1. 36.3 Transpiration drives transport of water and minerals from roots to shoots
Amarisa Miles

2. Terms to know
Apoplast – everything external to the plasma membrane of a plant cell, including cell walls, intercellular spaces, and the space within dead structures such as xylem vessels and tracheids
Endodermis – in plant roots, the innermost layer of the cortex that surrounds the vascular cylinder. Last checkpoint for selective passage of minerals from cortex into vascular cylinder.
Stele – the vascular tissue of a stem or root
Casparian strip – a water-impermeable ring of wax in the endodermal cells of plants that blocks the passive flow of water and solutes into the stele by way of cell walls
Cohesion-tension hypothesis – Explains the ascent of xylem sap. States that transpiration exerts pull on xylem sap, creating tension, and that the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots
Print corresponding Word document for this slide on flash drive

3. Absorption of Water and Minerals by Root Cell
Root hairs absorb the soil solution
Drawn into hydrophilic walls of epidermal cells
Passed along cell wall and extracellular space into root cortex
Soil solution made up of water molecules and dissolved mineral ions not tightly bound to soil particles
Epidermal cells located at end of roots

4. Overview of Transpiration

5. Transport of Water Minerals into the Xylem
Water and minerals enter xylem before being transported to the rest of the plant
Minerals already in the symplast when they reach the endodermis continue through the plasmodesma of endodermal cells and pass into the vascular cylinder
Xylem located in vascular cylinder
Plasmodesma is an open channel through the cell wall that connects the cytoplasm of adjacent plant cells allowing water, small solutes, and some larger molecules to pass between the cells

6. Transport of Water Minerals into the Xylem
Minerals that reach the endodermis via the apoplast encounter the Casparian strip that blocks their passage into the vascular cylinder
Works by forcing water and minerals to cross the plasma membrane of the endodermal cell before they can enter the vascular cylinder

7. Transport of Water Minerals into the Xylem
Endodermis prevents solutes in the xylem from leaking into the soil solution
Acts as an apoplastic barrier between the cortex and vascular cylinder
Transportation needs minerals from the soil in xylem
Keeps many unneeded or toxic substances out
An apoplastic barrier is

8. Transport of water and minerals from root hairs to the xylem
Apoplastic route
Symplastic route
Transmembrane route
The endodermis
Transport in the xylem
Soil solution is taken in by the hydrophilic walls of root hairs and allowed into the apoplast where water and minerals are then diffused into the cortex along the cell walls and extracellular spaces into the xylem
Minerals and water cross the plasma membranes of root hairs rather than moving around cell walls. The water and minerals are taken to the symplast.
Soil solution moves along the apoplastic route and along the way releases some water and minerals into the protoplasts of cells of the epidermis and cortex as they continue to move inward through the symplast.
This is a route that only minerals already in the symplast or entering the symplastic route can travel by crossing the plasma membrane of an endodermal cell and detours around the Casparian strip and right into the vascular cylinder.
Endodermal cells and other living cells within the vascular cylinder discharge water and minerals into their apoplastic walls. The xylem vessels then transport the water and minerals by bulk flow upward into the shoot system.

9. Bulk Flow Transport via the Xylem
From the vascular cylinder, xylem sap is transported long distances by bulk flow to the veins of each leaf
Bulk flow is much faster than diffusion or active transport
Process of transporting xylem sap involves transpiration
Transpiration is important because transpired water must be replaced by absorbed water, or the plant will die

10. root pressure
Root cells pump mineral ions into xylem while Casparian strip prevents ions from leaking into the cortex and soil
Results in accumulation of minerals, lowering the water potential in vascular cylinder
Water flows in from root cortex, creating root pressure and pushing xylem sap
Root pressure can cause water to enter leaves which then is transpired which results in guttation
Only pushes water a few meters since positive pressures are overcome by negative pressures (gravity, tension)
1. Guttation is the release of water droplets that can be seen in the morning on the tips or edges of some plant leaves (not to be confused with dew which is condensed atmospheric pressure)

11. The Cohesion- Tension Hypothesis
Explains how xylem sap can travel against gravity up to the shoots of a plant
Transpiration of water off of the leaves provides a pull (tension) for the ascent of the xylem sap
The cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots

12. Cohesion-Tension Hypothesis
The title is Transpiration but it really just explains the cohesion-tension hypothesis

13. Ascent of Xylem Sap Figure 36.13 Page 775 in textbook
Hydrogen bonding forms an unbroken chain of water molecules extending from leaves to the soil. The force driving the ascent of the xylem sap is a gradient of water potential. For bulk flow over long distances, the water potential gradient is due to a gradient of the pressure potential. Transpiration results in the pressure potential at the end of the leaf of the xylem being lower than the pressure potential at the root end. The water potential may vary during daylight but the direction of the water potential gradient remains the same.
Figure 36.13
Page 775 in textbook