The structural protein collagen is the main component of the extracellular matrix (ECM) of the cells of the gas exchange system. The ECM is the supporting external environment of cells. In the gas exchange system, the two main cell types that produce and secrete collagen are fibroblasts and chondrocytes. Fibroblasts are found throughout the gas exchange system, but chondrocytes are localised within a particular type of tissue found in the trachea and in the bronchi.

Name the tissue located in the trachea and bronchi that contains chondrocytes.

2(a)(ii)Gas ExchangeMedium3 marks

Describe features of the tissue named in 2(a)(i) and outline the role of this tissue in the trachea and bronchi.

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2(b)Mitosis Cell CycleMedium2 marks

In certain conditions, fibroblasts divide by mitosis to produce more cells. During this time, there are increased levels of an enzyme that acts to maintain the length of the telomeres. Explain why maintaining telomere length is important for fibroblasts that carry out mitosis.

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2(c)Mitosis Cell CycleEasy1 mark

Name the cell structures involved in the formation of the mitotic spindle in a fibroblast that are not present for the formation of the mitotic spindle in a plant cell.

2(d)Mitosis Cell CycleMedium2 marks

The structure of a chromosome changes during mitosis. Describe the structure of a chromosome as it appears at telophase of mitosis.

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2(e)(i)Gas ExchangeEasy1 mark

Figure 2.1 shows part of an alveolus and an alveolar capillary separated by a very thin ECM space known as the interstitium.

Figure 2.1, a diagram of part of an alveolus and an alveolar capillary: the capillary wall encloses a red blood cell, a fibroblast lies outside the capillary, the interstitium is the thin space between the capillary wall and the alveolar wall, the alveolar wall carries a surfactant layer on its inner surface, and the alveolar air space lies inside the alveolus.

Name the type of cell shown in Figure 2.1 that forms the capillary wall.

2(e)(ii)Gas ExchangeMedium2 marks

When damage occurs to the alveolar wall or capillary wall, fibroblasts become more active and increase the production of ECM components. Suggest the change that will occur to the interstitium owing to an increased fibroblast activity and explain how this will result in less oxygen being transported away from the capillary.

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2(f)(i)Proteins Amino AcidsEasy1 mark

A collagen molecule is composed of more than one collagen polypeptide. The primary structure of the collagen molecule contains a repeat sequence of three amino acid residues (amino acid triplet). Each amino acid triplet contains glycine (gly), as shown in Figure 2.2.

Figure 2.2, a diagram of an amino acid triplet in a collagen polypeptide: three linked circles labelled gly, X and Y are joined in a chain, with the join between gly and X bracketed and labelled 'covalent bond', the three residues together bracketed and labelled 'amino acid triplet', and the Y residue labelled 'amino acid residue (amino acid after bond formation)'.

Name the type of covalent bond that links the amino acid residues shown in Figure 2.2.

2(f)(ii)Proteins Amino AcidsMedium3 marks

Three different amino acids are commonly found in the X and Y positions of the amino acid triplet. One of these amino acids, alanine, has a similar role to glycine in the structure of a collagen molecule. The molecular structure of alanine is shown in Figure 2.3.

Figure 2.3, the molecular structure of alanine: an amine group (two H atoms and an N) on the left bonded to a central carbon that carries an H atom above and a CH3 side chain below, with the central carbon bonded on the right to a carboxyl carbon double-bonded to O and single-bonded to OH.

With reference to Figure 2.2 and Figure 2.3, explain why a high proportion of glycine and alanine residues is important for the structure of a collagen molecule.

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