DP Chemistry (last assessment 2024)

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Question 23M.3.HL.TZ2.7

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Date May 2023 Marks available [Maximum mark: 8] Reference code 23M.3.HL.TZ2.7
Level HL Paper 3 Time zone TZ2
Command term Describe, Explain, State Question number 7 Adapted from N/A
7.
[Maximum mark: 8]
23M.3.HL.TZ2.7

Proteins are large polymers of 2-amino acids.

(a)

Describe the interactions between amino acids occurring at the primary, secondary and tertiary levels within a protein.

Structure Level Interactions between amino acids
Primary ...........................................................
Secondary ...........................................................
Tertiary ...........................................................

[3]

Markscheme
Structure Level Interactions between amino acids
Primary covalent bonding
OR
peptide bond
OR
amide bond ✓
Secondary hydrogen bonding ✓
Tertiary interactions between R groups/side chains
OR
ionic/electrostatic «attraction»
OR
hydrogen bonding
OR
hydrophobic interactions
OR
disulfide bridges
OR
London/dispersion/van der Waals/«instantaneous» induced dipole-induced dipole ✓

 

Do not accept “amino acid sequence” for M1.

Do not accept “alpha helix” OR “beta sheets” for M2.

Accept “covalent bonding” for M3.

(b)

Explain how paper chromatography can separate and identify mixtures of amino acids.

[2]

Markscheme

Any two of:

sample spotted on paper/stationary phase AND solvent moves up the paper
OR
continuous cycles of adsorption and desorption/dissolution
OR
analyte moves when in solvent AND does not move when on paper ✓

different/depends on attractions to mobile phase AND stationary phase/paper
OR
«amino acids» separated based on solubilities in/affinity to the two phases
OR
separated based on polarities/polar attractions/molar masses of «amino acids» ✓

developed with ninhydrin/reagent/locating agent
OR
become identified with UV «light» ✓

«amino acids» identified by Rf/retention factor «values»
OR
Rf/retention factors «values» compared with known samples ✓

(c)

Explain the concept of product inhibition in metabolic pathways.

[2]

Markscheme

product of reaction is inhibitor of enzyme
OR
product binds/bonds to allosteric site of enzyme ✓

regulates own production
OR
sets up feedback loop to control concentration/production ✓

The following diagram shows a protein calibration curve.

(d)

State the concentration, in mg cm−3, of a protein sample with an absorbance of 0.80.

[1]

Markscheme

0.520 «mg cm−3» ✓

 

Accept any value in range 0.510-0.530 «mg cm−3».