Biological Molecules: Carbohydrates (AQA A Level Biology): Exam Questions

Name the type of reaction that forms polysaccharides from their monomers.

Explain how two features of starch allow it to act as a storage substance.

A muscle cell was tested to detect the presence of starch.

State and explain the result of this test.

Sucrose is formed from monomers X and Y. Figure 1 shows the structure of sucrose and monomer Y.

Figure 1

Draw monomer X.

Figure 2 shows a column containing inert beads. The enzyme sucrase is fixed to these inert beads. A student pours a sugar solution containing sucrose, lactose and glucose into the top of the column.

Figure 2

Name the compound(s) you expect to be present in the products.

Describe how the student could separate out the compounds in the ‘product’ solution.

Within animal cells fructose can be converted into glucose. 

Describe how glucose is stored within animal cells.

Define the term disaccharide.

Maltose is a disaccharide which can be broken down by the enzyme maltase.

Maltose + water        + glucose

The formula for glucose is C₆H₁₂O₆.

State the formula for maltose.

A solution of the enzyme maltase was added to a tube containing maltose solution. 

• Sample 1 was taken from the tube immediately after the solutions were mixed

• The remaining solution was then incubated at 37 °C for an hour before sample 2 was removed

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

The chemical test in part (c) was carried out on both samples 1 and 2. Both tubes were then left for 15 minutes to allow the precipitate to settle.

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

Name the monomers from which a lactose molecule is made.

Describe what happens when lactose is broken down into its monomers.

A teacher produced a dilution series of a lactose solution so that they could plot a calibration curve. They started with a stock solution of lactose of concentration 0.75 mol dm⁻³ and distilled water, with which they made a series of dilutions from 0.1 to 0.5 mol dm⁻³.

Complete Table 1 by identifying the headings, units and the concentration of the lactose solution.

  Table 1

The teacher performed a Benedict's test on the five lactose dilutions ranging from 0.1 mol dm⁻³ to 0.5 mol dm⁻³. A sample of each solution was placed in a colorimeter and the light absorbance was measured. Table 2 below shows the light absorbance for each sample.

Table 2

Use the data from Table 2 to sketch a graph and predict the concentration of lactose that has a light absorbance of 0.4 arbitrary units.

Figure 1 shows a section of a cellulose molecule.

Name parts J and K.

State the role of cellulose in plant cell walls and explain how its structure is related to this role.

Plants store sugars as starch.

Describe how the presence of starch can be detected in plant cells. 

Compare and contrast the structures of cellulose and starch.

Figure 1 shows the structures of two monosaccharides; alpha glucose and fructose. These combine to form the disaccharide sucrose.

Figure 1

Draw the structure of sucrose:

• Label the bond between the two monosaccharides

• Show any by-products formed when the monosaccharides combine 

Sucrose can be hydrolysed with the enzyme invertase. In an experiment to measure the effect of temperature on human invertase activity, researchers treated solutions of sucrose with invertase solution and measured glucose production over a range of temperatures; 4°C, 25°C, 37°C, 80°C and 100°C.

Measurement involves treating the experimental mixture with a dye and measuring its absorbance of visible light. The greater the absorbance, the higher the invertase activity observed. 

Suggest four variables that the researchers would have to control, and state how each variable would be controlled.  

Use your knowledge of enzyme activity to predict the results that the researchers would have obtained. Complete the table below, using a system of zeros and crosses as follows: 

• 0 = no activity

• + = minimal activity

• ++ = moderate activity

• +++ = maximal activity

Explain your predictions in part c.

Glycogen is a storage carbohydrate.

Explain three features of glycogen that make it well suited to its role as an energy store. 

Starch is another storage carbohydrate.

Describe the structure of starch

Soluble forms of starch are used in laboratory experiments, despite the fact that starch is known as an insoluble molecule.

Suggest and explain how starch might be altered for use as a soluble laboratory reagent.

Participants and coaches involved in competitive sport have taken part in research into the optimum formulation of sports drinks. One such study is set out below.

Nine male cyclists took part in a study in which they fasted for 12 hours and were then given either water, glucose solution or a 2:1 glucose:fructose drink immediately prior to competing in a 10-mile time-trial race. The results are shown in Table 1

Table 1

Calculate the percentage improvement in average finishing time for cyclists consuming the  2:1 glucose:fructose mixture, in comparison to those who consumed glucose solution on its own.

(i) Suggest explanations for the results in Table 1

[2]

(ii) Criticise the scientific methods used in the study

[2]

Figure 1 below shows the structure of fructose.

Figure 1

Compare and contrast the structures of glucose and fructose.

Scientists may need to break down di- or polysaccharides in order to study carbohydrates in the laboratory.

Briefly describe one experimental methods by which the glycosidic bond in a disaccharide can be broken. 

A biologist set up partially permeable dialysis tubing as shown in Figure 1.

Figure 1

After leaving the experiment for 20 minutes, the biologist removed samples of liquid from inside the dialysis tubing and also from the water surrounding the tubing. The samples were added to several different reagents/indicators.

The blank results table used to record their results is shown below.

Table 1

Predict the observations that the biologist would have made. Do not make reference to positive or negative tests. 

Explain the predictions that you made in part b.

Starch is stored as granules inside plant cells, e.g. in potato tubers or carrot roots.

Explain the advantages of storing carbohydrates as starch, rather than as sugars, in plant cells.