Transport in Plants: Question 8
Syllabus 8.1, 8.4
A student cuts a thin cross-section through a plant stem and, under a microscope, identifies two different vascular tissues within the same vascular bundle: one made of dead, hollow, lignified vessels, and the other made of living cells connected end to end through perforated cross-walls, each living cell lying next to a smaller companion cell.
(a) State two structural differences between the xylem vessels and the phloem sieve tube elements described above. [2]
(b) State the main substances carried by each tissue, and the direction in which each transports its main substances through the plant. [2]
(c) Sieve tube elements are living cells, but each one has a companion cell attached to it, connected by many fine strands of cytoplasm. Explain why sieve tube elements need a companion cell alongside them, given the structural differences you described in (a). [3]
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Worked solution
Part (a): Structural differences
The xylem vessels are dead cells: once mature, they have no cell contents at all, and adjoining vessel cells have no cross walls between them, forming one continuous open tube. Their walls are also thickened with lignin.
The phloem sieve tube elements, by contrast, are living cells, but adjoining sieve tube elements are separated by cross walls that are perforated (sieve plates), rather than having no cross wall at all, and their walls are not lignified in the same way.
Part (b): Substances carried and direction of transport
Xylem carries water and dissolved mineral ions. It transports these in one direction only: upward, from the roots, through the stem, to the leaves.
Phloem carries dissolved sucrose and amino acids. Unlike xylem, its direction of transport is not fixed. It moves substances from a source, where they are produced or released (such as a photosynthesising leaf), to a sink, where they are used or stored (such as a growing shoot tip or a root). Because a sink can lie above or below a source, phloem can transport these substances upward or downward, depending on the plant’s needs at the time.
Part (c): Why sieve tube elements need a companion cell
Because sieve tube elements are living but have very reduced cell contents compared with an ordinary cell, they are poorly equipped to carry out all the cellular activities needed to keep themselves alive and to actively load and move sucrose along the phloem, a process that requires energy. The companion cell, attached alongside each sieve tube element and linked to it by fine strands of cytoplasm, retains a nucleus and has many mitochondria. It can therefore carry out the metabolic activities, including releasing energy by respiration, that the sieve tube element cannot efficiently perform for itself, effectively supporting and keeping the sieve tube element functioning.
Final answers
- (a) Xylem vessels are dead with no cross walls and lignified walls; sieve tube elements are living, connected by perforated sieve plates, and not lignified.
- (b) Xylem: water and mineral ions, always upward. Phloem: sucrose and amino acids, direction depends on the position of the source and sink (up or down).
- (c) Sieve tube elements have reduced cell contents and cannot carry out all their own metabolic activities, so the companion cell, with its nucleus and mitochondria, supplies the energy and cellular support they need.