what is transpiration pull theory?

Answer 1

ASCENT of XYLEM SAP:
Water transported up from the roots must replace water lost by transpiration.

WATER POTENTIAL: Xylem sap rises against gravity, driven by a gradient of water potential (*) Water flows from an area of high * to an area of low *. Water Potential is expressed in units of pressure: 1 bar = pressure needed to push a column of water up 10 meters. 1 megapascal= 10 bars. Pure water has * = 0. Addition of pressure increases *. Addition of solutes decreases *.
ROOT PRESSURE: When transpiration is low, ions pumped into the stele decrease * and cause water uptake by the stele. This uptake force is called root pressure. Cannot keep pace with transpiration, can force water up only a few meters.

TRANSPIRATION-COHESION-ADHESION THEORY:
a). water leaves leaf through stomata.
b). this water loss is replaced by evaporation from mesophyll cells, lowering their water potential, causing them to take water from neighboring cells.
c). Process continues back to the tracheids causing water up take from the xylem sap.
d). Water goes fro tracheids to air following a water potential gradient.
e). Cohesion and adhesion of water. H bonds of water, hydrophilic walls of xylem and small diameter of xylem aid in the movement of water up the tube.
f). This pull decreases water pressure in the xylem causing the roots to take water from the soil.

How Stomata Open and Close:
Turgid guard cells open the stomata, while flaccid cells close them.
The K+ ion is responsible for the stomatal action.
Uptake of K+ causes the cell to become turgid- decreases water potential.
Stomata open at dawn because light induces the cells to take in K+. An internal clock (circadian rhythm) will make them open even if in they are kept in the dark.
Guard cells will close due to: a water deficiency, High temperatures due to an increase of CO2

Answer 2

The Cohesion Tension theory: Given by Dixon in 1914.
This Theory has essentially three parts.
1. Continous Water Column: The water runs as a continous column through the tracheary.
2. Cohesion: The water molecules remain attached to each other due to the hydrogen bonds. This is called the Cohesion force. Its theoretical value is 1500 atm, but experimental value is 45 - 207 atm. The water column doesnot break because of an additional force called adhesion, between the walls of tracheary and the water molecules.
3. Transpirational Pull: The mesophyll cells are always saturated wit water vapors. As the stomata open to carry out transpiration, a vacuum is created in the cell, this empty space is filled by the water that rises due to the tension experienced by it due to the loss of water in the mesophyll cells.

Answer 3

transpiration is the sweating equivilent for plants.. water evaporates from the plant's leaves due to heat/exposure to air/etc

transpiration pull is something like when u dont have a pump, u suck water from one end of the hose and when it comes out, the water just flows by it self

that is, when water evaporates from the top of the plant/leaves, a suction like force is created cos vaccum canot be sustained by it self, thus the suction 'sucks' water from the bottom--the roots and the roots intake water from the surroundings

Answer 4

Simply, it occurs due to difference in the osomtic gradient. So that water moves from high water potential ( the leaf ) to low water potential ( the atmosphere ). That's why light, temerature, wind and humidity affect the rate of transpiration.
If u need more information about that, u can e-mail me or IM me.

Answer 5

correct answering already done

Answer 6

See: http://en.wikipedia.org/wiki/Transpiration_pull