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Date November 2020 Marks available 4 Reference code 20N.2.HL.TZ0.6
Level Higher level Paper Paper 2 Time zone TZ0 / no time zone
Command term Outline Question number 6 Adapted from N/A

Question

Proteins are an important group of chemicals found in all living organisms.

Draw a molecular diagram to show the formation of a peptide bond.

[3]
a.

Outline how proteins are digested and the products of protein digestion absorbed in humans.

[4]
b.

Explain how polypeptides are produced by the process of translation.

[8]
c.

Markscheme

a. two amino acids correctly drawn;

b. removal/production of H2O molecule shown;

c. peptide bond labelled between C of C=O and N of N-H;

Accept specific examples of amino acids correctly drawn for example glycine with H instead of R.

mpc can be awarded if the peptide bond is shown as in the dipeptide diagram but there are errors elsewhere.

a.

a. digested by peptidases/proteases;

b. pepsin/pepsinogen/endopeptidase secreted by stomach (lining)/digests proteins in stomach;

c. pancreas secretes/pancreatic juice contains endopeptidase/trypsin/peptidase;

d. endopeptidase digest proteins/polypeptides to shorter chains of amino acids/shorter peptides;

e. amino acids absorbed by active uptake/transport;

f. in small intestine/ileum;

g. villi increase the surface area for absorption;

h. absorbed into bloodstream/into capillaries;

Peptidase can be accepted instead of endopeptidase in mpc, but not in mpb or mpd.

b.

a. mRNA is translated;

b. mRNA binds with ribosome/with small subunit of ribosome;

c. tRNA-activating enzymes/aminoacyl tRNA synthetases attach specific amino acid to tRNA;

d. anticodon of 3 bases/nucleotides on tRNA;

e. start codon/AUG on mRNA;

f. tRNA carrying first amino acid/methionine binds to P/peptidyl site (when large subunit binds);

g. anticodon (on tRNA) binds to codon (on mRNA);

h. complementary base pairing (between codon and anticodon);

i. tRNA for next codon binds to A site/amino acyl site;

j. peptide bond forms between amino acids (on tRNAs) at P and A sites;

k. ribosome moves along mRNA to next codon/by three bases/in 5’ to 3’ direction;

l. tRNA released from E/exit site;

m. process/cycle repeats to elongate the polypeptide/until stop codon is reached;

n. release of polypeptide and mRNA/disassembly of ribosome complex at stop codon;

Marks can be awarded in an annotated diagram.

Accept UAA, UAG or UGA instead of stop codon in mpm or mpn but do not accept terminator sequence.

Do not award mpk for the ribosome moving to the start codon in a 5’ to 3’ direction.

c.

Examiners report

Mostly full marks were gained although some had the correct diagrams but failed to label the peptide bond.

a.

Nearly all candidates had some knowledge of protein digestion and many scored 3 or 4 of the 4-mark total. The relatively low mark allocation reflects that the program does not require detailed study of protein digestion or absorption. Few students referred to endopeptidases, which are stipulated in guidance for sub-topic 6.1 and also few stated that villi in the small intestine increase the surface area for absorption.

b.

Explanations of translation were generally good, with many of the significant events included. Fewer candidates than in the past included unnecessary information about transcription. A common omission was the presence of a three-base (or nucleotide) anticodon on tRNA and a frequent small error was the idea that during initiation the first tRNA binds to the A site and then moves to the P site, rather than binding to the P site immediately.

c.

Syllabus sections

Core » Topic 6: Human physiology » 6.1 Digestion and absorption
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