Plants: Water and Transport

Water potential moves from high to low.water from one cell with a high water potential moves to another cell with a low water potential

[solute] is inverse to [water]-as solute increases water potential decreases
as pressure decreases (cell wall expands) water potential decreases as well, allowing water to flow in easier

As the cell wall is degraded by enzymes, water potential decreases pump transports sodium out of the cytoplasm into the surrounding solution increasing water potential

soil: water fills crevices between soil particles (solute driven)-relatively few solutes-not under much pressure-water potential tends to be higher (relative to water in plant's roots
roots: high in solutes (solute driven)-under considerable turgor pressure-water potential is less than water potential in soil
air: water potential extremely low relative to the moist interior of a leaf
water potential starts high (soil) goes to low (air)
-plants gain water from soil and lose it in atmosphere

When soil dries...-water no longer fills the spaces between soil particles -instead of free floating water, all water that is left adheres tightly to soil particles ----creates negative pressure that lowers pressure potential of soil water
-when water potential in soil is low water is less likely to move from soil into plants

wilting: an entire tissue loses turgor turgor: cells that are firm and that experience wall pressure
when cells in a leaf or stem lose water to the atmosphere faster than it can be replaced then the plasma membrane contracts and the cells shrivel --if cells do not regain turgor-- risk for dehydration-- wilt

Apoplastic:cell walls, which are porous and spaces that exist between cells
Symplast: continuos connection through cells that exists via plasmodesmata
Casparian strip: narrow band of wax secreted endodermal cells tightly packed--composed of suberin which forms a water repellent cylinder at the endodermis
-temporarily blocks apoplastic pathway by preventing water from creeping around the walls of endodermal cells and going into vascular tissue
-for water and solutes to reach vascular tissue--have to cross membrane of an endodermal cell (acts as a filter)

Positive Pressure (push to force water up)-as water flows into xylem from other root cells it forces fluid up the xylem
-enough water can move to force water droplets out of the leaves--guttation
Negative Pressure( Pull force)-creating low pressure at the top
-cohesion-tension theory: negative force generated at air-water interface is transmitted through the water outside of the leaf cells----water in xylem goes into water in vascular tissues into roots and then water into soil
--continuous transmission of pulling force possible 1. water present throughout plant2. water is hydrogen bonded to one another in a continuous fashion

-water is pulled to the tops of trees along a water potential gradient via forces generated by transpiration at leaf surfaces
1. spaces in the middle of the leaf-- fill with moist air due to evaporation from the surfaces of the surrounding cells (water diffuses from inside of the leaf to the atmosphere)
2. water leaves leaf-- humidity of the spaces inside the leaf drops causing water to evaporate
3. Tension pulls water that surrounds nearby cells out of the xylem
4. tension from water in leaf pulls water through the stem all the way to the root xylem by cohesion (continuous H-bonding)
----water molecules pulled up because the tension at the surface is much greater than H-bonding that pulls molecules to the side or down
5. Tension pulls water from root cortex cells--root xylem
6. Tension pulls water from soil into roots

Daily changes in diameter of tree trunks supports CT-theory?--expand at night, shrink during the day-tension created in xylem is great enough to make tree trunks shrink in diameter-on hot summer days, xylem is under enough tension to shrink tree trunk slightly-expand at night when no transpiration occurs
Do daily changes in the water potential of leaf tissue support CT?--added pressure to root systems of corn using root bomb-released pressure on root system (altering transpiration rates)--increased transpiration during the day lead to lower water potential in shoot tissues-- low water potential enough to pull water up from roots and soil (where there is high water potential)-increased transpiration---- increased tension on xylem sap

Sources: any tissue, site, or location where a substance is produced or enters circulation
Sink: any tissue, site, or location where an element or a molecule is consumed or taken out of circulation
-During growing season:-mature leaves/stems that are actively photosynthesizing and producing sugar( SOURCES)
------moves to sinks [sugar use is HIGH and production is LOW]
-apical meristem-lateral meristem-developing leaves-flowers-developing seeds and fruits- storage cells in roots
-Early in growing season:-storage cells in roost act as sources and developing leaves act as sinks

Xylem--conducts water and dissolved ions from the root system to the shoot system--vessel-elements-dead-short,wide-have perforations -tracheids-pits (hole in primary and secondary cell wall)-dead
Phloem-- conducts sugar, amino acid, chemical signals and other substances throughout the plant body -sieve-tube elements -alive at maturity-long, thin, cells that have perforated ends (sieve plates)-lack nuclei
-companion cells-provide materials to maintain the cytoplasm and plasma membrane of sieve-tube members

1. Sucrose enters companion cells along with protons2. Membrane protein in companion cells hydrolyzes ATp and uses the energy release to transport H+ across the membrane to the exterior of the cell 3. Increase proton pump or H+-ATPases-establish large differences in charge and proton concentration on the two sides of the membrane
-electrochemical gradient favors entry of protons into the cell
-co-transporter-- protons and sucrose enter the cell together -protons along electrochemical gradient - sucrose against its concentration gradient

-water potential in the phloem in the source is high -sinks have a low water potentialFLOWS FROM SOURCE TO SINK
-water from the xylem flows into the phloem when there is a high concentration of sugar near the source
-creates a high pressure near the source
-high pressure drives sugar down from source to sink
-water leaves phloem back into xylem when the sugar is actively pumped out

Phloem is part of trees back and sugars can't be transported between shoots and roots
-if no phloem... roots will starve (HIGH sugar concentration in the phloem)
-no phloem...1. no way for sugars to be transported2. cutting cambium containing xylem and phloem means eventually tree cannot receive water and nutrients