Section 3A Concise

Flowering Plants: Transport

• Key Concepts

  • Adaptations for resource acquisition were critical for vascular plant evolution (36.1)

  • Various transport mechanisms function over short/long distances (36.2)

  • Transpiration drives water and mineral transport from roots to shoots via xylem (36.3)

  • Stomata regulate transpiration rate (36.4)

  • Sugars are transported through phloem from sources to sinks (36.5)

Adaptations for Acquiring Resources

• Evolution of xylem and phloem enabled long-distance transport (36.1)

• Xylem: transports water/minerals from roots to shoots

• Phloem: transports photosynthetic products from sources to sinks

Transport Routes for Water and Solutes

• Transport routes:

  • Apoplastic route: through cell walls and extracellular spaces

  • Symplastic route: through cytosol, requires one plasma membrane crossing

  • Transmembrane route: across membranes multiple times

• Bulk flow necessary for efficient long-distance transport (36.2)

  • Occurs in tracheids, vessel elements (xylem) and sieve-tube elements (phloem)

Water and Mineral Absorption

• Absorption mostly occurs at root tips via root hairs

• Root hairs absorb soil solution, water, and minerals into the cortex

• Active transport accumulates essential minerals (e.g., K+, NO3-)

Transport of Water and Minerals to Xylem

• Endodermis: last checkpoint for selective mineral passage into vascular cylinder

  • Minerals in symplast pass through plasmodesmata

  • Minerals in apoplast face Casparian strip barrier

• Only minerals entering symplast may cross endodermis

• Endodermis transports essential minerals into xylem, blocking toxic substances

Xylem Transport Mechanism

• Water and minerals move from endodermal protoplasts to cell walls

• Xylem conducts water and minerals to the shoot by bulk flow

• Tracheids and vessel elements lack protoplasts and are part of the apoplast

Practice Question

• The primary function of all leaves is photosynthesis. True or False?

  • Answer: TRUE