Water Movement

main driver of water movement in plant

tension created via transpiration

5 forces that move water

diffusion, osmosis,capillary forces, hydrostatic pressure, gravity

diffusion

molecules in fluids move from areas of higher to areas of lower concentrations

osmosis

diffusion of water across a differentially permeable membrane• In cells, it creates hydrostatic pressure(turgor)

capillary forces

cohesion and adhesion

cohesion and adhesion

These forces can create large tension (negative hydrostatic pressure) They can flatten the water and pull it into small spaces such as between cells or soil particles. In narrow tubes, they combine into capillary forces capable of pulling water up.

The leaf-transpiration

• Intercellular air spaces in the leaves are near equilibrium with the solution in the cellulose – this means they are nearly saturated with water vapor

• In comparison, the air outside is quite dry!

• Water vapor diffuses from the leaf to the atmosphere via the open stomata

• To prevent excessive diffusion, leaves form a boundary layer: a relatively undisturbed gradient of water vapor

leaf transpiration cond.

Larger the boundary layer → gentler gradient → slower rate of diffusion

-Wind makes the boundary layer thinner, increasing transpiration

-Plant adaptations to prevent extreme water loss due to wind include:

• Dense layer of trichomes

• Stomatal crypts – or sunken stomata

leaf transpiration cond. cond.

In most plants, stomatal opening is partly controlled by light.

-Under illumination, the concentration of solutes in the vacuoles of the guard cells increases:

• Starch is converted to malic acid

• Proton pumps expel H+, creating a gradient across the plasma membrane:

• This leads to K+ and Cl- to move into the cell → osmosis brings water in → turgor

inside the leaf

Loss of water vapor reduces the water potential of intercellular spaces ->The increase in ΔΨ leads to evaporation of water from cell walls

-> Drier cell walls have increased tension, bringing water from surrounding areas (including inside and outside cells)When ΔΨ is transferred to cells near vascular tissue,water will move in from the xylem

vascular tissue- the stem

As water moves from xylem to mesophyll in the leaves, hydrogen bonds pull in the rest of the water column (cohesion) - transferring the tension.

• Adhesion also transfer the tension to the walls of the xylem - but walls are too rigid to move.

-The amount of tension that can be induced is inversely proportional to the cross-sectional area:

• Tracheids can support greater tension than vessel elements!

root transport

Eventually, water loss from the xylem in the leaves, reduces the water potential of xylem in the roots

-The xylem is directly connected to the apoplast of the stele (central region inside the endodermis)

-Water loss from the stele is replaced with water from the cortex, which in turn is replaced with water from the soil:

• Because of the casparian strip, water must cross 2 plasma membranes to get to the xylem

water in soil

The same capillary forces affecting water inside the plant also affect water in the soil

• Most water is in capillary spaces between soil particles, held by hydrogen bonds

• As water is drawn into the roots, the hydrostatic tension in soil around the root increases, drawing in water from soil particles further away.

• If soil is too dry, the hydrostatic pressure is too large for plants to be able to obtain water -permanent wilting!