The veins of a leaf contain transport tissues. Fig. 2.1 is a drawing made from an electron micrograph showing a cross-section of the transport tissue in a leaf vein. The cells labelled A are modified companion cells, known as phloem transfer cells. Transfer cells move sucrose and other assimilates from mesophyll cells, B, into the phloem sieve tube element, C. [Figure 2.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

1. [5] award mp1 and mp8 anywhere in each section of the answer
2. cell B - mesophyll cells mp1 photosynthesis / described ; e.g. conversion of carbon dioxide to sugars using light energy synthesis of complex organic compounds from inorganic compounds using light energy
3. adaptation – mp2 and one from mp3 to mp7
4. mp2 chloroplasts / chlorophyll / chloroplast pigments to absorb light ;
5. any one from: mp3 large vacuole to keep chloroplasts at the periphery ;
6. mp4 starch grains as store of products of photosynthesis ;
7. mp5 large / moist, surface / cell wall for evaporation of water for transpiration (stream); A provides water for photosynthesis in context of water supply transpiration
8. mp6 thin cell wall gas exchange / diffusion of gases ;
9. mp7 isodiametric / (roughly) spherical shape prevents close packing / gives large air spaces;
10. cell C - sieve tube element mp8 transport / movement of, (named) sugars / (named) assimilates / photosynthates / organic substances ; A translocation
11. adaptation – any two from: mp9 peripheral / described, cytoplasm allowing maximum, volume / space, for transport of, phloem sap / assimilates; A 'at the edge' / 'thin layer' A less / little, resistance to flow / AW
12. mp10 no nucleus / few organelles / little cytoplasm allowing maximum, volume / space, for transport of, phloem sap / assimilates ; R 'no organelles' A less / little, resistance to flow / AW
13. mp11 elongate / elongated (cells, so end to end) to make (sieve) tubes ;
14. mp12 plasmodesmata (un)loading from / AW, companion cells / cell A; I 'between cells A and C'
15. mp13 sieve pores A flow between sieve tube elements; AW
16. mp14 sieve plates prevent, bursting / bulging; A maintaining hydrostatic pressure
17. mp15 AVP for C; e.g. phloem proteins and defence against pathogens / sealing wounds e.g. cell walls with cellulose microfibrils that run around the cells and to prevent bursting
18. [3] mp1, 2, 4 and 6 needs idea of increases/more; this is not needed for mp3 and mp5
19. any three from: 1 increases surface area:volume of cell / increases surface area of (cell surface) membrane ;
20. 2 idea of more space for / increased area for / more, proton pumps / carrier proteins in context of moving protons;
21. 3 pumping protons, from cytoplasm / into cell wall / into apoplast / AW ; A create / increase, proton / electrochemical / concentration, gradient
22. 4 idea of more space for / increased area for / more, cotransporter proteins / carrier proteins in context;
23. 5 cotransport of sucrose / described, into companion cell / into transfer cell / into cell A / from mesophyll cell / from cell B ; A secondary active transport A movement of sucrose and protons for cotransport
24. 6 more space for plasmodesmata (between cell A and cell C);