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

Structure of Transport Tissues (Cambridge International AS & A Level Biology – Topic 7)

Plants move water, minerals and organic nutrients through two specialised vascular tissues:

• Xylem – conducts water and dissolved mineral ions from roots to aerial parts.
• Phloem – distributes photosynthates (mainly sucrose) from sources (leaves, young stems) to sinks (roots, fruits, growing tips).

1. Distribution of Xylem and Phloem in Dicots

In dicot herbaceous plants the vascular bundles are arranged as a radial cylinder. The pattern is the same in stems, roots and leaves, but the relative positions of xylem and phloem differ slightly.

Organ (transverse section)	Relative position of xylem & phloem	Diagram (plan view)
Stem	Central cylinder (stele) contains several vascular bundles arranged in a ring. Each bundle: phloem (outside) – cambium – xylem (inside). Ground tissue (cortex & pith) occupies the inter‑bundle region.
Root	Vascular cylinder is central; xylem occupies the dorsal (upper) side, phloem the ventral (lower) side. Both are surrounded by pericycle and then the endodermis.
Leaf (mid‑rib)	Vascular bundles are collateral: phloem on the abaxial (lower) side, xylem on the adaxial (upper) side. Bundles are embedded in mesophyll; the mid‑rib shows a clear central vein.

2. Xylem Vessel Elements – Structure & Function

Vessel elements are the principal water‑conducting cells of xylem in most angiosperms. Their anatomy is a direct response to the need for rapid, low‑resistance bulk flow under negative pressure.

• Elongated, tube‑like cells – several millimetres long; provide a long, continuous conduit.
• Thick, lignified secondary walls – give mechanical rigidity and prevent collapse when the water column is under tension.
• Perforation plates at both ends; in mature vessels these are simple pores that allow uninterrupted flow between adjacent elements.
• Absence of protoplasm (dead at maturity) – the lumen is empty, minimising hydraulic resistance.
• Bordered pits in lateral walls – enable lateral water movement between neighbouring vessels and tracheids, and provide a route for pit‑membrane‑mediated cavitation repair.

These features create a rigid, low‑resistance pipeline that can transmit the tension generated by transpiration (cohesion‑tension theory).

3. Phloem Sieve‑Tube Elements – Structure & Function

Sieve‑tube elements form the conducting strand of phloem. Their design permits rapid, bidirectional movement of dissolved sugars and signalling molecules.

• Sieve plates – modified end walls with numerous sieve pores that maintain cytoplasmic continuity while allowing bulk flow.
• Reduced cytoplasm – most organelles (nucleus, vacuole, large plastids) are absent; a thin layer of cytoplasm lines the wall, lowering resistance.
• Callose deposits – can be rapidly laid down at sieve pores to seal the tube after injury.
• Plasmodesmata to companion cells – numerous symplastic connections enable exchange of sugars, amino acids, ATP and signalling molecules.

The combination of large‑diameter tubes, porous sieve plates and a thin cytoplasmic layer maximises the volumetric flow rate predicted by the Hagen–Poiseuille equation.

4. Companion Cells – Structure & Function

Companion cells are specialised parenchyma cells that remain alive and metabolically active, supporting the largely anucleate sieve‑tube elements.

• Large nucleus and abundant organelles – mitochondria (ATP supply), rough ER and ribosomes (protein synthesis), and a well‑developed Golgi apparatus (production of transport proteins).
• Dense plasmodesmata network – provides a high‑capacity symplastic pathway for loading/unloading sugars and signalling molecules.
• High surface‑to‑volume ratio – facilitates rapid exchange with surrounding phloem parenchyma and with the xylem‑derived water entering the sieve tube.

Companion cells actively transport sucrose into the sieve tube (via H⁺‑sucrose symporters), creating the osmotic gradient that drives the pressure‑flow mechanism.

5. Structure → Function Summary

6. The Pressure‑Flow (Mass‑Flow) Model

1. Loading at source – Companion cells actively import sucrose into sieve‑tube elements (H⁺‑sucrose symport). This raises solute concentration C, lowers water potential Ψ_w, and draws water from the adjacent xylem.
2. Generation of turgor pressure – Influx of water increases turgor pressure ΔP in the source region.
3. Transport – The pressure gradient between source (high ΔP) and sink (low ΔP) drives bulk flow of sap through the sieve tubes.
4. Unloading at sink – Sucrose is removed (active or passive) by sink tissues, decreasing C, raising Ψ_w, and causing water to exit the phloem back into the xylem or apoplast.
5. Continuation – As long as sources load and sinks unload, the pressure differential is maintained, allowing continuous flow.

Quantitatively, the volumetric flow rate Q can be approximated by the Hagen–Poiseuille equation:

\$\$

Q = \frac{\pi r^{4}}{8\eta}\,\frac{\Delta P}{L}

\$\$

where r = radius of the sieve tube, η = viscosity of the phloem sap, and L = length of the transport pathway. The large radius and low resistance of sieve tubes, together with active loading by companion cells, maximise Q.

7. Suggested Diagrams for Exam Answers

• Plan view of a dicot stem, root and leaf transverse section showing the radial arrangement of xylem and phloem.
• Close‑up labelled diagrams of:

  • Vessel element (perforation plate, lignified wall, bordered pits).
  • Sieve‑tube element (sieve plate, reduced cytoplasm, callose).
  • Companion cell (nucleus, mitochondria, ER, Golgi, plasmodesmata).

• Simple schematic of the pressure‑flow model indicating source, sink, loading, unloading and direction of bulk flow.

These sketches, together with concise bullet‑point descriptions, satisfy the Cambridge International AS & A Level Biology requirement to “draw plan diagrams, label cell types and relate structure to function”.