1. Transpiration and Unusual Plants
Chapter 29
Transpiration and Unusual Plants

2. You Must Know
How the transpiration cohesion-tension mechanism explains water movement in plants.

3. Epiphytes, Parasitic Plants, and Carnivorous Plants
Some plants have nutritional adaptations that use other organisms in nonmutualistic ways.
Three unusual adaptations are
Epiphytes
Parasitic plants
Carnivorous plants
Indian pipe, a nonphoto-
synthetic parasite of
mycorrhizae
Pitcher plants
Epiphytes grow on other plants and obtain water and minerals from rain, rather than tapping their hosts for sustenance.
Parasitic plants absorb water, sugars, and minerals from their living host plant.
Some species also photosynthesize, but others rely entirely on the host plant for sustenance.
Some species parasitize the mycorrhizal hyphae of other plants. Carnivorous plants are photosynthetic but obtain nitrogen by killing and digesting mostly insects.
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4. Plants can move a large volume of water from their roots to shoots.
Concept 29.5: Transpiration drives the transport of water and minerals from roots to shoots via the xylem
Plants can move a large volume of water from their roots to shoots.
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5. Absorption of Water and Minerals by Root Cells
Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water.
Root hairs account for much of the absorption of water by roots.
After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals.
The concentration of essential minerals is greater in the roots than in the soil because of active transport.
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6. Cell compartments and routes for short-distance transport
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Water can cross the cortex via the symplast or apoplast.
Plasmodesma
Apoplast
Plasma membrane
Symplast 6

7. Vessels (xylem) Casparian strip 1 Apoplastic route Plasma membrane
Figure 29.16a
Vessels
(xylem)
Casparian strip 1
Apoplastic
route
Plasma
membrane
Apoplastic
route 1 2 3
Symplastic
route 2 5
Water and minerals can travel to the vascular cylinder through the cortex via
The apoplastic route, along cell walls and extracellular spaces.
The symplastic route, in the cytoplasm, moving between cells through plasmodesmata .
The transmembrane route, moving from cell to cell by crossing cell membranes and cell walls.
The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder.
Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder.
The endodermis regulates and transports needed minerals from the soil into the xylem.
Water and minerals move from the protoplasts of endodermal cells into their cell walls.
Diffusion and active transport are involved in this movement from symplast to apoplast.
Water and minerals now enter the tracheids and vessel elements.
Symplastic
route
Root
hair 3
Transmembrane
route
Epidermis
Endodermis
Vascular
cylinder
(stele)
Cortex 5
Transport in the xylem 7

8. Bulk Flow Transport via the Xylem
Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow, the movement of a fluid driven by pressure.
The transport of xylem sap involves transpiration, the loss of water vapor from a plant’s surface.
Transpired water is replaced as water travels up from the roots.
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9. Pulling Xylem Sap: The Cohesion-Tension Hypothesis
According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots.
Xylem sap is normally under negative pressure, or tension.
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10. Ascent of xylem sap Water molecule Root hair Soil particle Water
Water uptake from soil 10

11. Adhesion by hydrogen bonding Xylem cells Cell wall Cohesion
Figure 29.18b
Adhesion by hydrogen
bonding
Xylem
cells
Cell wall
Cohesion
by hydrogen
bonding
Cohesion and adhesion in the ascent of xylem sap: Water molecules are attracted to each other through cohesion.
Cohesion makes it possible to pull a column of xylem sap. Water molecules are attracted to hydrophilic walls of xylem cell walls through adhesion. Adhesion of water molecules to xylem cell walls helps offset the force of gravity.
Cohesion
and adhesion
in the xylem 11

12. Xylem sap Mesophyll cells Stoma Water molecule Atmosphere
Figure 29.18c
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Transpiration
Transpirational pull is generated when water vapor in the air spaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata.
As water evaporates, the air-water interface retreats farther into the mesophyll cell walls and becomes more curved.
Due to the high surface tension of water, the curvature of the interface creates a negative pressure potential.
This negative pressure pulls water in the xylem into the leaf.
The pulling effect results from the cohesive binding between water molecules.
The transpirational pull on xylem sap is transmitted from leaves to roots. 12

13. Outside air  = -100.0 MPa Leaf  (air spaces) = -7.0 MPa
Figure 29.18
Xylem sap
Outside air 
Mesophyll cells =
MPa
Stoma
Leaf  (air spaces)
Water molecule =
-7.0 MPa
Atmosphere
Transpiration
Leaf  (cell walls)
Adhesion by hydrogen
bonding =
-1.0 MPa
Xylem
cells
Cell wall
Water potential gradient
Trunk xylem 
Cohesion
by hydrogen
bonding
−0.8 MPa 
Cohesion
and adhesion
in the xylem
Water molecule
Trunk xylem 
Root hair
−0.6 MPa 
Soil particle
Water
Soil 
Water uptake from soil
−0.3 MPa  13

14. Integrating Chapter 29 (Pearson Web Page) – due Thursday 11:59 pm