Plant Circulation and Transport

1. Plant Circulation and Transport Chapter 25

2. Xylem • primary xylem • apical meristemsprocambium primary xylem • formed early in development • herbaceous and woody plants • transport of water • secondary xylem • formed later in development from vascular cambium • woody plants only (annual rings in wood) • structural support; fair amount of transport • xylem is a complex tissue • tracheidsand vessel elements • transport of H2O • hollow and dead at maturity; only cell walls remain • stacked on top of each other, forming pipelines • parenchyma cells • storage of H2O and minerals • xylem fibers • provide structural support

3. Fig. 24.6 Xylem structure

4. how is H2O moved upwards in a plant? • many trees are very tall and H2O moves againstgravity • root pressure • H2O moves into plant’s roots from soil (osmosis) • H2O pressure in roots increases and pushes H2O up • not strong enough to account for rise of H2O in tall trees • strongest influence is in small plants in spring • cohesion-tension model • often begins with root pressure • plants only use a fraction of the H2O they absorb • most evaporates into the air and is lost • water moves up plants through transpiration • evaporation of water from plant parts exposed to air • H2O is pulled up by the drying power of air • top H2O molecules are evaporated off by transpiration • this E pulls up the rest of the water column • evaporation occurs through the stomata in leaves • water moves up a plant (roots  stems  leaves) • transpiration creates a negative pressure in xylem • extends downward from the leaves to roots • similar to sucking liquid up a straw

5. The process of transpiration

6. rate of transpiration • regulated by stomata and their guard cells • affected by env. factors • wind, heat, dryness • only works if H2O column is unbroken and narrow • cohesion and adhesion Fig. 25.12 Cohesion-tension model of xylem transport

7. Phloem • transport in phloem is much slower than in xylem • primary phloem • apical meristemsprocambium primary phloem • formed early in development • herbaceous and woody plants • transport of nutrients (sugars) • secondary phloem • formed later in development from vascular cambium • found only in woody plants (part of the bark) • structural support; fair amount of transport • phloem is a complex tissue • sieve-tube members • transport nutrients • function as living cells, lack a nucleus and most organelles • stacked on top of each other, forming pipelines • companion cells • lie adjacent to sieve-tube members • continually nourish sieve-tube members  keeps them alive • one companion cell per sieve-tube member • parenchyma cells – storage of nutrients • phloem fibers – provide structural support

8. Fig. 24.7 Phloem structure

9. materials flowing through phloem • mainly sugars (sucrose) • small amounts of other materials • proteins, hormones, chemical defenses, lipids, wastes, etc. • phloem sap • how nutrients are moved within a plant • phloem sap moves from sources to sinks • sources • areas where sap is produced or stored • leavesand roots (winter only) • sinks • areas that require sap • all other areas of the plant • phloem sap moves primarily down from the leaves

10. phloem sap moves through translocation • accomplished through a fluid pressure-flow mechanism • process • involves buildup of sap at sources  transported into phloem • phloem loading • involves release of sap at sinks  nutrients to tissues • phloem unloading • sap moves from high pressure at sources to low pressure at sinks • inflow of H2O from neighboring xylem cells assists the process • ATP needed for this is provided by companion cells • sieve-tube members, companion cells, and xylem all work together

11. The process of translocation

12. Translocation

13. Fig. 25.15 Pressure-flow model of phloem transport (translocation)