Question 19M.3.hl.TZ2.9
| Date | May 2019 | Marks available | [Maximum mark: 10] | Reference code | 19M.3.hl.TZ2.9 |
| Level | hl | Paper | 3 | Time zone | TZ2 |
| Command term | Compare and contrast, Explain, Outline, State, State and explain, Suggest | Question number | 9 | Adapted from | N/A |
Proteins have structural or enzyme functions.
Some proteins form an α-helix. State the name of another secondary protein structure.
[1]
β/beta pleated/sheet [✔]
This question was well answered with many scoring the mark although there were quite a few incorrect responses that answered “beta-helix” rather than “beta-pleated sheet”.
Compare and contrast the bonding responsible for the two secondary structures.
One similarity:
One difference:
[2]
One similarity:
hydrogen bonding
OR
attractions between C=O and N–H «on main chain» [✔]
One difference:
α-helix has hydrogen bonds between amino acid residues that are closer than β-pleated sheet
OR
H-bonds in α-helix parallel to helix axis AND perpendicular to sheet in β-pleated sheet
OR
α-helix has one strand AND β-pleated sheet has two «or more» strands
OR
α-helix is more elastic «since H-bonds can be broken easily» AND β-pleated sheet is less elastic «since H-bonds are difficult to break» [✔]
Note: Accept a diagram which shows hydrogen bonding between O of C=O and H of NH groups for M1.
Accept “between carbonyl/amido/amide/carboxamide” but not “between amino/amine” for M1.
Almost all the candidate’s stated hydrogen bonding as the similarity between the 2 types of secondary structures but lost marks on the difference between them.
Explain why an increase in temperature reduces the rate of an enzyme-catalyzed reaction.
[2]
enzyme denatured/ loss of 3-D structure/conformational change
OR
«interactions responsible for» for tertiary/quaternary structures altered [✔]
shape of active site changes
OR
fewer substrate molecules fit into active sites [✔]
This question was well answered where most candidates received one mark for identifying that the enzyme will denature with an increase in temperature. However, many candidates did not continue with the explanation that the shape of the active site changes.
State and explain how a competitive inhibitor affects the maximum rate, Vmax, of an enzyme-catalyzed reaction.
[2]
Vmax unchanged [✔]
at high substrate concentration substrate outcompetes inhibitor/need a higher
substrate concentration to reach Vmax [✔]
Note: Accept suitable labelled diagram.
Many candidates stated correctly that Vmax remains unchanged but only some mentioned that a higher substrate concentration was required to reach Vmax for the second mark.
Oil spills are a major environmental problem.
Suggest two reasons why oil decomposes faster at the surface of the ocean than at greater depth.
[2]
Any two of:
surface water is warmer «so faster reaction rate»/more light/energy from the sun [✔]
more oxygen «for aerobic bacteria/oxidation of oil» [✔]
greater surface area [✔]
Many candidates received two marks for this part while some candidates only suggested one reason or repeated the same reason (for example - heat and energy from the sun) even though the question clearly asked for two reasons.
Oil spills can be treated with an enzyme mixture to speed up decomposition.
Outline one factor to be considered when assessing the greenness of an enzyme mixture.
[1]
Any one of:
non-hazardous/toxic to the environment/living organisms [✔]
energy requirements «during production» [✔]
quantity/type of waste produced «during production»
OR
atom economy [✔]
safety of process [✔]
Note: Accept “use of solvents/toxic materials «during production»”.
Do not accept “more steps involved”.
The candidates struggled with this part and gave journalistic or vague answers that cannot be awarded marks. Atom economy was mentioned correctly by a few candidates.