A-LevelBiologyBiological molecules9700/23 · May/June 2023 · Q410 Marks

Food crops such as barley and wheat contain gluten. Gluten contains two proteins, glutenin and gliadin.

(a)(i)Table 4.1 contains descriptions of the structures of glutenin and gliadin. Complete Table 4.1 by writing the level of protein structure that applies to each description. Table 4.1 description level of protein structure a gliadin protein is a single polypeptide that forms a compact structure 20% of the amino acids in a glutenin molecule are glycine gliadin and glutenin molecules contain regions of ẞ-pleated sheets[3]
(a)(ii)Many genes in eukaryotic cells contain introns. The genes that code for gliadin do not contain introns. Explain how a lack of introns in a gliadin gene affects the production of mRNA from the primary transcript.[1]
(b)(i)Coeliac disease is a condition in which the immune system of a person responds to gluten in their diet. In coeliac disease, there is a response to the presence of peptides (short chains of amino acids) that are produced as a result of gliadin digestion. The gliadin peptides produced as result of digestion are often as large as 33 amino acids in length. Intestinal cells take up large numbers of these peptides at the same time. Suggest and explain how gliadin peptides are transported into intestinal cells.[2]
(b)(ii)The presence of gliadin causes the immune system of a person with coeliac disease to respond by producing anti-gliadin antibodies. Describe the sequence of events that results in the immune system producing anti-gliadin antibodies.[4]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[3] 1 mark per correct row ;;; description level of protein structure a gliadin protein is a single polypeptide that forms a compact structure tertiary 20% of the amino acids in a glutenin molecule are glycine primary gliadin and glutenin molecules contain regions of ẞ-pleated sheets secondary
[1] any one relevant: e.g. RNA/gene, splicing, not needed / does not occur ; R DNA splicing ref. to all primary transcript is a coding sequence ; A primary transcript has the same sequence as the mRNA no nucleotides removed, from the primary transcript / after transcription ; no process of exon joining / AW; no non-coding sequences, so introns to not need to be removed ; no alternative RNA splicing / exons not joined in different combinations; A only one type of mRNA formed / mRNA formed always the same idea that only other post transcriptional processing occurs ; e.g. 5' G (guanine) cap / poly A tail
[2] endocytosis; A pinocytosis A described detail; e.g. invagination of membrane / AW use of ATP A active process / energy needed formation of (endocytotic / pinocytotic) vesicle if mp1 gained from description, mp2 must be another detail accept other named transport mechanism, with detail
[4] any four from: 1 ref. presence of, non-self / foreign, antigen or gliadin acts as antigen or ref. to antigen presentation (by macrophages); 2 recognition / activation / AW, of (specific) B- / T-lymphocytes ; A clonal selection 3 clonal expansion of B-lymphocytes (and T-lymphocytes) ; AW e.g. form a clone of cells (by mitosis) / divide many times by mitosis 4 T-helper cells, secrete / release, cytokine; 5 cytokine stimulates, humoral / B-lymphocyte, response; if mp4 and mp5 not gained allow one mark for T-helper cells stimulate B-lymphocyte response 6 B-lymphocytes (differentiate to) form plasma cells ; 7 plasma cells, produce / secrete, (anti-gliadin) antibodies;