Lec8-Xylem+and+phloem_Handout_3perPage.pdf - 10/15/18...

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Unformatted text preview: 10/15/18 Biology 2601: Physiology of Organisms Water and fluid transport: Moving fluids in plants Further Readings from TZMM 6th Ed Assigned Reading: Chapter 3, pp 84-93 • Structure & Properties of Water • Diffusion & Osmosis • Water Potential of Plant Cells Further Reading: Chapter 4 pp 104-110 • Water Transport through the Xylem Chapter 11 Translocation in the Phloem pp 285-314 water move through vascular system comprise d of xylem and phloem trees and woody plants are different due to their secondary growth An epidermal layer on outside and cortical layer and vascular resides in center of root with 3 or 4 main tissue types critical: Xyle phloem, endodermis imp for solute control ans pericycle/cambrium involved in cell division Plant vascular anatomy xylem and phloem arranged in bundles with xylem on inside and phloem outside and seperated by cambdium where new cell division takes place (actively dividing cells) diff between herbacious and woody is how much xylem in core xylem on inside, cambdium then phloem for both ib.bioninja.com.au 1 10/15/18 Xylem • Translocates water and inorganic nutrients from the roots to the leaves (unidirectional) • Dead cells at maturity xylem function as water translocation vessels(short and fat) tracheids (long and flat) at maturity they are dead cells (no cytoplasm no cellular contents just open tubes) cell wall are heavily with ligand function is to transport water as soon as xylem differentiate into tracheid and vessel they die by contrast: phloem is a LIVING tissue not dead and transports photpsynthate and other signaliing molecules and transports them throughout plant BIDIRECTIONAL up and down Tracheids & Vessels Phloem • Transports products of photosynthesis, signalling molecules between source and sink tissues (bidirectional) • Living cells connected through sieve plates Sieve Tube Elements & Companion Cells TZMM Fig. 11.3 Sieve element cell • No nucleus, vacuole, filaments, microtubules or ribosomes Sieve plates • Holes in cell wall between sieve elements source making photosynthetic product and sink is using it does NOT mean transporting cells in different direction at SAME time it just means either up or down depending on what tissue acting as source or sink Temporal, seasonal, lifetime difference living cells with sieve tubes and companion cells associated very imp for function of phloem contain most cellular component whereas sieve tube have reduced cytoplasmic content biosynthetically active but no nucleus so not expressing genes sieve can NOT function without companion • Covered by smooth ER • Have mitochondria, plastids and smooth ER companion function as loading and unloading of sieve lot of energy with nuclei and fully functional cells • Under high turgor pressure (lots of solutes) Companion cells sieve plates are connection point bw sieve elements covered by smooth ER and connected to metabolism of cell • Phloem loading and unloading • ATP production & protein synthesis See also TZMM Fig. 11.5 2 10/15/18 Water moves down a pressure gradient • Described as a water potential (Ψ) measured in MPa (pressure units) • The water potential (Ψw) of a cell is defined by: Pressure potential Ψw = Ψs + Ψp + Ψg Osmotic (or solute) potential driving force moving water: pressure gradient buildup to direct flow water potential —> osmotic (movement of water of SOLUTES from low to high through semi permeable membrane) cell wall of plants can experience internal pressure so water can move if one cell has high water pressure and the other doesnt - once u get higher, gravity has impact cell to cell movement of water mostly gravitational potential is negligent Gravitational potential Moving water: pressure most living cells have turgor pressure in xylem theres negative pressure tension which pulls water in instead of out like turgor other cells under high pressure push water out and xylem in negative • A positive pressure in a cell is caller turgor pressure (as found in living cells) • Turgor pressure would tend to push water out of the cell • In xylem, the negative pressure in cells is called tension – it pulls water in The physics of pulling fluid through tubes: Water moves down a pressure gradient Evaporation here creates low pressure (Ψw= -2 MPa) water evaporate leaving from stomates create negative potential - in roots, water potential is close to zero - more negative in leafs than roots - means that water will flow (continious connection) - lower in leaves as water evaporating - BOTH flow of water from roots to shoots follow very large negative gradient Bulk water flow from roots to shoots - tension bw lower and upper parts Which pulls water up tubes from here (Ψw= -0.1 MPa) 3 10/15/18 Cohesion-tension theory • Evaporation at the leaves causes negative pressure - to explain how water moves in plants - water has lots of cohesion molecules so forms continuous column of water - u should always cut flowers under water to last longer - cut them in open air , tension causes air to suck up - lots of tension in plants shoots • The cohesive properties of the water molecules transfer this tension through the length of the water column • Requires a continuous water column • This is a lot of tension • Dense wood to support xylem • Danger of cavitation Capillary action (capillarity) • ‘Wettable’ walls • Strong adhesion between liquid and walls of vessel cavitation is a problem capillary help to prevent cavitation to keep liquid cohesive for same amount of atm pressure, theres different diameter of tubes its higher in more narrow tube • Surface tension • Interactions between water molecules • Related to diameter of tube • Could account for a few metres... Evaporation of water from leaves creates a significant negative hydrostatic pressure at the surface of mesophyll cells as curvature is smaller, hydrostatic gets more negative that pulls water out of cells if water continious then its pulling that water out of soil TZMM Fig. 4.8 4 10/15/18 more negative pressure, water tends to evaporate - most stable state of water at low pressure - water remain liquid at low pressurre means its in meta-stable Cavitation • Xylem is under a lot of tension (negative pressure) • At these low pressures, the lowest energy state for water should be as a vapour • Water in xylem is meta-stable if u have nucleating sites, bubbles can form - can listen to bubble of xylem • At a nucleus, a bubble can spontaneously form • This is common! • It is detected by the noise of the bubbles forming Consequences of cavitation • Formation of a bubble blocks the xylem • Embolism • Bubble continues to grow once nucleated • No longer a continuous column of water no too many embolism , cavitation results in continious column of water TZMM Fig. 4.7 Safety-efficiency trade-off A large vessels can accomodate much higher flow FOURTH power - twice radius allows 16 times flow of water - from efficiency, larger vessels are better B Q = ΔPπr4 8Lη • Flow is proportional to the xylem radius to the fourth power – 16X faster flow in A than B • Wide xylem means efficient transport 5 10/15/18 but u lose lots of water very quickly when xylem cavitates Safety-efficiency trade-off A big loss B Q = ΔPπr4 8Lη • But also less redundancy! • If xylem cavitates, huge costs compared to an inefficient plant with more xylem pathways Oak Maple Spruce Quercus rubra Acer rubrum Picea abies More efficient spruce tree have much safer transport of water oak has more efficient but ambelism is bigger problem Safer From Insidewood website at NCSU apoplast- hydrodrating cell wall, dynamic which involves ACTIVe transport, crossing cell wall meaning it has to go through water movement through phloem is through pressure membrane which TAKES ENERGY- sugar dont move through of water potential equaition which requires ACTIVE semi permeable membrane w/o energy expenditure METABOLIC INPUT so build up in companion and sieve can be much higher than - require active metabolism NOT passive process surrounding as we are expending energy - even movement of sugars through diffusion which is Phloem loading passive process - how sugars move from mesophyll cells to vascular - plant cells are connected by plasmodesma • Companion cell (membrane line pores that link cytoplasm from one imports sucrose cell to another (active) - mesophyll generate sugar through carbon fixation and sugar diffuse to next cell and so on • Loads phloem either -smyplastic —> moving through cytoplasm via symplast or - passive in a sense that diffusion is passive but apoplast active in sense sugar has to be formed in an active state TZMM Fig. 11.14 6 10/15/18 How to maintain a concentration gradient in symplastic loading Phloem transport mechanisms one mole of glycogen esxert same hydrostatic pressure as on one mole of glycogen might have 10k moelcues of glucose - dropping no. of solutes = keep osmotic pressure low -no. of carbon atoms at far right is higher • Note that the source and sink tissues don’t have to be leaves and roots Polymer trapping TZMM Fig. 11.17 Phloem transport mechanisms • Phloem loaded with sucrose by companion cells at source tissues • May be symplastic (via plasmodesmata) or apoplastic (active transport) • Osmotic gradient in phloem creates pressure differential and moves both water and solutes TZMM Fig. 11.10 solute concentration is roughly same VERY LOW so doesnt contribute to water potential - main driving force is difference in water potential active loading of phloem increase solute conc in phloem and osmosis drives water from xylem in phloem as solute conc much higher in phloem and osmosis move water from high to low conc of solutes - more solutes mean more negative value (how concentrated solution) - pressure increasing in phloem cells as so much water coming in - other end is unloading - decreasing solute potential(taken all solutes out so water leaves cell again) pressure is built by pumping sugar 7 10/15/18 Pressure potential Ψw = Ψs + Ψp + Ψg Osmotic (or solute) potential Gravitational potential • Osmotic potential of phloem becomes more negative (as solutes accumulate) • Water potential declines, leading to water flow from xylem • This generates a turgor pressure in the phloem Phloem transport mechanisms water and solute TOGETHER same RATE • The phloem sap then flows by bulk flow • Pressure gradient driven movement of water and solutes • Much faster than diffusion • Can be against the water potential gradient Phloem transport mechanisms • Phloem unloaded at sink tissue • Concurrent water flow in and out of xylem driven by water potential gradients TZMM Fig. 11.10 8 10/15/18 Phloem & Xylem transport mechanisms are interconnected Shoot Xylem Source Phloem Ψw= -0.8 MPa Ψp= -0.7 MPa Ψs= -0.1 MPa Ψw= -1.1 MPa Ψp= +0.6 MPa Ψs= -1.7 MPa 25 Ψw = Ψs + Ψp + Ψg Ψw= -0.6 MPa Ψp= -0.5 MPa Ψs= -0.1 MPa Root Xylem Ψw= -0.4 MPa Ψp= +0.3 MPa Ψs= -0.7 MPa Sink Phloem Phloem & Xylem transport mechanisms are interconnected Shoot Xylem Source Phloem Ψw= -0.8 MPa Ψp= -0.7 MPa Ψs= -0.1 MPa Ψw= -1.1 MPa Ψp= +0.6 MPa Ψs= -1.7 MPa 26 Ψw = Ψs + Ψp + Ψg Ψw= -0.6 MPa Ψp= -0.5 MPa Ψs= -0.1 MPa Root Xylem Ψw= -0.4 MPa Ψp= +0.3 MPa Ψs= -0.7 MPa Sink Phloem Phloem unloading • As with loading, unloading can be symplastic (plasmodesmata) or apoplastic (active transport) • Fruits and sugar-rich storage tissues use apoplastic unloading 9 ...
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