DP Chemistry (last assessment 2024)

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Question 23M.3.HL.TZ1.8

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Date May 2023 Marks available [Maximum mark: 11] Reference code 23M.3.HL.TZ1.8
Level HL Paper 3 Time zone TZ1
Command term Describe, Determine, Explain, Predict, State Question number 8 Adapted from N/A
8.
[Maximum mark: 11]
23M.3.HL.TZ1.8

The amino acids in a protein can be separated using paper chromatography. The Rf values using a solvent of butanol and ethanoic acid are given.

Amino acid Rf value
Lysine 0.14
Glutamic acid 0.30
Threonine 0.35
Tyrosine 0.45
Asparagine 0.5
Methionine 0.55
Valine 0.61
Tryptophan 0.66
Leucine 0.73

[Source: BioTopics. n.d. Chromatography of amino acids. [online] Available at:
https://www.biotopics.co.uk/as/amino_acid_chromatography.html
[Accessed 20 May 2021]. Source adapted.]

(a)

The following diagram shows a chromatogram.

(a.i)

Determine the identity of the amino acid creating spot C by calculating the Rf value from the chromatogram.


Identity of spot C: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

 

[1]

Markscheme

Identity of spot C:
leucine ✓

(a.ii)

Predict, referring to the structure of the amino acids, whether spot X on the chromatogram in part (a)(i) is more likely to be serine or phenylalanine. Use the table of Rf values and section 33 of the data booklet.

[2]

Markscheme

serine AND more polar «than phenylalanine»
OR
serine AND OH group «on side chain» ✓

hydrogen bonding/greater affinity with stationary phase
OR
less soluble/poor affinity in solvent/mobile phase ✓


M2 may be scored without M1.

(b)

One role of proteins in the body is to catalyse reactions. Describe how enzymes catalyse reactions in the body.

[2]

Markscheme

bind to substrate at active site ✓
«provide» alternative pathway with lower «activation» energy ✓


Must have idea of binding to substrate AND a specific active site for M1.

(c)

State one industrial use of enzymes.

[1]

Markscheme

additives to detergents/washing powders/liquids
OR
breakdown oil spills/industrial waste ✓


Accept other industrial use.

Do not accept non-industrial uses, such as pregnancy testing.

(d)

Explain how a non-competitive inhibitor affects the Michaelis constant, Km, and Vmax of a reaction. Refer to the reaction between the inhibitor and the enzyme in your answer.


Effect on Km: 

 


Effect on Vmax: 

 

 

Explanation for Km: 

 

 


Explanation for Vmax:

 

 

 

[3]

Markscheme

Effect on Km: remains the same/no change AND
Effect on Vmax: decreases/reduced ✓

Explanation for Km:
no decrease in affinity of enzyme for substrate ✓

Explanation for Vmax:
binds at allosteric site
OR
binds away from active site
OR
changes shape of active site
OR
renders active site ineffective ✓

(e)

Determine the concentration, in mol dm−3, for a protein sample with absorbance of 0.50 at 240 nm. Use section 1 of the data booklet. 

Molar extinction coefficient = 0.75 dm3 cm−1 mol−1 
Path length = 1.0 cm

[2]

Markscheme

« log10I0I=εlc and log10 Io/I = A »

εlc = 0.50 ✓

«c = 0.50 / (0.75 dm3 cm−1 mol−1 × 1 cm)»
0.67 « mol dm−3 » ✓