A-LevelBiologyGas exchange9700/22 · May/June 2019 · Q211 Marks

Some tissues of the gas exchange system include cells that are able to produce and secrete mucins. Mucins are stored in vesicles in these cells, ready for secretion. Once outside the cell, mucins adsorb water to form mucus.

(a)Name the structures in the gas exchange system that produce and secrete mucins.[2]
(b)Mucins are described as glycosylated proteins. The process of glycosylation involves the addition of sugar components after polypeptides are synthesised. Suggest one location in the cell where glycosylation of mucin could occur.[1]
(c)The processes that occur in the production and secretion of mucins are listed. translation exocytosis glycosylation transcription Complete Table 2.1 by writing the processes in the correct order in which they would take place. [Table 2.1][2]
(d)Chloride ions move out of the mucin-producing cells at the same time as mucin is secreted. Suggest and explain how the exit of chloride ions helps the formation of mucus from mucin.[3]
(e)The gas exchange system includes some cells that are able to divide by mitosis. Explain why it is important to have these cells in the gas exchange system.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[2] mucous gland cells / mucous gland(s) ; I mucosa
goblet cell(s);
list containing trachea and/or bronchus and/or bronchioles I and allow to two max
BUT if alveoli stated or incorrect cell types (e.g. ciliated cells) max 1
[1] any one from:
Golgi (body/ complex/apparatus);
rough endoplasmic reticulum; A rough ER/RER R SER I ER
[2] two marks to complete table
first process: transcription
second process: translation
third process: glycosylation
fourth process: exocytosis
allow one mark if transcription first process and exocytosis last process but other two wrong way round
allow one mark if transcription and translation wrong way round and other two correct
[3] accept symbol Y for water potential
three from:
1 chloride presence outside cell causes water potential (outside cells) to, decrease / become more negative / become lower or chloride leaving cell causes water potential (in cell) to increase / AW;
2 water potential gradient, created / present (between inside and outside of cell);
3 water leaves cell by osmosis (to be taken up by mucin to form mucus);
4 from higher to lower water potential / AW ; R if term gradient included
[3] any three from:
to produce, new/daughter, cells...
1 that are genetically identical;
2 to replace, old / damaged / dead, cells; A named cells
3 to repair (damaged / injured / diseased) tissue ; A named tissue
if mp 2 and 3 not gained, allow 1 mark for replacing damaged tissue
4 ref.to function; in context
5 AVP; idea of increase in number of cells / growth, to accommodate increase in size of gas exchange system in growing infants / AW