Biological Molecules: Proteins (AQA AS Biology)

Exam Questions

2 hours15 questions
1a2 marks

Complete Table 1 to indicate which of the substances shown in Table 1 are proteins (or contains protein). 

Table 1

Substance

Protein / Not Protein 

Starch granules

 

Hormones 

 

Stomach enzymes 

 

Glycogen

 

Muscle tissue

 

Haemoglobin 

 
1b1 mark

Figure 1 shows the structure of an amino acid. Identify the group labelled X in

Figure 1

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1c1 mark

Explain the meaning of the term ‘R’ in Figure 1.

1d1 mark

Figure 2 shows a dipeptide.

Figure 2

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Outline the peptide bond in the dipeptide shown in Figure 2.

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2a2 marks

Chromatography is a technique that can be used to separate a mixture into individual components on the basis of their solubility in a particular solvent. A small amount of the mixture to be separated is dropped onto a piece of chromatography paper at a position marked as the baseline, before the paper is suspended in a solvent.  The solvent travels up the paper and the components appear as dots on the paper at the point at which they come out of solution. 

Figure 1 shows the results obtained when a mixture of two amino acids is separated using chromatography. 

Figure 1

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One way of identifying the components of a mixture is to calculate a value known as the Rf value. Use the equation provided below to calculate the Rf value of alanine in
Figure 1

                               Rf = fraction numerator D i s tan c e space m o v e d space b y space t h e space a m i n o space a c i d s over denominator D i s tan c e space b e t w e e n space t h e space b a s e l i n e space a n d space t h e space s o l v e n t space f r o n t end fraction

2b2 marks

Table 1 shows the properties of amino acids that determine their Rf values for a particular solvent and chromatography medium.

Table 1

Property

Predicted Rf value (High OR Low)

High solubility

 

Small molecule

 

Using the terms ‘High’ or ‘Low’, suggest predictions for the Rf values of different amino acid properties to complete Table 1

2c1 mark

Suggest how a scientist might use the results of a chromatography experiment to identify an unknown amino acid. 

2d1 mark

Suggest one branch of science where chromatography techniques, like the one used to separate a mixture of amino acids, have a practical application.

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3a3 marks

Table 1 shows selected aspects of protein structure and the level of protein structure to which they each belong. 

Table 1

Aspect of protein structure

Level of structure /

primary, secondary, tertiary, quaternary

α-helix

secondary

Disulfide bridge

 

Sequence of amino acids

 

?-pleated sheet

 

Separate polypeptide chains interacting

 

Hydrogen bonds 

 

Ionic bonds

 

Complete Table 1 using the terms, primary, secondary, tertiary and/or quaternary. The first row has been completed for you. 

3b2 marks

Two types of protein are shown in Figure 1. They are labelled Protein A and Protein B.

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Identify which of A or B is a globular protein and give a reason for your choice.

3c1 mark

The Biuret test is a biochemical test for proteins. State the colour change observed in a positive Biuret test. 

3d2 marks

The Biuret test is a qualitative test for proteins. Explain why the Biuret test is referred to as ‘qualitative’. 

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4a1 mark

The amino acid serine has the structure shown in Figure 1.

Figure 1

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Draw a box or circle around the R group of serine in Figure 1.

4b1 mark

Serine, shown in Figure 1, is able to form hydrogen bonds with other amino acids. Suggest what this indicates about the polarity of the R group

4c2 marks

The amino acid phenylalanine is non-polar. Its structure is shown in Figure 2. State whether phenylalanine is more likely to be found at the centre of a globular protein or at its surface. Give a reason for your answer. 

Figure 2

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4d2 marks

Table 1 shows some different types of chemical bond that form in proteins.

Table 1

Bond type

Bond strength /

 1 = strongest, 3 = weakest

Hydrogen bonds 

 

Ionic bonds

 

Disulfide bonds

 

Complete Table 1 by ranking the bonds 1, 2, and 3 according to their bond strength. 

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5a1 mark

State the main role of the protein haemoglobin. 

5b2 marks

In the hereditary disease sickle cell anaemia, a mutation results in the amino acid valine (which is non-polar) replacing glutamic acid (which is polar), making haemoglobin less soluble. Molecules of sickle-cell haemoglobin do not bind well to oxygen.

Explain why a sickle cell patient’s haemoglobin is less soluble than a non-sufferer’s haemoglobin. 

5c2 marks

Each haemoglobin molecule can bind 4 oxygen molecules. There are 300 million haemoglobin molecules in each red blood cell. An adult human is estimated to have 3 x 1013 red blood cells in circulation at any one time. 

Calculate the theoretical maximum number of oxygen molecules that could be carried in the red blood cells of an adult human. 

5d2 marks

Figure 1 below shows the structure of collagen protein at various levels of detail. 

Figure 1

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Use Figure 1 to suggest two features of collagen that enable it to be a strong, structural protein.

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1a3 marks

Describe how a dipeptide would form from two amino acids. 

1b3 marks

Polypeptides are formed from hundreds of amino acids joined together. A polypeptide
contains 288 peptide bonds.

What would be the total number of amino acids in this polypeptide? 

Explain your answer. 

1c3 marks

How does a functional protein differ from a polypeptide?

1d1 mark

There is a general structure that can be applied to all amino acids. Figure 1 below
shows the structure of the amino acid valine.

Figure 11-3-aqa-m1d

Draw a circle around the part of the molecule that would be identical in all amino
acids

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2a2 marks

Describe how a polypeptide is formed.

2b5 marks

DNA polymerases are a vital group of enzymes. Different variations are found in all living
cells. DNA polymerases are responsible for replicating DNA during semi-conservative
replication. 

Explain why most cellular enzymes like DNA polymerases are made predominantly from protein.

2c1 mark

Some washing powders contain molecules that break down and hydrolyse the
peptide bonds between amino acids. The degree to which a protein is broken down can
be called the degree of hydrolysis (DH). The DH value is calculated using the equation

                      bold DH bold space equals space fraction numerator bold 100 bold space bold cross times bold space bold Number bold space bold of bold space bold peptide bold space bold bonds bold space bold hydrolysed over denominator bold Total bold space bold number bold space bold of bold space bold peptide bold space bold bonds bold space bold present end fraction

Washing powder was applied to a molecule of protein. The DH value after exposure to
the washing powder was 28 and a total of 140 peptide bonds were hydrolysed. Calculate
the number of amino acids in the protein.
2d1 mark

Suggest why it is beneficial for washing powders to contain molecules with the ability to
break down proteins.

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3a3 marks

Collagen protein molecules have a quaternary structure and are vital in providing
structural support in a wide variety of organisms. There are different types of collagen but
all types share the same basic features. 

One of the polypeptide chains that forms a molecule of collagen contains 1049 peptide
bonds. Assuming all the chains that form collagen are identical in length, how many
amino acids would be found in a single molecule of collagen?

Explain your answer. 

3b2 marks

Explain why collagen is described as having a quaternary structure rather than a tertiary
structure. 

3c3 marks

Scientists wanted to test for the presence of protein while developing a new meal
replacement shake. Describe the test that they should use and state what a positive test
result would look like.

3d2 marks

Compare and contrast the different chemical elements found in proteins and
carbohydrates.

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4a4 marks

Explain how a polypeptides’ tertiary structure is determined by bonds between amino
acids. 

4b2 marks

Haemoglobin and collagen are both two important proteins found in the human body. Fill
in Table 1 below to accurately describe their properties.

Table 1 

 

Haemoglobin

Collagen

Globular or fibrous

   

Solubility in water

   

No. of polypeptide chains

   

Type of helix present

   

Prosthetic group present

   
4c2 marks

Describe the function of haemoglobin and collagen. Explain how their structure is
related to their function.

4d1 mark

Haemoglobin is made of alpha and beta polypeptides. Each alpha polypeptide has 141
amino acids and each beta polypeptide has 146 amino acids. What is the total number
of peptide bonds present in one alpha polypeptide and one beta polypeptide.

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5a3 marks

Figure 1 below shows a single amino acid. Draw out the mechanism by which two
amino acids form a dipeptide.

Figure 1

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5b2 marks

A lab technician investigating the amino acid composition of new food supplement
decided to use paper chromatography to separate out and identify the different amino
acids present.

The technician placed a concentrated drop of the food supplement mixture on the origin
line. She then placed concentrated drops of known amino acids at other points along the
origin line. The paper was placed in a tank containing solvent for 2.5 hours. When the
solvent reached the top of the paper, she marked the solvent front using a pencil. The
paper was then sprayed with ninhydrin and heated in a 100oC oven. The chromatogram
results can be seen in Figure 2 below.

Figure 2

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Which amino acids are not present in the food supplement. Explain your answer.

5c1 mark

The Rf value of amino acid C was 0.2. Using the equation below work out the distance in
mm that the amino acid moved from the origin line. 

5d5 marks

The influenza A virus remains a prevalent human pathogen partly because of its ability
to evade human antibodies. The antibodies produced by humans in response to
influenza, are specific to the influenza attachment protein hemagglutinin (HA). Changes
in the influenza HA gene segment prevents human antibodies from binding. Explain how.

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1a1 mark

Figure 1 shows the molecular structure of the amino acid isoleucine.

Figure 1

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Circle the central carbon atom (∝-carbon) in Figure 1

1b3 marks

A section of the plasma membrane is shown in Figure 2.

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Suggest and explain whether isoleucine would be more likely to occupy Region A or Region B of the membrane protein.  

1c2 marks

Isoleucine is described as an essential amino acid for humans. Define the term  ‘essential’ in this context.

1d4 marks

Draw the structure of the dipeptide formed from two molecules of isoleucine. Name and label the bond that forms between the two molecules and any by-products formed.

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2a2 marks

Mixtures of amino acids can be separated by two-way thin-layer chromatography. In this technique, the mixture is placed on a thin layer of inert, solid material spread over a flat aluminium plate and run with Solvent A until the solvent reaches the top of the plate. The plate is then dried, turned 90° and run with Solvent B. The plate is then re-dried and developed with ninhydrin spray to expose the individual amino acids. Explain why different solvents have to be used in each stage of this technique.

2b1 mark

Table 1 sets out the Rf values of a mixture of selected amino acids using two separate solvents in thin-layer chromatography.  Abbreviations have been used for the amino acid names.  

Table 1

Amino acid

Rf values

Isopropanol

Methanol

Ala

0.25

0.73

Arg

0.08

0.42

Asp

0.11

0.38

Glu

0.19

0.59

Gly

0.13

0.45

Leu

0.60

0.83

Lys

0.07

0.47

Phe

0.46

0.74

Ser

0.12

0.50

Thr

0.19

0.61

Val

0.46

0.76

Explain why Rf values do not have units.

2c4 marks

Using the information from Table 1, identify two pairs of amino acids that would not be separated by two-way thin layer chromatography.  The solvents used are isopropanol and methanol, in that order. Justify your selections

2d4 marks

Describe and explain the factors that affect the Rf value when separating amino acids out by thin-layer chromatography.  Assume the same solvent, layer thickness, layer medium, temperature and depth of solvent are being used.

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3a4 marks

Explain how the secondary structure of a protein contributes to its overall 3-D shape.

3b2 marks

Collagen is the most abundant protein in the human body, and is an important element in connective tissue such as skin, bones, teeth and tendons. Collagen contains repeating triplets of the amino acids glycine, proline and hydroxyproline, resulting in a tightly-coiled helix. Glycine is the simplest amino acid, with a single hydrogen atom as its R group, whereas proline and hydroxyproline have large organic R groups.  

Suggest and explain a role for glycine as part of the repeating structure.

3c3 marks

The gene coding for a bacterial protein contains 579 base pairs of DNA.  Bacteriologists have measured that 5.6 molecules of the protein can be synthesised per minute per ribosome. Calculate the rate of translation and express your answer, to the nearest whole number, in amino acids s-1.

3d5 marks

Identify the main aspects of the tertiary structure labelled A, B, C and D in Figure 1 and deduce what all four aspects have in common.
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4a2 marks

A molecule of haemoglobin contains four prosthetic groups containing iron. Define the term ‘prosthetic group’. 

4b6 marks

Describe the quaternary structure of haemoglobin and explain how its structure relates to its function in mammals.

4c4 marks

A 70 kg male athlete when at rest, uses oxygen at a rate of 16 cm3 kg-1 min-1.  Haemoglobin binds to oxygen at 1.34 cm3  g-1. Calculate the mass of haemoglobin in grammes per 100 cm3 of athlete’s blood. Assume that his total blood volume is 5 dm3 and that his blood circulates his body once a minute on average whilst at rest.

4d4 marks

The amino acid sequence of haemoglobin chains has been determined in many species. Llamas and high-soaring migratory birds have an 18% difference in their amino acid sequence compared to that of humans.  Suggest and explain the differences between llama haemoglobin and human haemoglobin structure.

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5a2 marks

Describe a biochemical test to prove that a solution contains protein and outline a positive result.   

5b1 mark

The molecular structure of the amino acid tryptophan is shown in Figure 1.

Figure 1

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Draw a circle on Figure 1 to highlight the R group of tryptophan.

5c3 marks

Figure 2 shows the structure of a different amino acid, cysteine. 

Figure 2

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Identify and explain the component of cysteine’s structure that plays an important role in a protein’s tertiary structure.  

5d1 mark

Draw out the structure of the bond that forms between a molecule of tryptophan and a molecule of cysteine.   

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