The Chapter 29 Homework is due on Thursday, March 14 The Chapter 29 Homework is due on Thursday, March 14. (We will be completely skipping section 29.3.) Integrating Chapter 29 Homework – due Tuesday, March 12 at 11:59 pm There will be a Test on Chapter 29 on Friday, March 15.

Chapter 29 Transpiration

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

Concept 29.1: Adaptations for acquiring resources were key steps in the evolution of vascular plants The success of plants depends on their ability to gather and conserve resources from their environment. The transport of materials is central to the integrated functioning of the whole plant. The evolution of adaptations enabling plants to acquire resources from both above and below ground sources allowed for the successful colonization of land by vascular plants. The algal ancestors of land plants absorbed water, minerals, and CO2 directly from surrounding water. Early nonvascular land plants lived in shallow water and had aerial shoots. Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport. © 2014 Pearson Education, Inc. 4

Figure 29.2-1 The plant does not have to expend any energy to transport water through the xylem H2O Xylem What substances do plants need for photosynthesis? The evolution of xylem and phloem in land plants made possible the development of extensive root and shoot systems that carry out long-distance transport. Xylem transports water and minerals from roots to shoots. What substances do plants need for respiration? H2O and minerals 5

O2 CO2 H2O O2 H2O and minerals CO2 Figure 29.2-2 Figure 29.2-2 An overview of resource acquisition and transport in a vascular plant (step 2) O2 H2O and minerals CO2 6

Phloem transports photosynthetic products from sources to sinks. Figure 29.2-3 CO2 O2 Light Sugar H2O Phloem transports photosynthetic products from sources to sinks. O2 H2O and minerals CO2 Video: water transport 7

60 L of water during a growing season! 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. 60 L of water during a growing season! A single maize plant transpires 60 L of water during a growing season. Peak velocities in the transport of xylem sap can range from 15 to 45 m/hr. 15 to 45 mph! © 2014 Pearson Education, Inc. 8

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. 9

Cell compartments and routes for short-distance transport Cell wall Apoplastic route Cytosol Symplastic route Transmembrane route Key Plasmodesma Water can cross the cortex via the symplast or apoplast. Apoplast Plasma membrane Symplast 10

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 Symplastic route Root hair 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. 3 Transmembrane route Epidermis Endodermis Vascular cylinder (stele) Cortex 5 Transport in the xylem 11

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, 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. Xylem sap is normally under negative pressure, or tension. © 2014 Pearson Education, Inc. 12

Ascent of xylem sap Water molecule Root hair Soil particle Water Water uptake from soil 13

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. 14

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. 15

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  16