Transport in multicellular plants

• The need for, and functioning of, a transport system in multicellular plants

Learning Outcomes

Candidates should be able to:

(a) explain the need for transport systems in multicellular plants and animals in terms of size and surface area to volume ratios;

(b) define the term transpiration (see section 5) and explain that it is an inevitable consequence of gas
exchange in plants;

(c) [PA] describe how to investigate experimentally the factors that affect transpiration rate;

(d) [PA] describe the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants;

(e) [PA] describe the structure of xylem vessel elements, phloem sieve tube elements and companion cells and be able to recognise these using the light microscope;

(f) relate the structure of xylem vessel elements, phloem sieve tube elements and companion cells to their functions;

(g) explain the movement of water between plant cells, and between them and their environment, in terms of water potential (no calculations involving water potential will be set);

(h) describe the pathways and explain the mechanisms by which water is transported from soil to xylem and from roots to leaves (includes reference to the symplast/symplastic pathway and apoplast/apoplastic pathway);

(i) outline the roles of nitrate ions and of magnesium ions in plants;

(j) [PA] describe how the leaves of xerophytic plants are adapted to reduce water loss by transpiration;

(k) explain translocation as an energy-requiring process transporting assimilates, especially sucrose,
between the leaves (sources) and other parts of the plant (sinks);

(l) explain the translocation of sucrose using the mass flow hypothesis;