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Transport in Vascular Plants
Chapter 36 Transport in Vascular Plants
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Figure 36.1 Coast redwoods (Sequoia sempervirens)
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Figure 36.2 An overview of transport in a vascular plant (layer 1)
Minerals H2O
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Figure 36.2 An overview of transport in a vascular plant (layer 2)
Minerals H2O CO2 O2
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Figure 36.2 An overview of transport in a vascular plant (layer 3)
CO2 O2 Light H2O Sugar H2O Minerals
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Figure 36.2 An overview of transport in a vascular plant (layer 4)
Minerals H2O CO2 O2 Sugar Light
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Figure 36.3 Proton pumps provide energy for solute transport
CYTOPLASM EXTRACELLULAR FLUID ATP H+ Proton pump generates membrane potential and H+ gradient. – +
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Figure 36.4 Solute transport in plant cells
+ CYTOPLASM EXTRACELLULAR FLUID Cations ( for example) are driven into the cell by the membrane potential. Transport protein K+ – (a) Membrane potential and cation uptake H+ NO3– NO3 – (b) Cotransport of anions Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. S (c) Cotransport of a neutral solute Cell accumulates anions (NO3 –, for example) by coupling their transport to the inward diffusion of H+ through a cotransporter.
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Figure 36.5 Water potential and water movement: an artificial model
Y = –0.23 MPa (a) 0.1 M solution (d) (c) (b) YP = 0 H2O YS = –0.23 Y= –0.23 MPa YS = –0.23 Y = 0 MPa YP = Y = –0.07 MPa YP = YS = 0 Y = –0.30 MPa YP = –0.30 YP = 0 Y = 0 MPa Pure water
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Figure 36.6 Water relations in plant cells
0.4 M sucrose solution: = 0 s = –0.9 = –0.9 MPa = 0 s = –0.7 = –0.7 MPa Initial flaccid cell: = 0 s = 0 = 0 MPa Distilled water: Plasmolyzed cell at osmotic equilibrium with its surroundings = 0 = –0.9 MPa = 0.7 s = –0.7 = 0 MPa Turgid cell at osmotic 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. 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.) (b)
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Figure 36.7 A watered Impatiens plant regains its turgor
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Figure 36.8 Cell compartments and routes for short-distance transport
Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Plasmodesma Vacuolar membrane (tonoplast) Plasma membrane Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. Key Symplast Apoplast The symplast is the continuum of cytosol connected by plasmodesmata. The apoplast is the continuum of cell walls and extracellular spaces. Transmembrane route Symplastic route Apoplastic route 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. Cell wall Cytosol Vacuole (a) (b)
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Figure 36.9 Lateral transport of minerals and water in roots
Casparian strip Endodermis Pathway along apoplast Pathway through symplast 1 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. Casparian strip Plasma membrane 2 Minerals and water that cross the plasma membranes of root hairs enter the symplast. Apoplastic route 1 3 Vessels (xylem) 2 4 5 3 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. Root hair Symplastic route Epidermis Cortex Endodermis Vascular cylinder 4 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 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.
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Figure 36.10 Mycorrhizae, symbiotic associations of fungi and roots
2.5 mm
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Figure Guttation
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Figure 36.12 The generation of transpirational pull in a leaf
Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film’s pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater. Cuticle Upper epidermis Mesophyll Lower Water vapor CO2 O2 Xylem Stoma Evaporation At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells. In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata. Airspace Cytoplasm Cell wall Vacuole Water film Low rate of transpiration High rate of Air-water interface Y = –0.15 MPa Y = –10.00 MPa 3 1 2
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Figure 36.13 Ascent of xylem sap
Outside air Y = –100.0 MPa Leaf Y (air spaces) = –7.0MPa Leaf Y (cell walls) = –1.0 MPa Trunk xylem Y = – 0.8 MPa Water potential gradient Root xylem Y = – 0.6 MPa Soil Y = – 0.3 MPa Mesophyll cells Stoma Water molecule Atmosphere Transpiration Adhesion Cell wall Cohesion, by hydrogen bonding Root hair Soil particle Cohesion and adhesion in the xylem Water uptake from soil
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Figure 36.14 Open stomata (left) and closed stomata (colorized SEM)
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Figure 36.15 The mechanism of stomatal opening and closing
Cells turgid/Stoma open H2O Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell K+ 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. (a) 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. (b) Cells flaccid/Stoma closed
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Figure 36.16 Structural adaptations of a xerophyte leaf
Lower epidermal tissue Trichomes (“hairs”) Cuticle Upper epidermal tissue Stomata 100m
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Figure 36.17 Loading of sucrose into phloem
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. (a) Mesophyll cell Cell walls (apoplast) Plasma membrane Plasmodesmata Companion (transfer) cell Sieve-tube member Phloem parenchyma cell Bundle- sheath cell High H+ concentration Cotransporter Proton pump ATP Key Sucrose Apoplast Symplast H+ 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. (b) Low H+ concentration S
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Figure 36.18 Pressure flow in a sieve tube
Vessel (xylem) H2O Sieve tube (phloem) Source cell (leaf) Sucrose Sink cell (storage Root) 1 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. 2 4 3 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. In the case of leaf-to-root translocation, xylem recycles water from sink to source. Transpiration stream Pressure flow
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Figure 36.19 What causes phloem sap to flow from source to sink?
Aphid feeding Stylet in sieve-tube member Severed stylet exuding sap Sieve- Tube To test the pressure flow hypothesis, researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink. EXPERIMENT The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. RESULTS The results of such experiments support the pressure flow hypothesis. CONCLUSION Sap droplet Stylet Sap droplet 25 m Sieve- tube member
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