The Chapter 29 Homework is due on Monday, March 30 at 11:59 pm The Chapter 29 Test is on Tuesday, March 31.

Transport and Water Potential Chapter 29 Transport and Water Potential

You Must Know How passive transport, active transport, and cotransport function to move materials across plant cell membranes. The role of water potential in predicting movement of water 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 7

Video clip of water transport.

Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss. Sugarcane Corn Switch grass C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound. An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon even when CO2 concentrations are low. These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle. CAM plants open their stomata at night, incorporating CO2 into organic acids. Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle. © 2014 Pearson Education, Inc. 9

Figure 29.5 Solute transport across plant cell plasma membranes Sucrose (neutral solute) (b) H+ and cotransport of neutral solutes H+/sucrose cotransporter S H+ CYTOPLASM EXTRACELLULAR FLUID H+ Hydrogen ion H+ H+ H+ H+ H+ H+ Proton pump H+ (a) H+ and membrane potential Nitrate (c) H+ and cotransport of ions H+/NO3− cotransporter NO3− H+ Potassium ion Ion channel (d) Ion channels K+ Plasma membrane permeability controls short-distance movement of substances. Both active and passive transport occur in plants. In plants, membrane potential is established through pumping H by proton pumps. Plant cells use the energy of H gradients to cotransport other solutes by active transport. Plant cell membranes have ion channels that allow only certain ions to pass. 10

Short-Distance Transport of Water Across Plasma Membranes To survive, plants must balance water uptake and loss. Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure. © 2014 Pearson Education, Inc. 11

Water potential determines the direction of movement of water. Water potential is a measurement that combines the effects of solute concentration and pressure. Water potential determines the direction of movement of water. Water flows from regions of higher water potential to regions of lower water potential. Potential refers to water’s capacity to perform work. © 2014 Pearson Education, Inc. 12

Water potential is abbreviated as  and measured in a unit of pressure called the megapascal (MPa)   0 MPa for pure water at sea level and at room temperature.   S  P Both pressure and solute concentration affect water potential. This is expressed by the water potential equation:   S  P. The solute potential (S) of a solution is directly proportional to its molarity. Solute potential is also called osmotic potential. Pressure potential (P) is the physical pressure on a solution. © 2014 Pearson Education, Inc. 13

Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast. Turgid Flaccid The protoplast is the living part of the cell, which also includes the plasma membrane. Water potential affects uptake and loss of water by plant cells. If a flaccid (limp) cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis. Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall. If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid (firm). Turgor loss in plants causes wilting, which can be reversed when the plant is watered. © 2014 Pearson Education, Inc. 14

(b) Initial conditions: cellular   environmental  Figure 29.6b Initial flaccid cell: P S  −0.7 Pure water:   −0.7 MPa P S  Turgid cell at osmotic equilibrium with its surroundings   0 MPa P S  0.7 −0.7 Figure 29.6b Water relations in plant cells (part 2: turgid cell)   0 MPa (b) Initial conditions: cellular   environmental  15

(a) Initial conditions: cellular   environmental  Figure 29.6a Initial flaccid cell: P S  −0.7 0.4 M sucrose solution:   −0.7 MPa  Plasmolyzed cell at osmotic equilibrium with its surroundings P S −0.9   −0.9 MPa P S  −0.9  Figure 29.6a Water relations in plant cells (part 1: plasmolyzed cell)  −0.9 MPa (a) Initial conditions: cellular   environmental  16