Chapter 36: Transport in Vascular Plants

Chapter 36: Transport in Vascular Plants 1. Where does transport occur in plants? Start with water….

H2O H2O Minerals Figure 36.2 An overview of transport in a vascular plant

CO2 O2 H2O H2O Minerals Figure 36.2 An overview of transport in a vascular plant

Figure 36.2 An overview of transport in a vascular plant CO2 O2 Light H2O Sugar H2O Minerals

CO2 O2 Light H2O Sugar O2 H2O CO2 Minerals Figure 36.2 An overview of transport in a vascular plant

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells?

CYTOPLASM EXTRACELLULAR FLUID – + H+ + – ATP H+ – + H+ Proton pump generates membrane potential and H+ gradient. H+ H+ H+ – H+ + H+ – + Figure 36.3 Proton pumps provide energy for solute transport

+ – EXTRACELLULAR FLUID CYTOPLASM + – K+ Cations ( for example) are driven into the cell by themembrane potential. K+ + – K+ K+ K+ K+ K+ – + K+ – + Transport protein (a) Membrane potential and cation uptake + – H+ H+ NO3 – Cell accumulates anions (NO3 –, for example) by coupling their transport to theinward diffusion of H+ through a cotransporter. – NO3– + H+ – + H+ H+ H+ H+ H+ NO3– + – NO3 – NO3 – – + H+ NO3– H+ H+ H+ – + (b) Cotransport of anions + – H+ H+ H+ S Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. – + H+ + H+ – H+ S S H+ H+ H+ H+ S S S + – H+ – + H+ S H+ + – (c) Cotransport of a neutral solute Figure 36.4 Solute transport in plant cells

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells? • What influences the movement of water? • Ψ = Ψs + Ψp • Water moves from HIGH  low (more  less)

(b) (a) (c) (d) 0.1 M solution Purewater H2O H2O H2O H2O YP = 0 YP = –0.30 YP = 0 YP = 0.30 YP = 0.23 YS = –0.23 YS = –0.23 YS = –0.23 YS = 0 YS = –0.23 Y = –0.30 MPa Y = –0.23 MPa Y = 0.07 MPa Y = 0 MPa Y = 0 MPa Y = 0 MPa Y= –0.23 MPa Y = 0 MPa Fig. 36.5 Water potential and water movement: an artificial model Ψ = Ψs + Ψp + solute decreases Ψs Water goes from high  low + pressure counteracts Ψs More pressure forces water across membrane (-) pressure also moves water

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells? • What influences the movement of water? • What does this mean for plant cells?

Initial flaccid cell: p = 0 s = –0.7 0.4 M sucrose solution: Distilled water: = –0.7 MPa p = 0 p = 0 Turgid cell at osmotic equilibrium with its surroundings Plasmolyzed cell at osmotic equilibrium with its surroundings s = 0 s = –0.9 = 0 MPa = –0.9 MPa p = 0 p = 0.7 s = –0.9 s = –0.7 = 0 MPa = –0.9 MPa Initial conditions: cellular  > environmental . The cell loses water and plasmolyzes. After plasmolysis is complete, the water potentials of the cell and its surroundings are the same. (a) (b) Initial conditions: cellular  < environmental . There is a net uptake of water by osmosis, causing the cell to become turgid. When this tendency for water to enter is offset by the back pressure of the elastic wall, water potentials are equal for the cell and its surroundings. (The volume change of the cell is exaggerated in this diagram.) Figure 36.6 Water relations in plant cells Plasmolysis – shrinking of a plant cell away from its cell wall due to water loss Turgid – plant cell full of water due to its high solute concentration (turgor pressure) Aquaporins allow water to move quickly across a membrane

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells? • What influences the movement of water? • What does this mean for plant cells? • What are the transport routes dissolved substances can take between cells?

Cell wall Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. Transport proteins in the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Cytosol Vacuole Plasmodesma Vacuolar membrane (tonoplast) Plasma membrane (a) Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. Key Symplast Apoplast Transmembrane route Apoplast The apoplast is the continuum of cell walls and extracellular spaces. The symplast is the continuum of cytosol connected by plasmodesmata. Symplast Symplastic route Apoplastic route (b) Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another. Fig 36.8 Cell compartments and routes for short-distance transport How does water get into the plant?

Casparian strip Endodermis Pathway along apoplast Pathway through symplast Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. 1 Casparian strip Plasma membrane Minerals and water that cross the plasma membranes of root hairs enter the symplast. 2 Apoplastic route 1 Vessels (xylem) 2 As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast. 3 3 Root hair Symplastic route Epidermis Endodermis Vascular cylinder Cortex Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system. Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder. 5 5 4 4 Figure 36.9 Lateral transport of minerals and water in roots • Why is the Casparian strip so important? • forces dissolved substances across a selectively permeable membrane • Keeps unwanted & unrecognized substances OUT of the plant

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells? • What influences the movement of water? • What does this mean for plant cells? • What are the transport routes dissolved substances can take between cells? • What is the mutualistic relationship between plant roots and • another biological organism?

2.5 mm Figure 36.10 Mycorrhizae, symbiotic associations of fungi and roots

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells? • What influences the movement of water? • What does this mean for plant cells? • What are the transport routes dissolved substances can take between cells? • What is the mutualistic relationship we discussed between plant roots • another biological organism? • How is xylem sap transported? (How can it defy gravity?) • Cohesion – water’s ability to stick to itself via hydrogen bonds • Adhesion – water’s ability to stick to other polar substances via H-bonds • WHY?? • electronegative oxygen creates polar covalent bond in water

Xylem sap Outside air Y = –100.0 MPa Mesophyll cells Stoma Leaf Y (air spaces) = –7.0MPa Water molecule Transpiration Atmosphere Leaf Y (cell walls) = –1.0 MPa Xylem cells Adhesion Cell wall Water potential gradient Trunk xylem Y = – 0.8 MPa Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root xylem Y = – 0.6 MPa Root hair Soil Y = – 0.3 MPa Soil particle Water uptake from soil Water Figure 36.13 Ascent of xylem sap Transpiration – loss of water vapor through leaves that pulls water up from roots What controls the loss of water? Stomata

20 µm Fig. 36.14 Open stomata (left) and closed stomata (colorized SEM) What controls the opening & closing of the stomata? - K+ in the guard cells

Cells turgid/Stoma open (a) Changes in guard cell shape and stomatal opening and closing (surface view). Guard cells of a typical angiosperm are illustrated in their turgid (stoma open) and flaccid (stoma closed) states. The pair of guard cells buckle outward when turgid. Cellulose microfibrils in the walls resist stretching and compression in the direction parallel to the microfibrils. Thus, the radial orientation of the microfibrils causes the cells to increase in length more than width when turgor increases. The two guard cells are attached at their tips, so the increase in length causes buckling. Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell (b) Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells. H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O Figure 36.15 The mechanism of stomatal opening and closing Cells flaccid/Stoma closed

Chapter 36: Transport in Vascular Plants • Where does transport occur in plants? Start with water…. • How are solutes transported between cells? • What influences the movement of water? • What does this mean for plant cells? • What are the transport routes dissolved substances can take between cells? • What is the mutualistic relationship we discussed between plant roots • another biological organism? • How is xylem sap transported? (How can it defy gravity?) • How is phloem sap transported?

High H+ concentration Cotransporter Sieve-tube member Companion (transfer) cell Mesophyll cell H+ Proton pump Cell walls (apoplast) S Plasma membrane Plasmodesmata Key ATP Sucrose H+ H+ Apoplast S Phloem parenchyma cell Bundle- sheath cell Low H+ concentration Symplast Mesophyll cell (a) Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells. (b) A chemiosmotic mechanism is responsible for the active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell. Figure 36.17 Loading of sucrose into phloem

Vessel (xylem) Sieve tube (phloem) Source cell (leaf) Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis. Sucrose 1 1 H2O H2O 2 2 This uptake of water generates a positive pressure that forces the sap to flow along the tube. The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink. Transpiration stream Pressure flow In the case of leaf-to-root translocation, xylem recycles water from sink to source. Sink cell (storage Root) 4 4 3 3 Sucrose H2O Figure 36.18 Pressure flow in a sieve tube

Chapter 36: Transport in Vascular Plants