Amino acids and protein structureEdexcel International A Level Biology: Subtopic test

1. 1
   All amino acids share a common structure: a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. The simplest amino acid, glycine, has a hydrogen atom as its R group. Alanine has a methyl group, CH₃, as its R group.

   (a)

   Which two groups react together when two amino acids join to form a peptide bond?

   [1 mark]

   • AThe carboxyl groups of both amino acids
   • BThe R groups of both amino acids
   • CThe amino group of one amino acid and the hydrogen atom of the other
   • DThe carboxyl group of one amino acid and the amino group of the other

   (b)

   A tripeptide is made from three amino acids. How many peptide bonds are formed, and how many molecules of water are released?

   [1 mark]

   • AThree peptide bonds and three molecules of water
   • BTwo peptide bonds and two molecules of water
   • CThree peptide bonds and two molecules of water
   • DTwo peptide bonds and three molecules of water

   (c)

   Describe how a dipeptide is formed from glycine and alanine.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Collagen and haemoglobin are both proteins. Collagen forms tough fibres in tendons: each molecule has three polypeptide chains wound round each other, and about one third of its amino acid residues are glycine. Haemoglobin is found inside red blood cells; each molecule has four polypeptide chains, each with a haem group that binds oxygen.

   (a)

   Which feature of collagen makes it suitable for its function in tendons?

   [1 mark]

   • AIts hydrophilic outer surface makes it soluble in water
   • BIt has an active site that binds a specific substrate
   • CIts chains are wound together and cross-linked, giving high tensile strength
   • DIt has haem groups that bind oxygen

   (b)

   Which feature of haemoglobin explains why it is soluble in the cytoplasm of red blood cells?

   [1 mark]

   • AHydrophilic R groups on the outside of the folded molecule
   • BHydrophobic R groups on the outside of the folded molecule
   • CIts long, thin fibre-like shape
   • DIonic bonds between the peptide bonds

   (c)

   Compare the structure and function of a fibrous protein and a globular protein, using collagen and haemoglobin as examples.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In sickle-cell disease, a single amino acid in the β polypeptide chain of haemoglobin is changed. A hydrophilic glutamic acid is replaced by a hydrophobic valine, at a position on the outside of the folded chain. The abnormal haemoglobin is much less soluble and, when oxygen concentration is low, its molecules stick together into long fibres.

   (a)

   Describe the primary, secondary and tertiary structure of a protein.

   [3 marks]

   (b)

   Explain how the change of a single amino acid can make the haemoglobin less soluble.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician tests four solutions with biuret reagent. Solution A is egg albumin, solution B is glucose, solution C is glycine and solution D is egg albumin that has been boiled for 10 minutes and cooled. Solutions A and D turned violet. Solutions B and C stayed blue.

   (a)

   Describe how the biuret test is carried out and explain the results for solutions A to D.

   [6 marks]

   (b)

   A student claims that all proteins have the same properties because they are all made of amino acids joined by peptide bonds. Evaluate this claim, using haemoglobin and collagen as examples.

   [6 marks]

   Total for question 4: 12 marks

End of questions