Sugar molecules enter cells through transport proteins. (a) Explain why transport proteins are required for the movement of sugar molecules, such as glucose and fructose, into cells. Some plant cells convert fructose and glucose into sucrose for transport from sources to sinks. Sucrose is moved into phloem sieve tubes as shown in Fig. 5.1. [Figure 5.1]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. sugar (molecules) / glucose / fructose, is polar / is water soluble / not lipid soluble / hydrophilic ;
2. cannot pass through, (phospholipid bilayer / hydrophobic core / fatty acid 'tails' / hydrocarbon 'tails'; A non-polar regions
3. accept H⁺ for proton throughout
4. 1 (at Y) protons, pumped out (of companion cell) / moved out by active transport / move out through proton pump;
5. A protons are moved out against concentration gradient
6. 2 creates a, proton gradient / electrochemical gradient ;
7. 3 protons go into the, cell wall / apoplast ; R mesophyll cell
8. 4 (at X) protons enter cell by facilitated diffusion ;
9. 5 (X is) cotransporter / cotransport protein;
10. 6 sucrose transported into (companion) cell together with protons ;
11. 7 (sucrose enters) against concentration gradient ;
12. 8 sucrose concentration, increases / maintained, in companion cell ;
13. 9 sucrose diffuses into sieve tube (element);
14. 10 through plasmodesmata ;
15. 11 AVP; e.g. ref. to, secondary / indirect, active transport
16. look for names of plant organs other than leaves, ignore names such as potato, iris, onions
17. R leaves unqualified
18. any two for max 1
19. root /root tip
20. stem/stem tip / shoot / shoot tip
21. tubers
22. bulbs
23. corms
24. rhizomes
25. buds
26. flowers
27. fruits
28. seeds
29. young/maturing / developing / infected, leaves
30. AVP