A-LevelBiologyThe mitotic cell cycle9700/21 · May/June 2019 · Q410 Marks

Meristematic tissue is found in the growing regions of plants, such as shoot tips. Meristematic cells have a similar role to stem cells in animals. [Figure 4.1] shows some of the stages in the formation of a mature phloem sieve tube element and companion cells from a meristematic cell.

(a)Cells E and F in Fig. 4.1 are daughter cells produced when the meristematic cell divides in the shoot tip. Explain why it is important that one of the daughter cells (cell E) is a meristematic cell.[1]
(b)Complete Table 4.1 to describe the changes that are shown in Fig. 4.1 between stages: • F and G • G and H • H and J. Table 4.1 stages description F and G G and H H and J[3]
(c)Explain how the structure of a mature sieve tube element is related to its function.[4]
(d)Describe the functions of companion cells in transport in the phloem.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

any one from:
idea that to provide cells that can, differentiate / divide;
for (continued) growth of the, shoot tip / (named) tissues ;
I 'to produce more meristematic cells'
one mark per row
stages: F and G, description: cell, elongates / enlarges / grows and, a vacuole forms / makes a tonoplast;
stages: G and H, description: cell divides (longitudinally) and one of the cells has a vacuole ;
stages: H and J, description: developing sieve tube cell, elongates / enlarges / grows and loses its nucleus or one (developing companion) cell divides (transversely) to form (two) companion cells ; A companion cell divides
any four from:
1 elongated cells to form, long tubes / AW ;
2 little, cytoplasm / cell contents / fewer organelles, to reduce resistance to flow ; A peripheral cytoplasm / no nucleus, to allow transport of maximum volume of, (named) assimilates / sap / nutrients A 'more space for .....' R no organelles R 'no cell contents'
3 sieve (plates have) pores, so little barrier to flow from cell to cell / easy (for phloem sap) to pass from cell to cell / allows mass flow ;
4 sieve plates, support / stop collapse of/ stop bulging of, sieve tube elements; A become plugged with, P-protein / callose, to prevent losses / after damage A maintain hydrostatic pressure (in sieve tubes)
5 plasmodesmata between sieve tube and companion cell for ease of, loading / unloading / AW ;
6 AVP; e.g. membrane around sieve tube for osmosis to occur / prevent loss of (named) assimilates
allow assimilates/AW for sucrose
1 movement of sucrose from, mesophyll / parenchyma / source, cells ; A movement of sucrose to (named) sink cells
2 move sucrose, into / out of, sieve tubes (through plasmodesmata);
3 pump, protons / hydrogen ions / H+, out of cell/ into cell wall/into apoplast;
A ref. to secondary active transport
4 provides, ATP / proteins, for sieve tubes ; A maintain metabolism of sieve tubes