state that some mineral ions and organic compounds can be transported within plants dissolved in water

Transport in Plants – Cambridge IGCSE/A‑Level (Topic 7)

Learning Objective

State that mineral ions and organic compounds are transported within plants dissolved in water, and describe the structures and mechanisms that enable this transport.

1. Structure of Transport Tissues (Syllabus 7.1)

1.1 Xylem – water‑conducting tissue

• Tracheids – long, tapered cells with thick lignified walls and pits; provide tensile strength and a continuous water‑conducting pathway.
• Vessel elements – short, wide cells with perforation plates; together form vessels that give high hydraulic conductivity.
• Fibres – heavily lignified, dead cells that reinforce the vascular strand.
• Parenchyma – living cells that store nutrients and allow radial movement of solutes.

1.2 Phloem – food‑conducting tissue

• Sieve‑tube elements – elongated, living cells; ends are cut off and connected by sieve plates (large pores) that allow bulk flow of sap.
• Companion cells – closely associated with each sieve‑tube element; rich in mitochondria and contain H⁺‑ATPases that power active loading and unloading of solutes.
• Phloem fibres – provide mechanical support.
• Phloem parenchyma – store carbohydrates and other metabolites.

1.3 Typical Distribution in Plant Organs

• Stem (dicot transverse section) – central xylem (often a star‑shaped ring) surrounded by phloem; both are enclosed by the pericycle and cortex.
• Root – central stele with xylem in the centre and phloem surrounding it; the endodermis with its Casparian strip forms the inner boundary of the cortex.
• Leaf – vascular bundles with xylem on the upper side of the bundle (adaxial) and phloem on the lower side (abaxial); bundles are embedded in mesophyll.

2. Water Potential and the Casparian Strip (Syllabus 7.2)

Water moves from higher to lower water potential (Ψ_w). The water‑potential equation is:

Ψ_w = Ψ_s + Ψ_p

• Ψ_s (solute potential) – always negative; the more solutes dissolved, the lower Ψ_s.
• Ψ_p (pressure potential) – positive when turgor or external pressure is present.

The Casparian strip (suberin‑filled band in the radial walls of endodermal cells) blocks the apoplastic pathway at the endodermis, forcing water and dissolved ions to cross a plasma membrane (symplastic route). This gives the plant selective control over which ions enter the stele and therefore the xylem.

3. Pathways for Solute Transport (Syllabus 7.2)

• Apoplastic route – movement through cell walls and intercellular spaces; no cytoplasmic contact.
• Symplastic route – movement from cell to cell via plasmodesmata; requires crossing a membrane at the Casparian strip.

4. Xylem Transport – Mineral Ions in Solution

1. Root hairs absorb water by osmosis; dissolved mineral ions follow the water into the root cortex (apoplastic).
2. The Casparian strip forces the water‑ion solution into the symplast, allowing selective uptake of ions.
3. Ions enter the xylem vessels of the central stele.
4. Transpiration from stomata creates a negative pressure (Ψ_p) in the leaf xylem; together with cohesion of water molecules, this generates the cohesion‑tension (transpiration‑pull) mechanism that pulls the aqueous solution upward.

5. Phloem Transport – Organic Compounds in Solution

5.1 Loading at the Source (Leaves)

1. H⁺‑ATPase in the plasma membrane of companion cells pumps H⁺ out, establishing an electrochemical gradient (low external Ψ_p, high external Ψ_s).
2. Sucrose‑H⁺ symporters use this gradient to import sucrose against its concentration gradient into companion cells.
3. In many species sucrose is converted to raffinose‑family oligosaccharides, lowering cytoplasmic sucrose concentration and maintaining the gradient.
4. The rise in solute concentration lowers Ψ_s in the sieve‑tube element; water follows osmotically, raising Ψ_p and generating a pressure gradient.

5.2 Pressure‑Flow (Mass‑Flow) Mechanism

• High Ψ_p at the source pushes the sap toward regions of lower Ψ_p (sinks).
• At sinks (roots, growing buds, fruits) sucrose and other solutes are removed (active or passive unloading), raising Ψ_s locally, allowing water to exit the phloem and lowering Ψ_p.

6. Integrated Step‑by‑Step Summary (Matches Syllabus Language)

1. Root uptake – water enters root hairs osmotically; mineral ions dissolve in the water.
2. Radial movement – water‑ion solution travels apoplastically until the Casparian strip; then it crosses the plasma membrane (symplastic route) into the stele.
3. Xylem ascent – ions enter xylem vessels; transpiration pull together with cohesion‑tension draws the aqueous solution upward.
4. Source loading (leaf) – photosynthesis produces sucrose; H⁺‑ATPase in companion cells creates a gradient that drives sucrose‑H⁺ symport (active loading).
5. Generation of pressure gradient – loading lowers Ψ_s, water enters, raising Ψ_p in sieve‑tube elements.
6. Mass flow – the pressure difference between source (high Ψ_p) and sink (low Ψ_p) drives bulk flow of the aqueous sap through the phloem.
7. Sink unloading – sucrose and other organic solutes are removed; water exits the phloem, completing the pressure‑flow cycle.

7. Key Points to Remember (Exam‑Friendly)

• Both mineral ions (xylem) and organic compounds (phloem) travel dissolved in water – water is the universal solvent.
• The Casparian strip forces radial transport into the symplast, giving the plant control over ion entry.
• Xylem transport relies on transpiration pull and the cohesion‑tension mechanism; it moves mainly upward.
• Phloem transport relies on active loading (H⁺‑ATPase, sucrose‑H⁺ symport) and the pressure‑flow (mass‑flow) mechanism; it can move in any direction depending on source–sink relationships.
• Students must be able to draw and label a xylem vessel element, a phloem sieve‑tube element with companion cell, and the typical arrangement of xylem/phloem in stems, roots, and leaves.