Download 36.3 Transpiration

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 36.13
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