]
• Boil / heat to 95 °C; [1 mark]
• For a positive result, a precipitate will form / colour change from blue to green/yellow/orange/red/brown; [1 mark]

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

Extract:

(i) The test for reducing sugar involves the following steps...

• Add Benedict's solution/reagent;
• Boil / heat to 95 °C;
• For a positive result, a precipitate will form / colour change from blue to green/yellow/orange/red/brown;

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

The test for reducing sugar can be modified as follows to test for non-reducing sugar...

• Heat a sample of the unknown solution with hydrochloric acid;
• Neutralise the acid / add an alkali/sodium hydroxide;
• Repeat the Benedict's test / mix with Benedict's solution and heat to a high temperature;

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 how iodine solution can be used to measure 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.

Extract:

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.

Extract:

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.

The biochemical test for triglycerides is...

• The (ethanol) emulsion test
• Mix the food sample with ethanol/alcohol
• Then add water / pour into water
• (If lipids are present) a white/milky emulsion forms

A positive result is that the clear solution will turn milky when the water is added.

Using your knowledge of the structure of triglycerides, explain the visual result of a positive test for the presence of lipids.

Seeds are a good source of lipids. They also contain starch which 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.

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.

Extract:

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

| Biological molecule tested | Test reagent(s) used | End results observed | | Glucose | (i) | Brick-red precipitate forms | | Sucrose | (ii) | Blue colour | | Protein | (iii) | Blue colour | | Lipids | (iv) | Cloudy suspension forms |

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.

Extract:

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

| Biological molecule tested | Test reagent(s) used | End results observed | | Glucose | (i) | Brick-red precipitate forms | | Sucrose | (ii) | Blue colour |

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

Extract:

Table 1

| Biological molecule tested | Test reagent(s) used | End results observed | | Glucose | (i) | Brick-red precipitate forms | | Sucrose | (ii) | Blue colour | | Protein | (iii) | Blue colour | | Lipids | (iv) | Cloudy suspension forms |

The test reagents used for each biological molecule were as follows...

• (i) Benedict's AND (ii) hydrochloric acid with Benedict's; [1 mark]
• (iii) Biuret AND (iv) ethanol/alcohol; [1 mark]

To get full marks for this question you must correctly identify all the reagents for four biological molecules tested. Take note that the test for non-reducing sugar requires the addition of hydrochloric acid first before Benedict's reagent is used. This is to hydrolyse the glycosidic bond between the two monosaccharides making up sucrose.

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 procedure to that outlined in part (c), a student wished to make up a solution of glucose of 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.

A group of students wanted to test a food sample to measure the concentration of reducing sugars.

They began their investigation by testing a series of samples of known glucose concentration using a colorimeter. Their results are shown in Table 1 below.

Table 1

Explain why the students began their investigation in this way.

Extract:

A group of students wanted to test a food sample to measure the concentration of reducing sugars.

They began their investigation by testing a series of samples of known glucose concentration using a colorimeter. Their results are shown in Table 1 below.

Table 1

| Glucose concentration / mmol dm^-3^ | % absorbance | | 1.0 | 69 | | 2.0 | 58 | | 3.0 | 46 | | 4.0 | 39 | | 5.0 | 25 | | 6.0 | 17 |

Calculate the percentage change in absorbance as the glucose concentration increases from 1.0 mmol dm^-3 to 6.0 mmol dm^-3. Give your answer to three significant figures.

Extract:

A group of students wanted to test a food sample to measure the concentration of reducing sugars.

They began their investigation by testing a series of samples of known glucose concentration using a colorimeter. Their results are shown in Table 1 below.

Table 1

| Glucose concentration / mmol dm^-3^ | % absorbance | | 1.0 | 69 | | 2.0 | 58 | | 3.0 | 46 | | 4.0 | 39 | | 5.0 | 25 | | 6.0 | 17 |

Use the data in Table 1 to plot a graph of glucose concentration against absorbance.

Extract:

A group of students wanted to test a food sample to measure the concentration of reducing sugars.

They began their investigation by testing a series of samples of known glucose concentration using a colorimeter. Their results are shown in Table 1 below.

Table 1

| Glucose concentration / mmol dm^-3^ | % absorbance | | 1.0 | 69 | | 2.0 | 58 | | 3.0 | 46 | | 4.0 | 39 | | 5.0 | 25 | | 6.0 | 17 |

Describe three measures that the students should take to ensure that their results are valid.

The effect of storage on the sugar content of winter squash fruits was studied. The results are shown in Fig. 1 below. Note that the error bars show standard deviation.

Fig. 1

Suggest how the squash may have been tested to assess sugar content.

Extract:

The effect of storage on the sugar content of winter squash fruits was studied. The results are shown in Fig. 1 below. Note that the error bars show standard deviation.

Fig. 1 shows changes in the levels of sucrose, glucose and fructose in fruit after 25, 50 and 75 days of storage.

Describe the effect of storage time on squash sugar content shown in Fig. 1.

Extract:

The effect of storage time on squash sugar content shown in Fig. 1:

• There is an overall decrease in sugar content as storage time increases
• The mass of sucrose decreases significantly between day 25 and day 50 and then increases (slightly) between day 50 and day 75
• The change in sucrose levels is greater than the change in levels of glucose and fructose
• There is significantly more sucrose present than glucose and fructose at all times during storage
• There is significantly more glucose than fructose present at day 25 and only slightly more glucose / similar levels of glucose and fructose present at day 50 and day 75

Suggest an explanation for the results observed in part (b).

Extract:

The effect of storage on the sugar content of winter squash fruits was studied. The results are shown in Fig. 1 below. Note that the error bars show standard deviation.

Fig. 1

Fig. 2 shows another set of data gathered during the investigation into the effect of storage time on squash biochemistry.

Fig. 2

The researchers concluded from this data that winter squash would become more appealing to consumers over time.

Discuss this conclusion.