| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.hl.TZ2.10 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Describe | Question number | 10 | Adapted from | N/A |
Question
Amino acids contain both acidic and basic functional groups.
An aqueous solution of glutamine contains 0.600 mol dm−3 of the zwitterion and 0.300 mol dm−3 of the anionic form.
pKa1 = 2.2 and pKa2 = 9.1
Outline which pKa value should be used when calculating the pH of the solution, giving your reason.
Calculate the pH of the glutamine solution using section 1 of the data booklet.
Describe what is meant by the genetic code and how it relates to protein synthesis.
Markscheme
pKa2 AND pH of solution > pH of isoelectric point «as anion present»
OR
pKa2 AND zwitterion has lost H+ to become anion «so in basic solution»
OR
pKa2 AND «only» anion «and zwitterion» present [✔]
«pH = 9.1 + log »
«= 9.1 + (–0.3)» = 8.8 [✔]
sequence of bases in DNA [✔]
codon/triplet code/each set of three bases codes for an amino acid [✔]
Examiners report
Several candidates stated correctly that pKa2 should be used with a reason whereas others wrote pKa1, which was incorrect.
Majority of the candidates calculated correctly the pH of the glutamine solution while other candidates managed the ECF mark from part a (i).
This part was poorly answered by many candidates and was unable to state that genetic code is a sequence of bases in DNA and each codon codes for an amino acid. Often complex answers were written which were incorrect.
Syllabus sections
-
17N.3.hl.TZ0.13:
The stability of DNA is due to interactions of its hydrophilic and hydrophobic components.
Outline the interactions of the phosphate groups in DNA with water and with surrounding proteins (histones).
-
17M.3.hl.TZ2.15a:
Outline how its structure allows it to be negatively charged in the body.
-
17M.3.hl.TZ1.15d:
The sequence of nitrogenous bases in DNA determines hereditary characteristics.
Calculate the mole percentages of cytosine, guanine and thymine in a double helical DNA structure if it contains 17% adenine by mole.
-
18M.3.hl.TZ1.6d:
Describe how DNA determines the primary structure of a protein such as insulin.
- 18N.3.hl.TZ0.7b: Suggest one concern about the use of genetically modified, GM, food.
- 18N.3.hl.TZ0.7a: State the feature of DNA that determines the primary structure of proteins synthesised by a...
-
16N.3.hl.TZ0.15b:
In 2010, scientists claimed that they had discovered a species of bacteria capable of incorporating arsenic in place of phosphorus into the bacterial DNA. This claim has since proved controversial. Suggest one technique or evidence that might help support the claim.
- 16N.3.hl.TZ0.15a: State the name of the component of DNA responsible for the migration of its fragments to the...
-
17M.3.hl.TZ2.15b:
Deduce the nucleotide sequence of a complementary strand of a fragment of DNA with the nucleotide sequence –GACGGATCA–.
- 19N.3.hl.TZ0.13b: Explain how the double-helical structure of DNA is stabilized once formed.
-
18M.3.hl.TZ2.12:
DNA is a biopolymer made up of nucleotides. List two components of a nucleotide.
-
19M.3.hl.TZ1.14a:
Outline what is meant by genetically modified organisms.
-
19N.3.hl.TZ0.13a:
List two components of nucleotides.
-
20N.3.hl.TZ0.8b(ii):
The diverse functions of biological molecules depend on their structure and shape.
Draw the nitrogenous base that is paired with guanine in DNA, showing the hydrogen bonds between the bases. Use section 34 of the data booklet.
-
19M.3.hl.TZ1.14b:
Outline one benefit of the use of these products.
