Cambridge A-Level Biology 9700 – Transport Mechanisms
Transport Mechanisms in Plants
Learning Objective
State that some mineral ions and organic compounds can be transported within plants dissolved in water.
Overview
Plants rely on the movement of substances in solution to distribute nutrients, hormones, and metabolites. Water acts as the solvent, carrying dissolved ions and organic molecules through specialised pathways.
Key Pathways for Solute Transport
Apoplastic route – movement through cell walls and intercellular spaces.
Symplastic route – movement from cell to cell via plasmodesmata.
Xylem transport – upward movement of water and dissolved mineral ions from roots to shoots.
Phloem transport – bidirectional movement of organic compounds (primarily sugars) and some ions.
Mineral Ions Transported in Solution
Mineral ions are taken up by root hairs and enter the xylem as part of the transpiration stream. They remain in solution because they are ionic and highly soluble in water.
Ion
Primary Function
Typical Concentration in Xylem Sap (µM)
K⁺
Regulates stomatal opening, enzyme activation
100–500
Ca²⁺
Cell wall stability, signalling
10–50
Mg²⁺
Chlorophyll synthesis, enzyme co‑factor
5–20
NO₃⁻
Primary nitrogen source
200–1000
PO₄³⁻
Energy transfer, nucleic acids
10–30
Organic Compounds Transported in Solution
Organic molecules such as sugars, amino acids, and hormones are soluble in the aqueous phase of the phloem sap. Their movement is driven by pressure‑flow (mass flow) mechanisms.
Compound
Role in Plant
Typical Concentration in Phloem Sap (mM)
Sucrose
Major transport carbohydrate
0.5–1.0
Amino acids (e.g., glutamine)
Protein building blocks
0.1–0.3
Auxin (IAA)
Growth regulator
≈0.001
Gibberellins
Stem elongation, seed germination
≈0.0005
Mechanism Summary
Water is absorbed by root cells through osmosis.
Mineral ions dissolve in this water and enter the xylem vessels.
Transpiration creates a negative pressure, pulling the solution upward.
Organic compounds are synthesised in source tissues (e.g., leaves) and loaded into the phloem.
High osmotic pressure in the source region draws water in, generating a pressure gradient that drives bulk flow toward sink tissues.
Key Points to Remember
Both mineral ions and organic compounds are transported as aqueous solutions.
The solubility of these substances in water allows them to move rapidly over long distances.
Different pathways (xylem vs. phloem) are specialised for distinct classes of solutes.
Understanding the transport of dissolved substances is essential for explaining nutrient distribution, growth regulation, and stress responses.
Suggested diagram: Flow of water, mineral ions, and organic compounds through xylem and phloem, showing apoplastic and symplastic routes.