7.1.3—Xylem and phloem cell drawings

Xylem vessel elements and phloem sieve-tube elements are specialised conducting cells, while companion cells maintain and support the associated sieve-tube elements. Their mature structures differ because water/mineral transport and assimilate transport impose different constraints.

Xylem vessel elements — water and mineral-ion pathway

• Large vessel elements join end to end; when mature they have no protoplasm and no end plates, leaving a relatively open path for upward mass flow.
• Thick lignified walls strengthen the vessel and help it resist collapse under the pressures involved in water transport; pits allow lateral water movement between conducting regions.
• The mature conducting element is therefore structurally strong and low in internal obstruction, matching one-way transport from roots towards the rest of the plant.

Phloem sieve-tube elements — assimilate pathway

• Sieve-tube elements join end to end to form a continuous tube. Sieve plates with pores connect neighbouring elements, allowing organic assimilates to move through the tube.
• Mature sieve-tube elements lack a nucleus, ribosomes and a large vacuole; thin cytoplasm leaves more space for assimilate flow. Their cellulose wall supports the tube without the lignified xylem pattern.
• Phloem transport can occur in either direction in the plant, according to source-to-sink relationships, rather than being a fixed upward stream.

Companion cells — sieve-tube support

• Each sieve-tube element is associated with a companion cell containing a nucleus and other organelles, including many mitochondria.
• Plasmodesmata link the companion cell to its sieve-tube element; transport proteins and ATP supply support loading and unloading of assimilates and the metabolism of the conducting partner.

Read the structures as an aligned comparison: conducting cell state — mature xylem vessel elements are non-living and open, whereas sieve-tube elements remain living but have reduced internal organelles; wall/connection — xylem uses lignified walls, no end plates and pits, whereas phloem uses cellulose walls and sieve plates; transport — xylem carries water and dissolved mineral ions mainly upward, while phloem carries organic assimilates up or down according to source and sink; support — xylem structure directly resists collapse, while companion-cell organelles and plasmodesmata support sieve-tube metabolism and loading/unloading.

Do not call companion cells the conducting tube or treat sieve plates as xylem end plates. Do not infer that every xylem cell has the same appearance: this objective centres on vessel elements, sieve-tube elements and their companion cells. Staff drawing brief: compare labelled longitudinal/transverse sketches of a vessel element, sieve-tube element and companion cell, showing only syllabus-supported walls, pores, organelles and connections; do not generate or bind an image.