Question 113 marks
Question
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. A B Fig. 1.1 (a) With reference to Fig. 1.1, state how it is possible to distinguish between a scanning electron micrograph and a transmission electron micrograph. …… [1] (b) The approximate length of a microvillus is 1 μm. Outline the method you would use to estimate the magnifications of the images shown in Fig. 1.1. …… [2] (c) One role of an intestinal epithelial cell is the absorption of glucose from the gut lumen into the circulatory system. This involves different membrane transport proteins. The events occurring in an intestinal epithelial cell during the absorption of glucose are summarised in Fig. 1.2. not to scale glucose gut lumen tissue capillary K+ fluid glucose Na+ Na+ Fig. 1.2 • Sodium ions (Na+) are removed from the cell by active transport through a transport protein known as a sodium-potassium (Na+ / K+) pump. • This decreases the concentration of Na+ in the cell compared to the gut lumen. • Glucose molecules are cotransported with Na+ into the cell from the gut lumen. • Glucose molecules are transported out of the cell into the tissue fluid down a concentration gradient. (i) Active transport involves water-soluble substances, such as Na+ and K+, and the use of ATP to provide the energy needed for their transport through carrier proteins. Outline other features of active transport. …… [2] (ii) Glucose molecules enter the cell through a membrane protein. Suggest why glucose molecules need to be cotransported with Na+ when it enters the cell through the membrane protein. …… [2] (iii) Explain how microvilli increase the uptake of glucose into an intestinal epithelial cell. …… [2] (d) Stem cells are also located in the wall of the small intestine. These cells divide by mitosis continuously. Suggest and explain the importance of mitosis by stem cells in the small intestine. …… [4] [Total: 13]
Mark scheme
1(a) I ref. to resolution 1 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 I thin shows, internal view of cell / contents of cell / AW does not have stated feature of SEM (see mp1) scanning (EM) stated plus feature and ; if Fig. 1.1, A / B not identified transmission (EM) stated plus feature 1(b) any two from: max 1 if stating to measure one of the sides of the figure 2 (for each image) measure a number of microvilli and calculate a mean ; 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 divide, microvillus / image, length by, actual length / AW / 1(μm) ; A idea that value for, microvillus / image, length is the magnification 1(c)(i) any two from: 2 (movement of substances) against concentration gradient / from low(er) to high(er) concentration ; (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 ; 1(c)(ii) any two from: 2 1 glucose is moving against a (concentration) gradient ; AW e.g. low to high 2 movement powered by inward movement of Na+ ; AW 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 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 1(c)(iii) increases / larger / large, surface area (over which glucose can enter cell per unit time) ; A increases / higher / high, SA:V 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 1(d) any four from: 4 suggestion max 3 explanation 1 to replace, old / dead / damaged / injured, cells 6 idea of function in context of mp1 or 2 or 3 ; or e.g. repair allows tissues to function to, repair / replace, damaged tissue ; specialisation / differentiation, allows, cells / tissue, to I repair cells function I replace old tissues genetically identical so cells can function as a tissue 2 cells are produced that can, differentiate / become specialised (into intestinal cells); look for other approaches e.g. I multipotent / AW ‘replace damaged cells that no longer function’ is mp1 and mp6 R if, stated / strongly implied that, all cells produced by …………………………………………………………………… mitosis become specialised 7 detail of function (intestines) ; e.g. absorption (of, glucose / nutrients) production of 3 produce cells that are genetically identical ; digestive enzymes A clones pass absorbed, nutrients / AW, to blood A identical / same, DNA / genetic material / genetic information I nutrition 4 growth ; R growth of cells 8 idea of development during, infancy / childhood ; 5 renew / replenish / restore number of / AW, stem cells ; 9 for, continued production / constant supply / AW, of intestinal cells I increase in stem cells or so that number of stem cells doesn’t decrease ;