Roots and vascular transport: structure and function
Organisation • Plant tissues, organs and systems
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Root hair cells: structure and function
Root hair cells have a long thin extension that increases the surface area of the root in contact with the soil, allowing greater water and mineral uptake . Root hair cells have thin cell walls and a large permanent vacuole, which reduces the diffusion distance and maintains a strong water potential gradient so osmosis moves water into the cell efficiently . High numbers of root hairs across roots maximise total uptake surface area and thereby increase water absorption from soil .
Uptake mechanisms: osmosis and active transport
Water enters root hair cells by osmosis because soil solution typically has a higher water potential than the cell sap; water moves from high to low water potential across a partially permeable membrane . Mineral ions enter root hair cells by active transport when external concentrations are lower than internal concentrations; the cells use metabolic energy to move ions against the concentration gradient and so maintain the solute levels required for growth and metabolism . Active uptake of ions increases the concentration of solutes in root cells, which lowers cell water potential and therefore increases water uptake by osmosis .
Xylem structure and water/mineral transport
Xylem vessels consist of dead cells joined end-to-end to form continuous hollow tubes that contain no end walls or cell contents, which allows unimpeded one-way flow of water and dissolved mineral ions from roots to leaves . Secondary cell walls are reinforced with lignin, which provides structural support and prevents vessel collapse under the negative pressure generated by transpiration . Continuous evaporation of water from leaf air spaces and stomata creates tension that pulls water up through xylem in a transpiration stream; cohesion between water molecules and adhesion to xylem walls maintain the continuous column of water .
Phloem structure and translocation
Phloem tissue contains living sieve tube elements aligned end-to-end with porous sieve plates, which permit flow of cell sap between elements; companion cells provide metabolic support and ATP for phloem loading and unloading . Translocation moves dissolved sugars (mostly sucrose) from source tissues (photosynthesising leaves) to sink tissues (roots, growing shoots or storage organs) by a pressure-flow mechanism: active loading of sugars at sources raises solute concentration, water influx by osmosis increases turgor pressure, and unloading at sinks reduces pressure so bulk flow moves sap along the sieve tubes fileciteturn0file2turn0file4.
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