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

Table 1 contains statements that could apply to three polysaccharides. Complete the table with a tick (✓) in each box if the statement correctly applies.

Table 1

Name the type of reaction that forms the carbohydrates in Table 1 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 compounds you expect to be present in the ‘Products’?

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

Within animal cells, fructose enters the gluconeogenesis pathway to form glucose. Describe how glucose is stored within the cell.

Define a 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. A sample (1) was taken from the tube after the solutions were mixed. The remaining solution was then incubated at 37°C. Sample 2 was removed after one hour of incubation. 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. 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.

Name the monomers from which a lactose molecule is made.

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

A teacher produces a dilution series of a lactose solution so they could plot a calibration curve. The teacher starts with a stock solution of lactose of concentration 0.75 mol dm⁻³ and distilled water with which they make a series dilutions from 0.1 to 0.5 mol dm⁻³. Complete Table 1 by giving all headings, units and the concentration of the lactose solution produced.

  Table 1

The teacher performed 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. 

Describe two similarities and two differences between the structures of cellulose and starch.

Figure 1 shows the structures of two monosaccharides, ∝-glucose and fructose. These combine to form the disaccharide sucrose. In the space below, draw the structure of sucrose. Label the bond between the two monosaccharides and show any by-products. 

Figure 1

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 the quantity of glucose produced at various temperatures; 4°C, 25°C, 37°C, 80°C and 100°C. The 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 Table 1 below, using a system of zeros and crosses as follows: 

0 = no activity

+ = minimal activity

++ = moderate activity

+++ = maximal activity

Table 1

Explain your predictions in part 1 (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 made of two polymers of ∝-glucose: amylose and amylopectin. Describe the structural differences between amylose and amylopectin and explain their effects on the starch molecule. 

Outline one structural similarity and one structural difference between amylopectin and glycogen. 

Soluble forms of starch are used in laboratory experiments, despite the fact that starch is well documented as an insoluble molecule. Suggest how starch is altered for use as a soluble laboratory reagent. 

Cellulose is thought to be the world’s most abundant carbohydrate. Humans and other large mammals do not possess the enzymes to extract the energy contained in cellulose. Use your knowledge of cellulose to explain why the biosphere is not overwhelmed with an accumulation of cellulose waste from dead plant matter. 

A section of a cellulose molecule is shown in Figure 1.

Describe and explain the feature of this structure that allows cellulose to form straight, strong fibres in biological tissues. 

The koala (Phascolarctos cinereus) is an herbivore that inhabits the forests of eastern Australia. The leaves of eucalyptus trees form the bulk of the koala’s diet. The leaves have a low calorific content. Use your knowledge of carbohydrates to explain the following two aspects of koala behaviour:

• Koalas are largely sedentary, sleeping up to 20 hours per day;

• Infant koalas ingest their mother’s faeces early in life.

Ruminants and other herbivores produce large volumes of methane which gets released into the atmosphere. Studies have shown that a cow produces 75 times more methane per kg of body mass than a human. Suggest why ruminants produce more methane per kg of body mass than humans. 

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.

Eight trained, competitive 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 beverage 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 finishing time from using the  2:1 glucose:fructose mixture, to using glucose solution on its own.

(i) Explain the results in Table 1

(2)

(ii) Criticise the scientific methods used in the study

(2)

Give one similarity and one difference between the structures of glucose and fructose.

High-fructose corn syrup (HFCS) is a widely used material in the food industry. Fructose is discerned as sweeter than glucose by human taste buds; this gives food manufacturers the opportunity to maintain food’s sweetness by using less carbohydrate material. Fructose is not metabolised in all cells (as glucose is) but is metabolised in the liver to form glycogen and can be subsequently converted to fat. Health campaigners have claimed that increased use of HFCS promotes obesity. Suggest why humans have a mechanism for metabolising fructose.

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

A biologist set up the apparatus as shown in Figure 1.

Figure 1

Dialysis tubing is partially permeable. 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). 

Table 1

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

Justify the predictions that you have made in question 5 (c)

Starch is stored as granules inside plant cells such as potato tubers or carrot roots.  Explain the advantages of storing food as starch, rather than as sugars, in plant cells.