Transpiration and Unusual Plants Chapter 29 Transpiration and Unusual Plants

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

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. © 2014 Pearson Education, Inc. 3

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. © 2014 Pearson Education, Inc. 4

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. © 2014 Pearson Education, Inc. 5

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

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

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. © 2014 Pearson Education, Inc. 8

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. © 2014 Pearson Education, Inc. 9

Ascent of xylem sap Water molecule Root hair Soil particle Water Water uptake from soil 10

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

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

Outside air  = -100.0 MPa Leaf  (air spaces) = -7.0 MPa Figure 29.18 Xylem sap Outside air  Mesophyll cells = -100.0 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

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