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

Structure of Transport Tissues

The vascular system of plants consists of two specialised transport tissues: xylem, which conducts water and mineral ions, and phloem, which distributes organic nutrients. Understanding how the cellular architecture of these tissues underpins their function is essential for A‑Level biology.

1. Xylem \cdot essel Elements

Vessel elements are the primary conductive cells of xylem. Their structure is adapted to move large volumes of water with minimal resistance.

• Elongated, tube‑like cells – length can be several millimetres, providing a long continuous conduit.
• Thick, lignified secondary walls – give mechanical strength and prevent collapse under negative pressure.
• Perforation plates at the ends of each element allow uninterrupted flow; in mature vessels these plates are often simple pores.
• Absence of protoplasm – the cell contents die at maturity, leaving a hollow tube that reduces resistance.
• Presence of pits – bordered pits in the lateral walls enable lateral water movement between adjacent vessels and tracheids.

These structural features enable the generation of a continuous water column that can be pulled upward by transpiration‑driven tension (cohesion‑tension theory).

2. Phloem Sieve Tube Elements

Sieve tube elements form the conducting strands of the phloem. Their design facilitates the rapid, bidirectional transport of photosynthates.

• Sieve plates – porous end walls composed of sieve pores that allow cytoplasmic continuity between adjacent elements.
• Reduced cytoplasm – most organelles are absent, leaving a thin layer of cytoplasm that reduces resistance to flow.
• Callose deposits – can be laid down at sieve pores to block transport in response to injury.
• Plasmodesmata connections – numerous plasmodesmata link sieve tube elements to companion cells, permitting extensive symplastic exchange.

The combination of sieve plates and a thin cytoplasmic layer creates a low‑resistance pathway for the bulk flow of solutes driven by osmotic pressure gradients.

3. Companion Cells

Companion cells are specialised parenchyma cells that are intimately associated with sieve tube elements. Their structure is tailored to support the metabolic needs of the otherwise largely anucleate sieve tubes.

• Large nucleus and abundant organelles – mitochondria, endoplasmic reticulum, and ribosomes provide ATP and proteins required for phloem loading and unloading.
• Numerous plasmodesmata – dense connections to the sieve tube element allow rapid transfer of sugars, amino acids, and signaling molecules.
• High surface‑to‑volume ratio – facilitates efficient exchange with the surrounding phloem parenchyma and xylem.

Companion cells actively load sucrose into the sieve tube using ATP‑dependent transporters, establishing the osmotic gradient that drives mass flow.

4. Structure–Function Summary

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

The pressure‑flow hypothesis explains phloem transport as follows:

1. Photosynthate (mainly sucrose) is actively loaded into sieve tube elements by companion cells, increasing the solute concentration \$C\$.
2. The rise in \$C\$ lowers the water potential \$\Psi_w\$ inside the sieve tube, causing water to enter osmotically from the adjacent xylem.
3. Inflow of water generates a turgor pressure increase \$\Delta P\$ at the source region.
4. At sink tissues, sucrose is unloaded, reducing \$C\$, raising \$\Psi_w\$, and causing water to exit the phloem.
5. The resulting pressure gradient \$\Delta P\$ drives bulk flow of the sap from source to sink.

Mathematically, the volumetric flow rate \$Q\$ can be expressed as:

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

where \$r\$ is the radius of the sieve tube, \$\eta\$ the viscosity of the sap, and \$L\$ the length of the transport pathway. The structural adaptations of sieve tubes (large radius, low resistance) maximise \$Q\$.