Biological Molecules: Carbohydrates (AQA A Level Biology)

Exam Questions

2 hours15 questions
1a2 marks

Name the monomer present in

(i)
Cellulose

(ii)
Starch
1b2 marks

Figure 1 shows a molecule of alpha glucose. Beta glucose is an isomer of alpha glucose.

Figure 1

screenshot-2023-05-19-104140

(i)
Define the term isomer.

(ii)
Draw a molecule of beta glucose.
1c2 marks

A disaccharide is formed by a condensation reaction between two monosaccharides.
Complete the word equation below and state the bond present in the disaccharide.

Alpha glucose + ____________ ⟶ Sucrose + _____________

1d2 marks

Cotton is a plant fibre used to make cloth. Explain how cellulose gives cotton it strength.

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2a2 marks

The equation shows the reaction catalysed by the enzyme maltase. State the name of the molecules A and B

Maltose    +  A    →   Glucose +  B

2b1 mark

Name the type of chemical reaction shown in the equation in part (a)

2c1 mark

A laboratory assistant added Benedict’s reagent to a solution of sucrose in a test tub and applied heat. Predict and explain the results.

2d2 marks

List two similarities and two differences between the structures of starch and glycogen.

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3a1 mark

Figure 1 shows a cell from a garden pea.

Figure 1

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Describe how you could determine if the granules contained starch.
3b1 mark

Name one polysaccharide other than starch found in a garden pea cell.

3c3 marks

Glucose is converted into starch for storage within plant cells, describe the reaction that takes place for this to occur. 

3d1 mark

Explain one reason why pea cells use starch as a storage molecule rather than glucose.  

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4a2 marks

The glycosidic bond is broken in a hydrolysis reaction. Disaccharides and polysaccharides are broken down in hydrolysis reactions. Figure 1 shows a molecule of maltose.

Figure 1

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Draw the reaction that occurs when a single molecule of maltose is broken down via a hydrolysis reaction.

4b1 mark

State the chemical formula of maltose.

4c3 marks

Describe how the student could separate a mixture of monosaccharides in solution.

4d2 marks

Figure 2 shows a chromatogram with an unknown mixture of monosaccharides.
Suggest how these monosaccharides could be identified.

Figure 2

3

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5a1 mark

Serial dilutions are created by taking a series of dilutions of a stock solution. The concentration decreases by the same quantity between each test tube. They can either be ‘doubling dilutions’ (where the concentration is halved between each test tube) or a
desired range (e.g. 0, 2, 4, 6, 8, 10 mmol dm-3).

Table 1

Glucose concentration / mmol dm-3

Volume of distilled H2O / cm3

Volume of glucose stock solution / cm3

0

5

 

2

4

 

4

3

 

6

2

 

8

1

 

10

0

 


Fill in
Table 1.

5b1 mark

Benedict’s solution can be used to carry out a semi-quantitative test on a reducing sugarsolution to determine the unknown concentration of reducing sugar present in a Sample. The intensity of any colour change seen relates to the concentration of reducing sugar present in the sample. Using serial dilution, standard solutions with know concentrations of a reducing sugar (such as glucose) are produced.

Each solution is then treated in the same way: the same volume of Benedict’s solution is added to each sample and heat is applied. Any colour change observed for each solution of a known concentration in given time can be attributed to the concentration of reducing sugar present in that solution

The same procedure is carried out on a sample with an unknown concentration of reducing sugar which is then compared to the stock solution colours to estimate the concentration of reducing sugar present

The colour change intensity can be compared between samples by eye or by using a colorimeter. Describe the relationship between colour change intensity and glucose concentration.

5c1 mark

Suggest why using a colorimeter to measure light absorbance would be favourable over a comparison done by eye.

5d3 marks
Table 2 contains statements that could apply to three polysaccharides. 
Complete the table with a tick (✓) in each box if the statement correctly applies.
 

Table 2

Statement

Glycogen

Cellulose

Starch

Contains 1-6 links

     

Contains α-glucose

     

Contains hydrogen bonds

     

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1a3 marks

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

Statement

Glycogen

Cellulose

Starch

Contains 1-6 links

     

Contains α-glucose

     

Contains hydrogen bonds

     
1b1 mark

Name the type of reaction that forms the carbohydrates in Table 1 from their monomers.

1c4 marks

Give two features of starch and explain how those features allow it to act as a storage substance.

1d2 marks

A muscle cell was tested to detect the presence of starch. State and explain the result of this test.

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2a1 mark
Sucrose is formed from monomers X and YFigure 1 shows the structure of sucrose and monomer Y.

Figure 1

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Draw monomer X.
2b2 marks

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

q2b

Name the compounds you expect to be present in the ‘Products’?

2c3 marks

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

2d4 marks

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

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3a2 marks

What is a disaccharide?

3b1 mark

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

Maltose + water    rightwards arrow with maltase on top   _________ + glucose

The formula for glucose is C6H12O6. What is the formula for maltose?
3c2 marks

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.

3d3 marks
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.

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4a2 marks
Name the monomers from which a lactose molecule is made.
4b3 marks
Describe what happens when lactose is broken down into its monomers.
4c2 marks

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

Concentration of lactose solution

/ ____________

Volume of 0.75 mol dm−3 lactose solution / cm3

_______________

/ ____________

____________

8

12

4d4 marks

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

Concentration of lactose

/ units

Light Absorbance

/ a.u.

0.1

0.05

0.2

0.21

0.3

0.25

0.4

0.48

0.5

0.74


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.

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5a2 marks

Figure 1 shows a section of a cellulose molecule.

screenshot-2023-05-20-112140

Name parts J and K.

5b3 marks

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

5c2 marks

Plants store sugars as starch. How can the presence of starch be detected in plant cells? 

5d4 marks

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

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1a4 marks

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

screenshot-2023-05-18-122236

1b4 marks

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.  

1c3 marks

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

Incubation temperature / ℃

Enzyme activity / arbitrary units

4

 

25

 

37

 

80

 

100

 
1d4 marks

Explain your predictions in part 1 (c)

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2a6 marks

Glycogen is a very effective storage carbohydrate. Describe and explain three features of glycogen that make it well suited to its role as an energy store. 

2b4 marks

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. 

2c2 marks

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

2d2 marks

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. 

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3a2 marks

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. 

3b2 marks

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

screenshot-2023-05-18-193442

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

3c2 marks

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.
3d2 marks

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. 

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4a2 marks

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

Drink

Total mass of monosaccharide ingested / g

Completion time of trial / 

minutes:seconds

Water

0

25:00

Glucose solution

30

23:32

2:1 glucose:fructose mixture

30

20:30

Calculate the percentage improvement in finishing time from using the  2:1 glucose:fructose mixture, to using glucose solution on its own.

4b4 marks
(i)
Explain the results in Table 1

(ii)
Criticise the scientific methods used in the study
4c2 marks

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

4d2 marks

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.

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5a4 marks

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

5b3 marks

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

Figure 1

screenshot-2023-05-18-195500

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

Biochemical test

Liquid from dialysis tubing

Liquid from the beaker

Biuret reagent

   

Iodine solution

   

Benedict’s solution

   


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

5c3 marks

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

5d3 marks

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.

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