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Transport in Plants - Translocation

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Translocation is the movement of organic solutes, primarily sucrose (C12H22O11C_{12}H_{22}O_{11}) and amino acids, through the phloem from sources to sinks.

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A 'Source' is a site of synthesis or mobilization of organic molecules (e.g., mature leaves or storage organs during spring), while a 'Sink' is a site of utilization or storage (e.g., roots, fruits, and developing buds).

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Phloem loading involves the active transport of H+H^+ ions out of companion cells using ATP. This creates an electrochemical gradient that drives the co-transport of sucrose into the companion cell-sieve tube complex via a H+H^+/sucrose symporter.

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The Mass Flow Hypothesis explains movement via a hydrostatic pressure gradient. High sucrose concentration at the source lowers the water potential (Psi\\Psi), causing water (H2OH_2O) to enter from the xylem by osmosis, increasing turgor pressure (PP).

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At the sink, sucrose is unloaded, increasing the water potential (Psi\\Psi) and causing H2OH_2O to move out. This creates a pressure difference (DeltaP\\Delta P) between the source and the sink, driving the bulk flow of phloem sap.

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Phloem tissue consists of sieve tube elements (lacking nuclei and ribosomes to reduce resistance) and companion cells (highly metabolic cells that support the sieve tubes).

📐Formulae

Ψ=Ψs+Ψp\Psi = \Psi_s + \Psi_p

ΔP=Psource−Psink\Delta P = P_{source} - P_{sink}

C12H22O11+H2O→C6H12O6+C6H12O6C_{12}H_{22}O_{11} + H_2O \rightarrow C_6H_{12}O_6 + C_6H_{12}O_6

💡Examples

Problem 1:

Calculate the water potential (Ψ\Psi) in a sieve tube at the source if the solute potential (Ψs\Psi_s) is −2.5 MPa-2.5\text{ MPa} and the hydrostatic pressure (Ψp\Psi_p) is 0.6 MPa0.6\text{ MPa}.

Solution:

Using the formula Ψ=Ψs+Ψp\Psi = \Psi_s + \Psi_p: Ψ=−2.5 MPa+0.6 MPa=−1.9 MPa\Psi = -2.5\text{ MPa} + 0.6\text{ MPa} = -1.9\text{ MPa}

Explanation:

The total water potential is the sum of the negative pressure exerted by solutes and the positive hydrostatic pressure generated by the entry of water into the rigid cell wall container.

Problem 2:

Explain the role of H+H^+ ions in the loading of sucrose into the phloem.

Solution:

ATPATP is used by proton pumps to move H+H^+ out of the companion cell. The resulting high concentration of H+H^+ in the cell wall creates a gradient. H+H^+ then diffuses back into the cell through a co-transporter protein, bringing sucrose (C12H22O11C_{12}H_{22}O_{11}) with it against its concentration gradient.

Explanation:

This is a form of secondary active transport. The kinetic energy of the H+H^+ moving down its gradient provides the energy needed to move sucrose into the phloem.

Translocation - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology