Smooth muscle is a tissue composed of smooth muscle cells. The cells contain cytoplasm packed with proteins that are involved in contraction and relaxation.

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

1. [1] (by) contracts / contracting / contraction, and, relaxes / relaxing / relaxation; must be in context of bronchioles R if includes ref. to stretch / expand / recoil R if in context of bronchus or trachea (then ecf in subsequent mps)
2. [1] diameter/lumen size, can be, controlled / changed; AW A contraction, decreases diameter (of lumen) / constricts A relaxation (after contraction) increases diameter (of lumen) / widens / dilates / expands R vasoconstriction / vasodilation
3. [1] idea of, controls / regulates, flow of air / volume of air flowing; AW I statements such as 'lets air in' / 'allows inhalation and exhalation' A oxygen in / carbon dioxide out, instead of air flow R oxygen out and carbon dioxide
4. [1] AVP; e.g. contract to, reduce air movement / prevent entry of contaminants idea of normally relaxed but can, contract/cause constriction, when needed max 1 if response relates function to inhalation and exhalation 0 marks if incorrectly stated as elastic tissue or elastic fibres
5. [1] stage micrometer / stage micrometer scale ;
6. [1] yes, qualified; e.g. 0.2 mm = 200 μm 0.2 mm/smallest student can see, is less than 0.25 mm 250 μm = 0.25 mm 250 µm / cell length, is longer than 200 µm the cell is 50 µm longer (than 0.2 mm) the cell is 0.05 mm longer (than 0.2 mm) allow if converted to standard form (2 × 10-4 v 2.5 x 10-4 m) must have units must include numerical values
7. [1] wall, of one layer / is one cell thick ; I capillary is one cell thick A thin wall / endothelial cells are a single layer A endothelium is, a single layer (of cells) / one cell thick
8. [1] idea that red blood cells transported in single file;
9. [1] idea that red blood cells diameter (approximately), as large / same, as capillary (lumen) ; A if diameter (e.g. 7µm) stated as being similar to capillary size
10. [1] idea of diameter of capillary narrower than smooth muscle cell;
11. [1] AVP; e.g. no muscle and elastic layer muscle fibres / elastic fibres / collagen, not present ref. to different shapes of red blood cell / red blood cell showing flexibility, to pass through (narrow) capillary
12. [1] named, substance / type of substance, that is supplied to smooth muscle cells or is a product removed from the cells ; e.g. (supply) oxygen / nutrients / glucose / amino acids / cell-signalling molecule e.g. (removes)_carbon dioxide / lactic acid / waste A exchange of respiratory gases (between capillary and cells) I plasma proteins into tissue fluid
13. [1] thin wall / AW, so short distance;
14. [1] endothelial pores / fenestrations, for more efficient passage / AW; A gaps, between / within, (endothelial) cells (of capillary wall) A increases, quantity / rate of supply (of substances) A endothelial pores for formation of tissue fluid (around cells) R if blood leaves to make tissue fluid
15. [1] (endothelial) pores / AW, for passage of, phagocytes / monocytes / macrophages / neutrophils;
16. [1] small (size) / narrow diameter, to reach cells / so (all) cells are close ; A value in range 5–10 µm A small size / narrow lumen, slows (rate of) blood flow
17. [1] detail about red blood cell and, respiratory gases / gas exchange / oxygen / carbon dioxide; e.g. narrow lumen slows red blood cell movement for uptake of oxygen slows red blood cell movement to increase time for exchange of gases only one red blood cell fits (at a time) so short diffusion distance red blood cells are close to (muscle) cells for gas exchange red blood cells squeeze through, (so) distance for oxygen to reach cells is minimised
18. [1] (a gene mutation is a) change in the sequence of base pairs in a DNA molecule; A change in the DNA, base / nucleotide, sequence I change in RNA base sequence I names of mutations
19. [1] codes for / may result in / results in / AW, an altered / a different / a changed, polypeptide ; A protein / amino acid sequence / primary structure, for polypeptide
20. [1] differences owing to, RNA / gene, splicing; A primary transcript for RNA A ref. to alternative splicing R DNA splicing R mutation occurring during gene splicing
21. [1] (differences occur during) removal of, introns / non-coding sequences ; R if introns described as bases/nucleotides / codons R different number of introns removed / some introns remain
22. [1] exons / coding sequences, detail ; e.g. joined differently / in a different combination in middle (of transcript), removed / not included for repeating sequence, removed / not included
23. [1] (so) messenger RNA / mRNA, formed is different ;
24. [1] (still have) similar, tertiary structure / binding site shape ; A similar 3D shape R same A idea of binding sites still complementary to attach to (the) proteins
25. [1] removed amino acids are not structural amino acids ;
26. [1] removed amino acids are not part of binding site ; A description
27. [1] R-group interactions (still) the, same / similar ;
28. [1] AVP; e.g. idea of more than one binding site and only, one / a few, changed A same type of binding site, but a different number