36.3 TRANSPIRATION DRIVES TRANSPORT OF WATER AND MINERALS FROM ROOTS

TO SHOOTS

AMARISA MILES

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

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

OVERVIEW OF TRANSPIRATION

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

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

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

TRANSPORT OF WATER AND MINERALS FROM ROOT HAIRS TO THE XYLEM

1. Apoplastic route

2. Symplastic route

3. Transmembrane route

4. The endodermis

5. Transport in the xylem

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

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)

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

COHESION-TENSION HYPOTHESIS

ASCENT OF XYLEM SAP

Figure 36.13Page 775 in textbook