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

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

Explain how polypeptides are produced by the process of translation.

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. 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.

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.

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

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.

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.

Describe how plants transport organic compounds from where they are made to where they are stored.

The processes of photosynthesis and respiration have some factors in common and others differ. Compare and contrast both processes for specific factors.

Humans rely on carbohydrates for much of their energy. Outline the process of digestion and absorption of starch in the human digestive system.

1. (overall) process is translocation / bidirectional / movement from source to sink;
2. sugars/sucrose/organic compounds produced in leaves;
3. (loaded by) active transport / passage by apoplast route;
4. loaded into companion cells / transported in phloem / sieve tubes;
5. high concentrations of solutes at the source cause uptake of water (by osmosis);
6. water provides hydrostatic pressure for transport (from source to sink);
7. unloaded / stored / used at sink;
8. lowers pressure at sink / creates pressure differential / water re-entry to xylem;

1. salivary amylase breaks down starch in the mouth
   OR
   pancreatic amylase breaks down starch in small intestine;
2. product is disaccharides / maltose;
3. maltase digest disaccharides into monosaccharides / glucose / simple sugars;
4. monosaccharides/glucose absorbed in the small intestine;
5. villi increase the surface area of the intestinal epithelium for greater absorption;
6. (monosaccharides are) absorbed by co-transport/active transport (into intestinal cell) / absorbed byfacilitated diffusion into blood (from intestinal cell);

Do not accept glucose or sugars for mpb.

Question 6 was the least popular in section B.

Most candidates provided correct and complete details of the process of translocation.

The comparison of the processes of cellular respiration and photosynthesis was done effectively. Some very good and well-organized answers were noted. It was common for students to use a table to organize their answers. Some correct answers contained full descriptions of the Calvin and the Krebs cycle, which was unnecessary.

Most candidates presented full and correct answers regarding the enzymes and molecules involved in starch digestion. The main recommendation here was to watch the accuracy of statements.

State the level of protein structure at which the polypeptide chains of hemoglobin are combined.

Explain the shape of the polypeptide chain at X.

Outline the role of tRNA in hemoglobin synthesis.

quaternary / fourth level;

1. alpha helix / $\alpha$ helix
2. type of secondary structure / second level of protein structure;
3. maintained by hydrogen bonds;
4. between C=O group and an N–H group;

1. decoding/translation (of the genetic code/RNA base sequence);
2. carries/brings one amino acid/a specific amino acid/the amino acid (corresponding to codon/anticodon);
3. tRNA has an anticodon which pairs with mRNA/is complementary to a codon (on mRNA);

Many candidates knew that association of polypeptide chains is quaternary structure. Any spelling of this term was accepted — only a minority got it correct.

This was one of the most successfully answered of questions, with better prepared candidates recognizing the alpha helix and explaining it in terms of hydrogen bonding.

There were varied answers to this question. The best made it clear that a tRNA molecule carries one specific amino acid corresponding to its anticodon and to the codon on mRNA to which it binds during translation.