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11/25/2014 Chapter 36: Resource Acquisition & Transport in Vascular Plants 1. Overview of Transport in Plants 2. Transport of Water & Minerals 3. Transport of Sugars 1. Overview of Transport in Plants CO2 O2 Sugar Light H2O H2O and minerals Resources Needed by Plants O2 CO2 1 11/25/2014 Resources Needed by Plants CO2 – carbon source used during photosynthesis of sugars and other organic molecules O2 – required for the synthesis of ATP by aerobic respiration Sunlight – source of energy for photosynthesis Water – obtained primarily from the soil Minerals & other Nutrients – obtained primarily from the soil Leaf Arrangement (Phyllotaxy) Leaf arrangement and orientation evolved to: • maximize light absorption • reduce self shading (blocking light to lower leaves) • avoid damage from intense light Ground area covered by plant Leaf area index represents % of ground area covered by plant: • commonly >1 due to multiple layers of leaves Plant A Leaf area = 40% of ground area (leaf area index = 0.4) Plant B Leaf area = 80% of ground area (leaf area index = 0.8) • plants self-prune structures that don’t receive enough light More on Leaf Arrangement… Plants must balance light absorption and water loss: • light absorption tends to correlate with water loss • greater surface area for light absorption = greater surface area for water loss • leaf shape & arrangement reflect a balance between the two: • low light & high moisture = larger, horizontal leaves • harsh light & low moisture = smaller, vertical leaves 2 11/25/2014 3 Modes of Transport Apoplastic – through extracellular spaces Symplastic – through cytosol, plasmodesmata Transmembrane – across multiple plasma membranes Cell wall Apoplastic route Cytosol Symplastic route Transmembrane route Key Plasmodesma Apoplast Plasma membrane Symplast Solute Transport Across Plant Cell Membranes CYTOPLASM ATP EXTRACELLULAR FLUID + − + − + − + H+ S H Hydrogen ion H+ H+ − + − + − + H+ H+ H+ H+ H+ S H+ H+ H+ H+ H+ H+ − Proton pump H+ S H+ + H+ − + H+/sucrose cotransporter (a) H+ and membrane potential − + − + − + H+ Sucrose (neutral solute) (b) H+ and cotransport of neutral solutes H+ H+ − + − + − + H+ K+ H+ H+ K+ H+ K+ Nitrate H+ − + − + − + Potassium ion K+ H+ K+ H+/NO3− cotransporter − + − + − + K+ K+ H+ H+ (c) H+ and cotransport of ions H+ Ion channel − + − + (d) Ion channels Transport Through Ion Channels • plants have gated ion channels that, when opened, allow ions to flow down the electrochemical gradient K+ K+ K+ − + − + − + − + − + Potassium ion K+ K+ K+ K+ Ion channel Ion channels 3 11/25/2014 The Role of H+ in Cotransport H+ ions are pumped by active transport to create an electrochemical gradient (membrane potential)… EXTRACELLULAR FLUID CYTOPLASM ATP − + − + − + H+ Hydrogen ion H+ H+ H+ H+ H+ Proton pump − + − + H+ H+ H+ and membrane potential H+ flow down its electrochemical gradient can be coupled to the active transport (movement from low to high conc.) of neutral solutes such as sugars… H+ S H+ S − + − + H+ − H+ H+ H+ + H+ H+ H+ H+ S H+/sucrose cotransporter − + − + − + H+ Sucrose (neutral solute) H+ and cotransport of neutral solutes …or the active transport of ions such as nitrate (NO3-) H+ H+ − + − + − + H+ H+ H+ H+ H+ H+/NO3− cotransporter − + − + H+ − + H+ Nitrate H+ H+ H+ and cotransport of ions 4 11/25/2014 The Transport of Water Osmosis is the diffusion of water across a cell membrane. The net direction of osmosis (water movement) in plants depends on 2 factors: • differences in the concentration of water & solutes across the membrane (water diffuses from high to low concentration) • differences in pressure (water moves from high to low pressure) The combination of these 2 factors (concentration & pressure) is called water potential. …more on Water Potential Initial flaccid cell: ψP = 0 ψ S = −0.7 Environment 0.4 M sucrose solution: ψP = 0 ψ S = −0.9 ψ = −0.7 MPa Water potential (y) = the sum of solute potential (yS) and pressure potential (yP) ψ = −0.9 MPa Final plasmolyzed cell at osmotic equilibrium with its surroundings: ψP = 0 ψ S = −0.9 ψ = −0.9 MPa (a) Initial conditions: cellular ψ > environmental ψ y = yS + yP For pure water yS = 0, the more solutes the more negative the yS yP can be + or – in relation to atmospheric pressure Initial flaccid cell: ψP = 0 ψ S = −0.7 Environment Pure water: ψP = 0 ψS = 0 ψ = −0.7 MPa ψ = 0 MPa Final turgid cell at osmotic equilibrium with its surroundings: ψP = 0.7 ψ S = −0.7 ψ = 0 MPa (b) Initial conditions: cellular ψ < environmental ψ Turgor Pressure in Plants The protoplast (interior part) of plant cells normally has a positive yP due to osmosis, a pressure called turgor pressure which keeps cells turgid (opposite of flaccid). Rate of osmosis is increased by aquaporins. Normal plant with turgid cells Wilted plant with flaccid cells In extracellular compartments such as xylem, yP is negative which aids in the movement of fluid up from the root system. 5 11/25/2014 2. Transport of Water & Minerals in Xylem From Root Hairs to Xylem Casparian strip Endodermal cell Pathway along apoplast 4 Pathway through symplast 5 Plasmodesmata 1 Apoplastic route Casparian strip 1 2 Symplastic route Water moves upward in vascular cylinder Plasma membrane Apoplastic route 3 2 Symplastic route 4 Root hair 3 Transmembrane route 4 The endodermis: controlled entry to the vascular cylinder (stele) Epidermis 5 Vessels (xylem) Endodermis Vascular cylinder (stele) Cortex 5 Transport in the xylem Water & Mineral Uptake by Roots The transport of water, minerals and other nutrients via xylem vessels begins at the interface of the root tip & root hair epidermis and the surrounding soil. • the root epidermal cells are permeable to the aqueous soil solution which freely passes along the cell walls (apoplastic route) to the root cortex • once this material reaches the endodermis, water and desired solutes are transported across the endodermal cells to the vascular cylinder (stele) • once in the stele, water and mineral nutrients enter the tracheids and vessel elements of the xylem as xylem sap to be transported throughout the plant 6 11/25/2014 How is Xylem Moved “Up”? Xylem sap moves upward in the plant due to a combination of the following: ROOT PRESSURE (a minor factor) • active transport of ions into the roots lowers the water potential resulting in water flowing in due to osmosis TRANSPIRATION (the major factor) • loss of water through the stomata of leaves • adhesion of water to xylem vessels & cohesion of water molecules to each other “pull” water up to replace water lost through transpiration Source of “Pull” in Transpiration Diffusion of water vapor out of stomata starts the “pull” which creates a negative water potential drawing water up: 4 Increased surface tension pulls water from cells and air spaces. 5 Water from xylem pulled into cells and air spaces. Cuticle Xylem Upper epidermis Mesophyll Air space 3 Air-water interface retreats. Microfibrils in cell wall of mesophyll cell 2 Water vapor replaced from water film. Lower epidermis Cuticle Stoma 1 Water vapor diffuses outside via stomata. Microfibril (cross section) Water Air-water film interface Transpiration Xylem sap Outside air ψ = −100.0 MPa Mesophyll cells Stoma Water molecule Atmosphere Leaf ψ (air spaces) = −7.0 MPa Transpiration Trunk xylem ψ = −0.8 MPa Water potential gradient Leaf ψ (cell walls) = −1.0 MPa Xylem cells Adhesion by hydrogen bonding Cell wall Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Trunk xylem ψ = −0.6 MPa Root hair Soil particle Water Soil ψ = −0.3 MPa Water uptake from soil 7 11/25/2014 Guard Cell Control of Stomata • when guard cells are turgid, they bend and as a result open the stomata • when guard cells are more flaccid the stomata are closed Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view) Regulation of Guard Cells Guard cell turgidity is controlled by K+ ions which move in response to changes in membrane potential due to active transport of H+: Guard cells turgid/ • Stoma pumping open H+ H2O K+ H2O Guard cells flaccid/ out ofStoma guard cells closed H2O H2O H2O H2O H 2O H2O H2O Role of potassium ions (K+) in stomatal opening and closing H2O • H+ pumped out of guard cells lowers the membrane potential (more negative) drawing K+ ions into the cell • the intracellular increase in K+ lowers the water potential and water flows in Plants open stomata by pumping H+ in response to light and low CO2 (provided there is enough water) 3. Transport of Sugars 8 11/25/2014 Sugar Translocation via Phloem The transport of photosynthetic products, a process called translocation, proceeds through phloem vessels in a direction opposite to that of xylem sap. • photosynthetic products such as sucrose are produced in photosynthetic organs such as leaves • they are transported in phloem sap to sites of sugar use or storage – sugar sinks • e.g., fruits, tubers, growing shoot and root tips • the transfer of sugars to phloem sieve tube elements or companion cells occurs through both symblastic and apoplastic routes… Loading Sugars into Phloem Sieve Tube Elements • transport from apoplast to sieve tube element symplast involves cotransport with H+ Apoplast Symplast Companion (transfer) cell Mesophyll cell High H+ concentration Cell walls (apoplast) Cotransporter H+ Proton pump Sieve-tube element Plasma membrane S Plasmodesmata ATP Mesophyll cell Bundlesheath cell H+ Phloem parenchyma cell Sucrose H+ S Low H+ concentration (a) Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube elements. (b) A chemiosmotic mechanism is responsible for the active transport of sucrose. Bulk Flow of Phloem Sap • this results in a net diffusion of sugar and movement of water towards the sinks H2O Sucrose H2O 1 1 Loading of sugar decreases water potential 2 Bulk flow by positive pressure • sugar concentration decreases near the sugar sinks due to usage for energy or addition to polymers such as starch Sieve Source cell tube (leaf) (phloem) Vessel (xylem) Bulk flow by negative pressure • unlike xylem sap, phloem sap flows toward sugar sinks due to positive pressure 2 Uptake of water increases pressure 3 Unloading of sugar Sink cell (storage root) 3 4 H2O and loss of water relieves the pressure 4 Recycling of water Sucrose 9