Chapter 36 Transport in Vascular Plants

Root hairs

Extensions of root epidermal cells; dramatically increase absorptive surface area; primary site of water and mineral uptake

Mycorrhizae

Mutualistic association between plant roots and fungi; fungal hyphae extend far into soil; greatly increase water and phosphorus uptake; ~90% of plant species

Apoplast

Continuum of cell walls and extracellular spaces; water moves through without crossing membranes; fast but non-selective

Symplast

Continuum of cytoplasm connected by plasmodesmata; selective water and solute movement between cells

Plasmodesmata

Channels through plant cell walls connecting adjacent cytoplasms; allow symplastic transport

Endodermis

Innermost layer of root cortex; contains the Casparian strip; controls what enters vascular cylinder

Casparian strip

Band of suberin (waxy material) in endodermal cell walls; blocks apoplast at endodermis; forces selective symplastic transport

Pericycle

Cell layer just inside the endodermis; source of lateral root development

Proton pumps (H⁺-ATPase)

Membrane proteins pumping H⁺ out of cells; create electrochemical gradient driving mineral ion uptake via cotransporters

Root pressure

Positive pressure in xylem caused by active ion pumping into roots → water follows osmotically; responsible for guttation

Guttation

Exudation of water droplets from leaf margin pores (hydathodes); caused by root pressure when transpiration is low

Cohesion-tension theory

Model explaining water movement up xylem; transpiration creates tension; cohesion of water molecules keeps column intact

Transpiration

Evaporation of water from leaf surfaces through stomata; creates the tension that drives xylem water transport

Cohesion

Tendency of water molecules to stick together via hydrogen bonds; keeps xylem water column intact under tension

Adhesion

Tendency of water to stick to hydrophilic xylem walls; contributes to capillary action and column support

Cavitation

Formation of an air bubble in xylem disrupting water flow; occurs when tension exceeds the strength of the water column; worsened by drought

Guard cells

Pairs of specialized cells flanking stomata; become turgid (open) or flaccid (closed) to regulate gas exchange

Stomatal opening mechanism

Blue light → H⁺-ATPase → K⁺ influx → water enters by osmosis → guard cells turgid → stoma opens

ABA (abscisic acid)

Plant hormone produced under drought stress; signals guard cells to close stomata by triggering K⁺ efflux

Stomatal closing

Triggered by darkness, high CO₂, drought, or ABA; K⁺ leaves guard cells → water leaves → cells flaccid → stoma closes

Source (phloem)

Any organ that produces or releases sugar into the phloem; e.g. mature leaves, storage organs releasing starch

Sink (phloem)

Any organ that uses or stores sugar from the phloem; e.g. roots, growing shoots, fruits, seeds

Phloem loading

Active transport of sucrose into sieve tubes at the source using H⁺ pumps and sucrose-H⁺ cotransporters; requires ATP

Phloem unloading

Removal of sucrose from sieve tubes at the sink; lowers osmotic pressure, causing water to leave phloem

Pressure-flow hypothesis

Model of phloem transport; high osmotic pressure (from sugar loading) at source drives bulk flow of sap toward low-pressure sink

Bulk flow

Mass movement of fluid driven by a pressure gradient; the mechanism of phloem transport in the pressure-flow model