Chapter 36: Resource Acquisition and Transport in Vascular Plants Mrs. Valdes AP Biology

Overview: Underground Plants Success of plants depends on their ability to gather & conserve resources from environment Diffusion, active transport, and bulk flow work together to transfer water, minerals, and sugars  “Stone Plants” live mostly subterranean with two leaf tips full of clear jelly that acts as lenses to channel light underground

H2O H2O and minerals Fig. 36-2-1 Figure 36.2 An overview of resource acquisition and transport in a vascular plant H2O and minerals

CO2 O2 H2O O2 CO2 H2O and minerals Fig. 36-2-2 Figure 36.2 An overview of resource acquisition and transport in a vascular plant O2 H2O and minerals CO2

CO2 O2 Light H2O Sugar O2 CO2 H2O and minerals Fig. 36-2-3 Figure 36.2 An overview of resource acquisition and transport in a vascular plant O2 H2O and minerals CO2

Concept 36.1: Land plants acquire resources both above and below ground Algal ancestors of land plants absorbed water, minerals, and CO2 directly from surrounding water Evolution of xylem and phloem in land plants  long-distance transport of water, minerals, and products of photosynthesis Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss Stems: conduits for water and nutrients; supporting structures for leaves Phyllotaxy: arrangement of leaves on stem; specific to each species

Light absorption affected by leaf area index total upper leaf surface of plant divided by surface area of land on which it grows affected by light absorption

Root Architecture and Acquisition of Water and Minerals Soil: resource mined by root system Taproot systems: anchor plants; characteristic of most trees Roots + hyphae of soil fungi form symbiotic associations called mycorrhizae Mutualisms with fungi helped plants colonize land

Concept 36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow Transport begins with absorption of resources by plant cells Movement of substances into/out of cells regulated by selective permeability Diffusion across a membrane = passive Pumping of solutes across a membrane = active AKA needs energy Most solutes pass through transport proteins in cell membrane Proton pump: most important transport protein for active transport plant cells create hydrogen ion gradient AKA form of potential energy that can be harnessed to DO WORK contribute to voltage known as membrane potential

Plant cells use energy stored in proton gradient and membrane potential to drive transport of many different solutes Cotransport: transport protein couples diffusion of one solute to active transport of another “Coattail” effect of cotransport: responsible for uptake of sugar sucrose by plant cells

Diffusion of Water (Osmosis) Osmosis: determines net uptake/water loss by cell; affected by solute concentration and pressure Water potential: measurement that combines effects of solute concentration and pressure Determines direction of movement of water (in/out) Water flows from regions of higher water potential to regions  lower water potential Symbol:Ψ Units: measure pressure; megapascals (MPa) Ψ = 0 MPa for pure water at sea level and room temperature

How Solutes and Pressure Affect Water Potential REMEMBER: Pressure and solute concentration affect water potential Solute potential (ΨS): proportional to number of dissolved molecules also called osmotic potential Pressure potential (ΨP): physical pressure on a solution Turgor pressure: pressure exerted by plasma membrane against the cell wall, and cell wall against the protoplast

Measuring Water Potential Water moves from higher water potential  lower water potential Addition of solutes reduces water potential Physical pressure increases water potential Negative pressure decreases water potential

(a) 0.1 M solution Pure water H2O ψP = 0 ψS = 0 ψP = 0 ψS = −0.23 Fig. 36-8a (a) 0.1 M solution Pure water Figure 36.8a Water potential and water movement: an artificial model H2O ψP = 0 ψS = 0 ψP = 0 ψS = −0.23 ψ = 0 MPa ψ = −0.23 MPa

(b) Positive pressure H2O ψP = 0 ψS = 0 ψP = 0.23 ψS = −0.23 ψ = 0 MPa Fig. 36-8b (b) Positive pressure Figure 36.8b Water potential and water movement: an artificial model H2O ψP = 0 ψS = 0 ψP = 0.23 ψS = −0.23 ψ = 0 MPa ψ = 0 MPa

(c) H2O ψP = 0 ψS = 0 ψP = ψS = −0.23 ψ = 0 MPa ψ = 0.07 MPa Fig. 36-8c (c) Increased positive pressure Figure 36.8c Water potential and water movement: an artificial model H2O ψP = 0 ψS = 0 ψP =   ψS = −0.23 0.30 ψ = 0 MPa ψ = 0.07 MPa

Negative pressure (tension) Fig. 36-8d (d) Negative pressure (tension) Figure 36.8d Water potential and water movement: an artificial model H2O ψP = −0.30 ψS = ψP = ψS = −0.23 ψ = −0.30 MPa ψ = −0.23 MPa

Water potential affects uptake and loss of water by plant cells Flaccid cell: placed in environment with higher solute concentration cell will lose water and undergo plasmolysis If same flaccid cell placed in solution with lower solute concentration cell will gain water and become turgid Turgor loss in plants causes wilting can be reversed when the plant is watered

Aquaporins: Facilitating Diffusion of Water Aquaporins: transport proteins in cell membrane that allow passage of water rate of water movement likely regulated by phosphorylation of aquaporin proteins

Three Major Pathways of Transport Transport also regulated by compartmental structure of plant cells 1: Plasma membrane directly controls traffic of molecules into/out of the protoplast 2: Plasma membrane is barrier between two major compartments, the cell wall and the cytosol 3: Vacuole, large organelle that occupies as much as 90% or more of protoplast’s volume Vacuolar membrane regulates transport between cytosol and vacuole In most plant tissues, cell wall and cytosol continuous from cell to cell Symplast: cytoplasmic continuum Plasmodesmata: cytoplasm of neighboring cells connected by channels Apoplast: continuum of cell walls and extracellular spaces

Water and minerals can travel through plant by three routes: Transmembrane route: out of one cell, across a cell wall, and into another cell Symplastic route: via continuum of cytosol Apoplastic route: via cell walls and extracellular spaces

Bulk Flow in Long-Distance Transport Bulk flow: movement of a fluid driven by pressure; required for efficient long distance transport of fluid Water and solutes move together through: tracheids and vessel elements of xylem sieve-tube elements of phloem Efficient movement possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm

Concept 36.3: Water and minerals are transported from roots to shoots Plants can move large volume of water from their roots to shoots Most water and mineral absorption occurs near root tips, where epidermis is permeable to water and root hairs are located Root hairs account for much of surface area of roots After soil solution enters roots, extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

Transport of Water and Minerals into Xylem Endodermis: innermost layer of cells in root cortex Surrounds vascular cylinder Last checkpoint for selective passage of minerals from cortex into vascular tissue Water can cross the cortex via the symplast or apoplast Casparian strip: waxy endodermal wall blocks apoplastic transfer of minerals from cortex to vascular cylinder

Bulk Flow Driven by Negative Pressure in the Xylem Transpiration: evaporation of water from a plant’s surface; Plants lose large volume of water Xylem sap: Water replaced by bulk flow of water and minerals; from steles of roots to stems and leaves Is sap mainly pushed up from the roots, or pulled up by the leaves?

Pushing Xylem Sap: Root Pressure At night, transpiration very low, root cells continue pumping mineral ions into xylem of vascular cylinder, THUS! Lowering water potential Positive root pressure relatively weak and is minor mechanism of xylem bulk flow Water flows in from the root cortex, generating root pressure sometimes results in guttation exudation of water droplets on tips or edges of leaves

Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism Water pulled upward by negative pressure in xylem Transpiration Pull: Water vapor in airspaces of leaf diffuses down water potential gradient  exits leaf via stomata Transpiration produces negative pressure (tension) in leaf exerts pulling force on water in xylem RESULT: pulling water into leaf

Cohesion and Adhesion in Ascent of Xylem Sap Transpirational pull on xylem sap transmitted from leaves to root tips and even into soil solution! facilitated by: cohesion of water molecules to each other adhesion of water molecules to cell walls Drought stress OR freezing can cause cavitation formation of water vapor pocket by a break in chain of water molecules

Water molecule Root hair Water Water uptake from soil Soil particle Fig. 36-15a Water molecule Root hair Soil particle Figure 36.15 Ascent of xylem sap Water Water uptake from soil

Adhesion by hydrogen bonding Fig. 36-15b Adhesion by hydrogen bonding Xylem cells Cell wall Figure 36.15 Ascent of xylem sap Cohesion by hydrogen bonding Cohesion and adhesion in the xylem

Xylem sap Mesophyll cells Stoma Water molecule Transpiration Fig. 36-15c Xylem sap Mesophyll cells Stoma Water molecule Figure 36.15 Ascent of xylem sap Transpiration Atmosphere

Xylem Sap Ascent by Bulk Flow Know… 1- Movement of xylem sap against gravity is maintained by transpiration-cohesion- tension mechanism 2- Transpiration lowers water potential leaves which generates negative pressure (tension) that pulls water UP through xylem 3- NO energy cost to bulk flow of xylem sap

Concept 36.4: Stomata help regulate rate of transpiration Leaves generally have broad surface areas and high surface-to-volume ratios THIS increases photosynthesis and increases water loss through stomata About 95% of water a plant loses escapes through stomata Each stoma is flanked by a pair of guard cells, which control diameter of stoma by changing shape

Mechanisms of Stomatal Opening and Closing Changes in turgor pressure open and close stomata Result from reversible uptake/loss of potassium ions by guard cells

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Fig. 36-17a Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Figure 36.17a Mechanisms of stomatal opening and closing Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view)

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed Fig. 36-17b Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed H2O H2O H2O H2O H2O K+ H2O H2O Figure 36.17b Mechanisms of stomatal opening and closing H2O H2O H2O (b) Role of potassium in stomatal opening and closing

Stimuli for Stomatal Opening and Closing Generally, stomata open during day and close at night to minimize water loss Stomatal opening at dawn triggered by: light, CO2 depletion Internal “clock” in guard cells All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

Effects of Transpiration on Wilting and Leaf Temperature Plants lose A LOT of water by transpiration Lose H2O and H2O not replace by H2O transport  plant wilt Transpiration also results in evaporative cooling: can lower temperature of leaf  prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

Adaptations That Reduce Evaporative Water Loss Xerophytes: plants adapted to arid climates have leaf modifications that reduce rate of transpiration crassulacean acid metabolism (CAM): form of photosynthesis where stomatal gas exchange occurs at night

Figure 36.18 Some xerophytic adaptations Ocotillo (leafless) Oleander leaf cross section and flowers Cuticle Upper epidermal tissue 100 µm Trichomes (“hairs”) Crypt Stomata Lower epidermal tissue Figure 36.18 Some xerophytic adaptations Ocotillo leaves Ocotillo after heavy rain Old man cactus

Concept 36.5: Sugars transported from leaves and other sources to sites of use or storage Translocation: process of transporting products of photosynthesis through phloem Phloem sap : aqueous solution high in sucrose travels from sugar source to sugar sink sugar source: organ that is a net producer of sugar Ex: mature leaves sugar sink: organ that is a net consumer or storer of sugar, such as a tuber or bulb Storage organ can be both sugar sink in summer and sugar source in winter Sugar MUST BE loaded into sieve-tube elements before being exposed to sinks Depending on species, sugar move by: symplastic both symplastic and apoplastic pathways Transfer cells: modified companion cells that enhance solute movement between apoplast and symplast

Phloem parenchyma cell Symplast Mesophyll cell Fig. 36-19a Mesophyll cell Cell walls (apoplast) Companion (transfer) cell Sieve-tube element Plasma membrane Plasmodesmata Key Figure 36.19a Loading of sucrose into phloem Apoplast Bundle- sheath cell Phloem parenchyma cell Symplast Mesophyll cell

In many plants, phloem loading requires active transport Proton pumping and cotransport of sucrose and H+ enable cells to accumulate sucrose At sink, sugar molecules diffuse from phloem to sink tissues and followed by water

Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms In studying angiosperms, researchers concluded sap moves through sieve tube by bulk flow driven by positive pressure Pressure flow hypothesis explains why phloem sap always flows from source to sink

Concept 36.6: Symplast is highly dynamic Symplast: living tissue responsible for dynamic changes in plant transport processes Plasmodesmata can change in permeability in response to: turgor pressure cytoplasmic calcium levels cytoplasmic pH Plant viruses can cause plasmodesmata to dilate Mutations that change communication within symplast can lead to changes in development

Electrical Signaling in the Phloem Phloem allows for rapid electrical communication between VERY separated organs Phloem is “superhighway” for systemic transport of macromolecules and viruses Systemic communication helps integrate functions of whole plant

You should now be able to: Describe how proton pumps function in transport of materials across membranes Define the following terms: osmosis, water potential, flaccid, turgor pressure, turgid Explain how aquaporins affect the rate of water transport across membranes Describe three routes available for short-distance transport in plants Relate structure to function in sieve-tube cells, vessel cells, and tracheid cells Explain how the endodermis functions as a selective barrier between the root cortex and vascular cylinder Define and explain guttation Explain this statement: “The ascent of xylem sap is ultimately solar powered” Describe the role of stomata and discuss factors that might affect their density and behavior Trace the path of phloem sap from sugar source to sugar sink; describe sugar loading and unloading