Chapter 36: Transport in Vascular Plants - Pathways for Survival

Chapter 36: Transport in Vascular Plants - Pathways for Survival For vascular plants, the evolutionary journey onto land involved differentiation into roots and shoots Vascular tissue transports nutrients in a plant; such transport may occur over long distances Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Physical forces drive the transport of materials in plants over a range of distances Transport in vascular plants occurs on three scales: Transport of water and solutes by individual cells, such as in root hairs Transport of H+ can drive transport of other solutes Short-distance transport of substances from cell to cell at the levels of tissues and organs Water potential drives transport Long-distance transport within xylem and phloem at the level of the whole plant A variety of physical processes are involved in the different types of transport Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Different things are transported throughout a CO 2 O 2 plant (tree) Sugar Light Minerals O 2 CO 2

Selective Permeability of Membranes and Proton pumps movement in and out of a cell The selective permeability of the plasma membrane controls movement of solutes into and out of the cell Specific transport proteins enable plant cells to maintain an internal environment different from their surroundings Proton pumps in plant cells create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work They contribute to a voltage known as a membrane potential Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Proton pumps generate membrane potential and H+ gradients. CYTOPLASM EXTRACELLULAR FLUID ATP Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes

Cations (K+, for example) are driven into the cell by the membrane potential. CYTOPLASM EXTRACELLULAR FLUID Membrane potential and cation uptake Transport protein

In the mechanism called cotransport, a transport protein couples the passage of one solute to the passage of another Cell accumulates anions (, for example) by coupling their transport to; the inward diffusion of through a cotransporter. Cotransport of anions

The coattail effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. Cotransport of a neutral solute

Effects of Differences in Water Potential Short distance transport To survive, plants must balance water uptake and loss Osmosis determines the net uptake or water loss by a cell - it is affected by solute concentration and pressure 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 Both pressure and solute concentration affect water potential The solute potential of a solution is proportional to the number of dissolved molecules The addition of solutes reduces water potential Pressure potential is the physical pressure on a solution Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The addition of solutes to water results in lower water potential and therefore, water flows into the region containing the solute Addition of solutes 0.1 M solution Pure water = 0 MPa P = 0 S = 0.23 P = 0.23 MPa

Physical pressure increases water potential and keeps water out of the region that, without pressure, would normally be of lower water potential Applying physical pressure = 0 MPa P = 0 S = 0.23 P = 0 MPa

Applying enough physical pressure will push the water out of the solute containing region Applying physical pressure = 0 MPa P = 0.30 S = 0.23 P = 0.07 MPa

Negative pressure Negative pressure affects water potential by pulling water P = 0.30 S = 0.23 P = 0.30 MPa P = 0.30 S = 0.23 P = 0.23 MPa

Water potential dramatically 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 become plasmolyzed If a flaccid cell is placed in and environment with a lower solute concentration, the cell will gain water and become turgid Plasmolyzed cell at osmotic equilibrium 0.4 M sucrose solution: P = 0 S = 0.9 P = 0.9 MPa Initial flaccid cell: P = 0 S = 0.7 P = 0.7 MPa Distilled water: P = 0 S = 0 P = 0 MPa Turgid cell at osmotic equilibrium P = 0 S = 0.9 P = 0.9 MPa P = 0.7 S = 0.7 P = 0 MPa

Aquaporin Proteins and Water Transport Aquaporins are transport proteins in the cell membrane that allow the passage of water Aquaporins do not affect water potential Transport is also regulated by the compartmental structure of plant cells The plasma membrane directly controls the traffic of molecules into and out of the cell The plasma membrane is a barrier between two major compartments, the cell wall and the cytosol Another major compartment in most mature plant cells is the vacuole, a large organelle that occupies as much as 90% or more of the protoplast s volume The vacuolar membrane regulates transport between the cytosol and the vacuole Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Plant cell compartments In most plant tissues, the cell walls and cytosol are continuous from cell to cell The cytoplasmic continuum is called the symplast The apoplast is the continuum of cell walls and extracellular spaces Cell wall Cytosol Vacuole Key Symplast Apoplast Cell compartments Plasmodesma Vacuolar membrane (tonoplast) Plasma membrane

Functions of the Symplast and Apoplast in Transport Water and minerals can travel through a plant by three routes: Transmembrane route: out of one cell, across a cell wall, into another cell Symplastic route: via the continuum of cytosol Apoplastic route: via the cell walls and extracellular spaces Key Transmembrane route Apoplast Symplast Apoplast Symplast Symplastic route Transport routes between cells Apoplastic route

Roots absorb water and minerals from the soil In bulk flow, movement of fluid in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes to drive long-distance transport Water and mineral salts from the soil enter the plant through the epidermis of roots and ultimately flow to the shoot system Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Roles of Root Hairs, Mycorrhizae, and Cortical Cells Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and root hairs are located 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 Most plants form mutually beneficial relationships with fungi, which facilitate absorption of water and minerals from the soil Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Endodermis: A Selective Sentry 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 Water can cross the cortex via the symplast or apoplast routes of transport The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Transport of water into the xylem in roots Apoplastic route Plasma membrane Casparian strip Symplastic route Root hair Vessels (xylem) Epidermis Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cortex Endodermis Stele (vascular cylinder)

Water and minerals ascend from roots to shoots through the xylem Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant The transpired water must be replaced by water transported up from the roots Xylem sap rises to heights of more than 100 m in the tallest plants Is the sap pushed upward from the roots, or is it pulled upward by the leaves? At night, when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential Therefore, water flows in from the root cortex, generating root pressure Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicots Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Pulling Xylem Sap: The Transpiration-Cohesion Tension Mechanism Water is pulled upward by negative pressure in the xylem Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata Such transpiration produces negative pressure (tension) in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution Transpirational pull is facilitated by cohesion and adhesion of the water molecules Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Pulling Xylem Sap: The Transpiration-Cohesion Tension Mechanism Cell wall Airspace Y = 0.15 MPa Y = 10.00 MPa Air-water interface Cuticle Upper epidermis Low rate of transpiration High rate of transpiration Cytoplasm Mesophyll Air space Evaporation Airspace Cell wall Lower epidermis Evaporation Water film Vacuole Cuticle CO 2 O 2 CO 2 Xylem O 2 Stoma

Water potential gradient Pulling Xylem Sap: The Transpiration-Cohesion Tension Mechanism Outside air = 100.0 MPa Leaf (air spaces) = 7.0 MPa Leaf (cell walls) = 1.0 MPa Transpiration Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Xylem cells Adhesion Cell wall Trunk xylem = 0.8 Mpa Cohesion and adhesion in the xylem Cohesion, by hydrogen bonding Root xylem = 0.6 MPa Soil = 0.3 MPa Water uptake from soil Water molecule Root hair Soil particle Water

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 absorption through the roots, the plant will lose water and wilt Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation (unfolding, and thus, inactivation) of various enzymes involved in photosynthesis and other metabolic processes Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Stomata: Major Pathways for Water Loss About 90% 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 Cells turgid/stoma open Radially oriented cellulose microfibrils Cells flaccid/stoma closed Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view)

Stomata help regulate the rate of transpiration Leaves generally have broad surface areas and high surface-to-volume ratios These characteristics increase photosynthesis and increase water loss through stomata 20 µm

Role of potassium in stomatal opening and closing Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions by the guard cells Cells turgid/stoma open Cells flaccid/stoma closed K +

Xerophyte Adaptations That Reduce Transpiration Xerophytes are specialized plants that are adapted to live in arid climates They have leaf modifications that reduce the rate of transpiration Their stomata are concentrated on the lower leaf surface, often in depressions that provide shelter from dry wind Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Xerophytes Cuticle Upper epidermal tissue Lower epidermal tissue Trichomes ( hairs ) Stomata 100 µm

Organic nutrients are translocated through the phloem Translocation is the transport of organic nutrients in a plant Phloem sap is an aqueous solution that is mostly 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 Sugar must be loaded into sieve-tube members before being exposed to sinks In many plant species, sugar moves by symplastic and apoplastic pathways Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Sucrose transport from leaves In many plants, phloem loading requires active transport Proton pumping and co-transport of sucrose and H + enable the cells to accumulate sucrose Key Apoplast Mesophyll cell Cell walls (apoplast) Plasma membrane Plasmodesmata Companion (transfer) cell Sieve-tube member Symplast High H + concentration Proton pump Cotransporter Sucrose Mesophyll cell Bundlesheath cell Phloem parenchyma cell Low H + concentration

Pressure Flow: The Mechanism of Translocation in Angiosperms In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure Vessel (xylem) Sieve tube (phloem) Source cell (leaf) Sucrose The pressure flow hypothesis explains why phloem sap always flows from source to sink Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms Sink cell (storage root) Sucrose

Experiments back up the pressure flow hypothesis as the mechanism of translocation in angiosperms 25 µm Sievetube member Sap droplet Stylet Sap droplet Aphid feeding Stylet in sieve-tube member (LM) Severed stylet exuding sap