9.3 Transport in Plants

Plants require efficient systems to move water, minerals, and organic compounds between different parts. Transport in plants occurs through two main vascular tissues: xylem (water and minerals) and phloem (organic solutes).

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1. Pathways of Water Absorption in Roots

Water absorbed by root hair cells can travel to the xylem via three pathways:

Apoplast Pathway

• Water moves through non-living components: cell walls and intercellular spaces.
• Does not cross any plasma membrane.
• Faster but unregulated.

Symplast Pathway

• Water moves through the living cytoplasm of cells.
• Cells are connected by plasmodesmata (cytoplasmic channels through cell walls).
• Allows selective regulation of solute movement.

Vacuolar Pathway

• Water moves from vacuole to vacuole through tonoplasts and plasma membranes.
• Considered a sub-type of the symplast pathway.

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2. The Casparian Strip

The Casparian strip is a band of suberin (a waxy, hydrophobic substance) deposited in the radial and transverse walls of endodermal cells in the root.

Function

• Acts as a barrier that blocks the apoplast pathway at the endodermis.
• Forces all water and dissolved minerals to pass through the symplast (living cytoplasm) of endodermal cells.
• This selective checkpoint allows the plant to control which minerals enter the xylem.

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3. Ascent of Sap — TACT Theory

Once water enters the xylem, it must be transported upward — sometimes to heights exceeding 100 m in tall trees. The TACT theory explains this passive process:

The result is a continuous, unbroken column of water from roots to leaves — the cohesion-tension mechanism.

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4. Root Pressure

Root pressure is a positive hydrostatic pressure that develops in the xylem of roots.

Mechanism

1. Mineral ions are actively transported into the xylem by root cells (requires ATP).
2. This lowers the water potential of xylem sap.
3. Water enters the xylem by osmosis from surrounding cells.
4. The accumulation of water creates a positive pressure that pushes sap upward.

Significance

• Contributes to ascent of sap, especially in short plants and at night when transpiration is minimal.
• Responsible for guttation — the exudation of water droplets from leaf tips through hydathodes.

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5. Stomatal Mechanism — $K^{+}$ Ion Influx Theory

Stomata are pores in the leaf epidermis surrounded by two guard cells. Their opening and closing regulates transpiration and gaseous exchange.

Stomatal Opening

1. In light, guard cells produce malic acid and actively pump $K^{+}$ ions inward from subsidiary cells.
2. Increased $K^{+}$ concentration lowers the water potential of guard cells.
3. Water enters guard cells by osmosis, increasing turgor pressure.
4. Due to the thicker inner wall, guard cells bow outward → stoma opens.

Stomatal Closing

1. In darkness or water stress, $K^{+}$ ions are pumped out of guard cells.
2. Water potential of guard cells rises → water leaves by osmosis.
3. Guard cells become flaccid → stoma closes.

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6. Translocation in Phloem — Pressure-Flow Hypothesis

Organic solutes (primarily sucrose) produced in leaves are transported to other parts of the plant through the phloem (sieve tube elements).

Mechanism (Mass Flow / Pressure-Flow)

At the Source (e.g., photosynthesizing leaves):

1. Sucrose is actively loaded into sieve tube elements from companion cells (requires ATP).
2. This lowers water potential in sieve tubes → water enters by osmosis.
3. High turgor pressure develops at the source end.

At the Sink (e.g., roots, fruits, storage organs):

1. Sucrose is unloaded from sieve tubes (actively or passively).
2. Water potential rises → water leaves sieve tubes.
3. Low turgor pressure at the sink end.

Result: A turgor pressure gradient from source (high) to sink (low) drives bulk flow of phloem sap.

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Summary Table