State the functions of xylem and phloem in transport of water, mineral salts and food.

Transport in Plants (IGCSE 0610 – Section 8)

Learning Objective

State the functions of xylem and phloem in the transport of water, mineral salts and food, and explain the pathways and mechanisms involved.

Key Points (exam‑style)

• Xylem transports water and dissolved mineral ions from roots to shoots (a one‑way system).
• Phloem transports organic nutrients (mainly sucrose) from source tissues to sink tissues (bidirectional flow).
• Both tissues form continuous vascular bundles in roots, stems and leaves.

Diagram Checklist (Core requirement)

• Root cross‑section – label epidermis, cortex, endodermis (showing the Casparian strip), pericycle, xylem (central), phloem (peripheral).
• Stem cross‑section – label pith, vascular bundle (xylem towards the centre, phloem towards the outside), cortex, epidermis.
• Leaf cross‑section – label upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis with stomata, and a vein showing xylem on the upper side of the vein and phloem on the lower side.

Structure – Function Links

Xylem

• Composed of long, dead vessel elements or tracheids with thick lignified walls and no cross‑walls – form continuous hollow tubes.
• Lignified walls give mechanical support and prevent collapse under the negative pressure generated by transpiration.
• The hollow, water‑filled tubes allow bulk flow of water and mineral ions.

Phloem

• Living sieve‑tube elements (lacking nuclei) closely associated with companion cells.
• Companion cells supply ATP for active loading and unloading of sugars.
• Sieve plates with pores permit movement of solutes while maintaining turgor pressure.

Functions of Xylem (exam‑style statements)

• Transports water absorbed by root hairs upward to leaves and growing points (transpiration pull).
• Carries mineral salts dissolved in the water from the soil to all aerial parts of the plant.
• Provides mechanical support because its cells are lignified and rigid.
• Operates as a **one‑way system** – movement is from roots → shoots only.

Functions of Phloem (exam‑style statements)

• Transports the products of photosynthesis (mainly sucrose) from source organs (e.g., mature leaves) to sink organs (e.g., roots, young leaves, fruits, seeds).
• Distributes other organic compounds such as amino acids, hormones and some minerals.
• Allows **bidirectional flow**; the direction depends on the relative position of source and sink.
• Consists of living cells (sieve‑tube elements with companion cells) that remain functional during transport.

Pathway of Water Through the Plant (Core requirement)

1. Root‑hair uptake – water moves into root‑hair cells by osmosis (soil water → root‑hair cell).
2. Radial movement through cortex – proceeds either:
   • Apoplast route: through cell walls and intercellular spaces.
   • Symplast route: from cell to cell via plasmodesmata.
3. Endodermis & Casparian strip – the Casparian strip blocks the apoplast, forcing water to cross the plasma membrane into the symplast, ensuring selective uptake of mineral ions.
4. Entry into the stele – water reaches the pericycle and then the xylem vessels of the central stele.
5. Ascent through the stem – water moves upward in the xylem by the cohesion‑tension mechanism (transpiration pull) and, to a lesser extent, by root pressure.
6. Leaf distribution & transpiration – water leaves the xylem into mesophyll cells, evaporates from stomata, and diffuses into the atmosphere.

Transpiration and Its Driving Forces

• Creates a water‑potential gradient: Ψsoil > Ψroot > Ψstem > Ψleaf > Ψair.
• Cohesion between water molecules and adhesion to xylem walls transmit the pull (cohesion‑tension theory).
• Factors that increase transpiration: high temperature, low relative humidity, wind, and large leaf surface area.
• Root pressure – generated by active uptake of ions into the xylem – can push water upward, especially at night or when transpiration is low.

Phloem Loading, Pressure‑Flow Mechanism and Unloading

1. Phloem loading (source) – sucrose is actively transported into sieve‑tube elements, usually via companion cells (ATP‑dependent).
2. Generation of turgor pressure – accumulation of solutes lowers the water potential, causing water to enter by osmosis and raise hydrostatic pressure in the source region.
3. Bulk flow (pressure‑flow) – the pressure gradient drives mass flow of sap from high‑pressure source to low‑pressure sink.
4. Phloem unloading (sink) – sucrose is removed from the sieve tube (active or passive), water follows osmotically, and pressure in the sink region falls.

Comparison of Xylem and Phloem

Suggested Investigation (optional)

• Water‑movement experiment – Add a few drops of food‑colouring to the watering solution of a potted bean plant. After 24 h, cut a stem cross‑section and observe the coloured front in the xylem to illustrate upward water movement.
• Phloem‑transport experiment – Place a leaf in a solution of ¹⁴C‑labelled sucrose, then after a suitable period detect radioactivity in distant sink tissues (e.g., roots). This demonstrates the pressure‑flow mechanism.

Summary

Xylem and phloem are the two specialised vascular tissues that sustain plant life. Xylem moves water and mineral salts upward from the roots, providing hydration, nutrient supply and structural support, and it operates as a one‑way system. Phloem distributes the organic products of photosynthesis to all parts of the plant, supporting growth, storage and metabolism, and it can transport in either direction depending on the location of sources and sinks. Mastery of their structures, functions, and the underlying transport mechanisms is essential for the Cambridge IGCSE Biology syllabus.