Epithelial cells in the small intestine have cell structures known as microvilli. The microvilli of these cells are found only on the surface that borders the gut lumen. Fig. 1.1 shows images of microvilli of intestinal epithelial cells. These images have been obtained using a scanning electron microscope and a transmission electron microscope. Fig. 1.1A is at a different magnification to Fig. 1.1B. [Figure 1.1]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

1. [1] I ref. to resolution any one from: Fig. 1.1A is scanning (EM), qualified ; e.g. shows, surface / contour / 3-D, views shows (only the), the outer / external, view (of microvilli) idea of good focus with different depths does not have stated feature of TEM (see mp2) Fig. 1.1B is transmission (EM), qualified ; e.g. showing, 2D / flat, views (of, microvilli / mitochondria) image in one plane showing section through, mitochondria / microvilli / cell shows, internal view of cell / contents of cell / AW does not have stated feature of SEM (see mp1) scanning (EM) stated plus feature and transmission (EM) stated plus feature I thin ; if Fig. 1.1, A / B not identified
2. [2] any two from: max 1 if stating to measure one of the sides of the figure (for each image) measure a number of microvilli and calculate a mean;
3. [2] same units for actual and measured images / example given ; e.g. if microvillus measurement is in mm convert, microvillus measured length to µm / actual length to mm R if extra detail given is incorrect e.g. divide by 1000 for mm to µm
4. [2] divide, microvillus / image, length by, actual length / AW / 1(μm); A idea that value for, microvillus / image, length is the magnification
5. [2] any two from: (movement of substances) against concentration gradient / from low(er) to high(er) concentration ;
6. [2] (protein has) binding site(s); specific; A description e.g. shape complementary to shape of substance can be in context of protein or binding site ref. to (binding causes protein), conformational change / change of shape;
7. [2] any two from: 1 glucose is moving against a (concentration) gradient ; AW e.g. low to high
8. [2] 2 movement powered by inward movement of Na+; AW
9. [2] 3 Na+ movement (into cell from gut lumen) down an electrochemical gradient or down a (Na+), concentration / diffusion, gradient or by facilitated diffusion; A diffusion if response includes ref. to the cotransport protein / cotranporter
10. [2] 4 AVP; e.g. idea that, cotransport needed so that all glucose will be removed from gut lumen ora facilitated diffusion of glucose will only be to equilibrium / AW cotransporter has (specific) binding site for glucose I cotransporter has a complementary shape idea that there is no specific active uptake protein for glucose
11. [2] increases/larger/ large, surface area (over which glucose can enter cell per unit time); A increases / higher / high, SA:V
12. [2] idea of (so) more, transport/carrier / cotransport, proteins A more cotransporters or more proteins, located in membrane / AW ; R pump proteins A idea of more space / places, where proteins can transport glucose
13. [4] any four from: suggestion max 3 1 to replace, old / dead / damaged / injured, cells or to, repair/replace, damaged tissue ; I repair cells I replace old tissues
14. [4] 2 cells are produced that can, differentiate / become specialised (into intestinal cells); I multipotent / AW R if, stated / strongly implied that, all cells produced by mitosis become specialised
15. [4] 3 produce cells that are genetically identical ; A clones A identical / same, DNA / genetic material / genetic information
16. [4] explanation 6 idea of function in context of mp1 or 2 or 3 ; e.g. repair allows tissues to function specialisation / differentiation, allows, cells / tissue, to function genetically identical so cells can function as a tissue look for other approaches e.g. 'replace damaged cells that no longer function' is mp1 and mp6
17. [4] 7 detail of function (intestines) ; e.g. absorption (of, glucose / nutrients) production of digestive enzymes pass absorbed, nutrients / AW, to blood I nutrition
18. [4] 4 growth; R growth of cells
19. [4] 8 idea of development during, infancy / childhood ;
20. [4] 5 renew / replenish / restore number of / AW, stem cells ; I increase in stem cells
21. [4] 9 for, continued production / constant supply / AW, of intestinal cells or so that number of stem cells doesn't decrease ;