Chapter 25: Transport of Water and Nutrients in Plants AP Biology

Plants require nutrients and water 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 © 2011 Pearson Education, Inc.

CO2 O2 Light Sugar H2O O2 H2O and minerals CO2 Figure 36.2-3 Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2 H2O and minerals CO2 3

Adaptations for acquiring resources were key steps in the evolution of vascular plants The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the 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 transportThe evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis Xylem transports water and minerals from roots to shoots Phloem transports photosynthetic products from sources to sinks © 2011 Pearson Education, Inc.

Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss © 2011 Pearson Education, Inc.

Plants Acquire Mineral Nutrients from Soil Ion exchange makes nutrients available to plants Soil organisms contribute to plant nutrition N-fixing bacteria (Rhizobium) Mycrorrhizae (symbiotic association with fungi) © 2011 Pearson Education, Inc. 6

Water and Solutes are Transported in the Xylem by Transpiration-Cohesion-Tension Differences in Water potential govern the direction of water movement (see next slides for review) Water and ions move across the root cell plasma membrane Membrane proteins required!! (need to move across hydrophobic membrane and often against gradient) Aquaporins Ion channels and proton pumps Ion exchange makes nutrients available to plants © 2011 Pearson Education, Inc. 7

Water Potential and 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 © 2011 Pearson Education, Inc.

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 Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa) Ψ = 0 MPa for pure water at sea level and at room temperature Water moves in the direction from higher water potential to lower water potential © 2011 Pearson Education, Inc.

How Solutes and Pressure Affect Water Potential Both pressure and solute concentration affect water potential This is expressed by the water potential equation: Ψ  ΨS  ΨP The solute potential (ΨS) of a solution is directly proportional to its molarity Solute potential is also called osmotic potential © 2011 Pearson Education, Inc.

Pressure potential (ΨP) is the physical pressure on a solution Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast The protoplast is the living part of the cell, which also includes the plasma membrane © 2011 Pearson Education, Inc.

Figure 36.8 Solutes have a negative effect on  by binding water molecules. Positive pressure has a positive effect on  by pushing water. Solutes and positive pressure have opposing effects on water movement. Negative pressure (tension) has a negative effect on  by pulling water. Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium H2O H2O H2O H2O Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm: Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm: In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water: Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm: Figure 36.8 Effects of solutes and pressure on water potential () and water movement. Positive pressure Positive pressure Negative pressure Pure water Solutes Solutes Membrane H2O H2O H2O H2O

Water Movement Across Plant Cell Membranes Water potential affects uptake and loss of water by plant cells If a flaccid 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 © 2011 Pearson Education, Inc.

Aquaporins: Facilitating Diffusion of Water Aquaporins are transport proteins in the cell membrane that allow the passage of water These affect the rate of water movement across the membrane © 2011 Pearson Education, Inc.

Water and Solutes are Transported in the Xylem by Transpiration-Cohesion-Tension Differences in Water potential govern the direction of water movement (see next slides for review) Water and ions move across the root cell plasma membrane Membrane proteins required!! (need to move across hydrophobic membrane and often against gradient) Aquaporins Ion channels and proton pumps Ion exchange makes nutrients available to plants © 2011 Pearson Education, Inc. 16

Short-Distance Transport of Solutes Across Plasma Membranes 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 In animals, membrane potential is established through pumping Na by sodium-potassium pumps Plant cell membranes have ion channels that allow only certain ions to pass © 2011 Pearson Education, Inc.

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 surface area of 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 soil because of active transport © 2011 Pearson Education, Inc.

Different mechanisms transport substances over short or long distances Three transport routes for water and solutes are: The apoplastic route, through cell walls and extracellular spaces The symplastic route, through the cytosol (plasmodesmata) The transmembrane route, across cell membrane This requires the use of transport proteins to allow passage of anything but gases Allows for selectivity!!! (only point in plant transport where selectivity can occur) © 2011 Pearson Education, Inc.

Cell wall Apoplastic route Cytosol Symplastic route Figure 36.6 Cell wall Apoplastic route Cytosol Symplastic route Transmembrane route Key Figure 36.6 Cell compartments and routes for short-distance transport. Plasmodesma Apoplast Plasma membrane Symplast

Water and ions pass to the xylem by way of the Apoplast and symplast: The apoplast consists of everything external to the plasma membrane It includes cell walls, extracellular spaces, and the interior of vessel elements and tracheids The symplast consists of the cytosol of the living cells in a plant, as well as the plasmodesmata Water can cross the cortex via the symplast or apoplast 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 © 2011 Pearson Education, Inc.

Pathway along apoplast Figure 36.10 Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast Plasma membrane Casparian strip Apoplastic route Figure 36.10 Transport of water and minerals from root hairs to the xylem. Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder (stele) Cortex

Transport of Water and Minerals into the Xylem The endodermis is the innermost layer of cells in the root cortex It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue © 2011 Pearson Education, Inc.

Animation: Transport in Roots Right-click slide / select “Play” © 2011 Pearson Education, Inc.

Transpiration drives the transport of water and minerals from roots to shoots via the xylem Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Transpirational Pull Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata As water evaporates, the air-water interface retreats further into the mesophyll cell walls The surface tension of water creates a negative pressure potential This negative pressure pulls water in the xylem into the leaf The transpirational pull on xylem sap is transmitted from leaves to roots © 2011 Pearson Education, Inc.

Outside air   100.0 MPa Leaf  (air spaces)  7.0 MPa Figure 36.13 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 Figure 36.13 Ascent of xylem sap. Water molecule Root hair Trunk xylem   0.6 MPa Soil particle Soil  Water Water uptake from soil  0.3 MPa 31

Adhesion and Cohesion in the Ascent of Xylem Sap Water molecules are attracted to cellulose in xylem cell walls through adhesion Adhesion of water molecules to xylem cell walls helps offset the force of gravity Water molecules are attracted to each other through cohesion Cohesion makes it possible to pull a column of xylem sap © 2011 Pearson Education, Inc.

Animation: Water Transport Right-click slide / select “Play” © 2011 Pearson Education, Inc.

Animation: Transpiration Right-click slide / select “Play” © 2011 Pearson Education, Inc.

Xylem Sap Ascent by Bulk Flow: A Review The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism Bulk flow is driven by a water potential difference at opposite ends of xylem tissue Bulk flow is driven by evaporation and does not require energy from the plant; like photosynthesis it is solar powered © 2011 Pearson Education, Inc.

Bulk flow is how solutes are transported through xylem Bulk flow differs from diffusion It is driven by differences in pressure potential, not solute potential It occurs in hollow dead cells, not across the membranes of living cells It moves the entire solution, not just water or solutes It is much faster © 2011 Pearson Education, Inc.

The rate of transpiration is regulated by stomata Leaves generally have broad surface areas and high surface-to-volume ratios These characteristics increase photosynthesis and increase water loss through stomata Guard cells help balance water conservation with gas exchange for photosynthesis © 2011 Pearson Education, Inc.

Stomata: Major Pathways for Water Loss About 95% of the water a plant loses escapes through stomata Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape Stomatal density is under genetic and environmental control © 2011 Pearson Education, Inc.

Mechanisms of Stomatal Opening and Closing Changes in turgor pressure open and close stomata When turgid, guard cells bow outward and the pore between them opens When flaccid, guard cells become less bowed and the pore closes This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells © 2011 Pearson Education, Inc.

Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Figure 36.15 Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view) H2O H2O H2O H2O Figure 36.15 Mechanisms of stomatal opening and closing. H2O K H2O H2O H2O H2O H2O (b) Role of potassium in stomatal opening and closing

Stimuli for Stomatal Opening and Closing Generally, stomata open during the day and close at night to minimize water loss Stomatal opening at dawn is triggered by Light CO2 depletion An internal “clock” in guard cells All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles Drought, high temperature, and wind can cause stomata to close during the daytime The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata © 2011 Pearson Education, Inc.

Effects of Transpiration on Wilting and Leaf Temperature Plants lose a large amount of water by transpiration If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes © 2011 Pearson Education, Inc.

Adaptations That Reduce Evaporative Water Loss Xerophytes are plants adapted to arid climates Some desert plants complete their life cycle during the rainy season Others have leaf modifications that reduce the rate of transpiration Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night © 2011 Pearson Education, Inc.

Oleander leaf cross section Figure 36.16 Ocotillo (leafless) Oleander leaf cross section Cuticle Upper epidermal tissue Ocotillo after heavy rain Oleander flowers 100 m Trichomes (“hairs”) Crypt Stoma Lower epidermal tissue Figure 36.16 Some xerophytic adaptations. Ocotillo leaves Old man cactus

Sugars are transported from sources to sinks via the phloem (Phloem Pressure Flow) The products of photosynthesis are transported through phloem by the process of translocation © 2011 Pearson Education, Inc.

Movement from Sugar Sources to Sugar Sinks In angiosperms, sieve-tube elements are the conduits for translocation Phloem sap is an aqueous solution that is high in sucrose It travels from a sugar source to a sugar sink A sugar source is an organ that is a net producer of sugar, such as mature leaves A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb © 2011 Pearson Education, Inc.

A storage organ can be both a sugar sink in summer and sugar source in winter Sugar must be loaded into sieve-tube elements before being exported to sinks Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways Companion cells enhance solute movement between the apoplast and symplast © 2011 Pearson Education, Inc.

Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow In many plants, phloem loading requires active transport Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water © 2011 Pearson Education, Inc.

The Pressure Flow Model

1 Loading of sugar 2 Uptake of water 3 Unloading of sugar 4 Figure 36.18 Sieve tube (phloem) Source cell (leaf) Vessel (xylem) 1 Loading of sugar H2O 1 Sucrose H2O 2 2 Uptake of water Bulk flow by positive pressure Bulk flow by negative pressure 3 Unloading of sugar Figure 36.18 Bulk flow by positive pressure (pressure flow) in a sieve tube. Sink cell (storage root) 4 Water recycled 4 3 Sucrose H2O

Companion (transfer) cell High H concentration Cotransporter Figure 36.17 Key Apoplast Symplast Companion (transfer) cell High H concentration Cotransporter Mesophyll cell Proton pump H Cell walls (apoplast) Sieve-tube element S Plasma membrane Plasmodesmata Figure 36.17 Loading of sucrose into phloem. ATP Sucrose H H S Bundle- sheath cell Phloem parenchyma cell Mesophyll cell Low H concentration (a) (b)

Animation: Translocation of Phloem Sap in Summer Right-click slide / select “Play” © 2011 Pearson Education, Inc.

Animation: Translocation of Phloem Sap in Spring Right-click slide / select “Play” © 2011 Pearson Education, Inc.