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Transport in Multicellular Plants: Part 2 Ch. 7 XYLEM and PHLOEM Function of XYLEM TISSUE Support Transport: Water and minerals Nitrates to make: Amino acids Organic bases (adenine) Proteins Nucleotides ATP Chlorophyll Magnesium to make: Chlorophyll Active sites of enzymes XYLEM CELLS in XYLEM TISSUE of Flowering Plants: Vessel Elements Transport of water No end walls = drain pipe Lignin in cell walls (except around plasmodesmata…create gaps we call PITS) Tracheids Transport of water Dead, lignified cells No open end walls; single enclosed cells with pits Fibres Pits NOT open pores; unthickened cell wall covers them Elongated cells Lignified walls Support Dead cells No contents Parenchyma cells Standard plant cells Unthickened cellulose walls All organelles present (except chloroplast in xylem parenchyma cells) Xylem Tissue “XYLEM VESSEL ELEMENTS” Arranged in long lines to form: No end walls= allows continuous movement of water by mass flow Dead, Empty cells no contents; allows easy movement of continuous column of water by mass flow xylem vessels long, hollow tubes that water flows through from roots to all other parts of the plant Mass Flow process of water moving through xylem vessels from roots to other parts When water molecules and any dissolved solutes move together, as a body of liquid (similar to water flowing in a river) (due to hydrostatic pressure…) Hydrostatic Pressure Pressure exerted by liquid At leaves, water is removed from xylem vessels via TRANSPIRATION…decreases pressure Pressure @ top of xylem vessel becomes lower than pressure @ bottom of xylem vessel Pressure difference creates pressure gradient Water moves up xylem vessels in solid column (think drinking straw) Root Pressure Plants raise pressure at base of vessels By active secretion of solutes into water in the xylem vessels in root Cells around xylem vessels use ACTIVE TRANSPORT to pump solutes across their membranes and into xylem Solutes in xylem decrease water potential of xylem and cause water to rush in from surrounding root cells Water pressure increases in xylem cells Helps move water up xylem vessels Not essential for water movement Water continues moving up plant even when dead Water movement passive Water into Roots Mutualism in Plants Mycorrhizas Networks of fibrous fungi that grows on roots Absorbs water and nutrients (phosphate) and transports to plant Plant provides nutrients for fungi to survive Rhizobium Nitrogen fixing bacteria living in nodules in roots Structure of Xylem Vessels Cell wall contains cellulose and lignin Lignin Makes cell walls impermeable to H2O Provides strength Vessel element will NOT collapse when there is negative pressure inside Pits In cell wall Allows movement of H2O out of the vessel element to other vessel elements OR to nearby plant tissue Narrow lumen Increases area of water in contact with wall Water molecules adhere to walls (ADHESION) Prevents breakage of water column How Water Moves From Soil to Air Moves down water potential gradient Water potential of soil higher than the air Water potential of the leaf kept LOWER than water potential in SOIL (water is always drawn up) Due to loss of water vapour through transpiration Transpiration maintains the water potential gradient 2 Ways Water Travels Symplast When water moves from one cell to another cell via osmosis, down a water potential gradient Apoplast When water moves from the cell wall on one cell to another cell, never enters cytoplasm; no osmosis involved How Water Moves From Soil to Air Water enters root hair cells by OSMOSIS Move down water potential gradient Water in air spaces between soil particles diffuses through cell surface membrane, into cytoplasm AND vacuole of the ROOT HAIR CELL Water moves from ROOT HAIR CELL to neighboring cell via OSMOSIS (down water potential gradient) SYMPLAST PATHWAY Water also seeps into cell wall of root hair cell No osmosis (no partially permeable membrane is crossed) Water seeps INTO and ALONG CELL WALLS of neighboring cells APOPLAST PATHWAY Carries MORE water than symplast path way How Water Moves From Soil to Air When water nears the centre of the root, it encounters the ENDODERMIS (blocked!) Cylinder of cells Centre of root Each cell in endodermis has a ring of IMPERMEABLE SUBERIN (waxy, waterproof band) Forms the CASPARIAN strip Prevents water from continuing to seep through cell walls Water must travel through these cells by SYMPLAST pass through cytoplasm of endodermal cells Water leaves the endodermis, crosses pericycle, and moves into the XYLEM VESSELS via PITS How Water Moves From Soil to Air Water moves up xylem vessels using MASS FLOW Water held together by adhesion and cohesion Hydrogen binds b/t each other Keeps water column UNBROKEN Low hydrostatic pressure at the top of the column Created by loss of water via TRANSPIRATION Lowering hydrostatic pressure causes a pressure gradient from the base to the top of the xylem vessel How Water Moves From Soil to Air In leaf: Water moves OUT of xylem vessels through pits Water then moves across the leave by the APOPLAST and SYMPLAST pathways Water evaporates from the wet cell walls into their leaf spaces Water then leaves the leaf spaces and diffuses out through the stomata Xerophytes Plant that is adapted to live in an environment where water is in short supply Adaptations: Leaves with small surface area to volume ratio 1. Reduces amount of surface area from which water vapour can diffuse Leaves with thick, waxy cuticle 2. Reduces the quantity of water that can diffuse through the surface of the leaf and into the air Methods of trapping moist air near stomata 3. Rolling of leaf with stomata on the inside Stomata in pits in leaf surface Having hairs around stomata All produce layer of HIGH WATER POTENTIAL around stomata = reduction in water potential gradient = decrease rate of diffusion of water vapour from air spaces inside leaf to the outside = plant preserves water Transport in Phloem Translocation movement of substances in phloem tissue Main substances moved (in solution): ASSIMILATES (substances that have been made by plants) Amino acids Sucrose CLICK! Phloem Tissue Cells Sieve Tube Elements LIVING cells Contain cytoplasm and a few organelles NO nucleus Walls are made of Cellulose (no lignin) Companion cells Associated with sieve tube elements Contains cytoplasm, many organelles(including nucleus and mitochondria) Sieve Tube Elements Cell wall with cellulose Cytoplasm mitochondria NO nucleus A few other organelles Creates SPACE for movement of phloem sap Sieve plate Perforated END WALL Allows mass flow of phloem sap through sieve pores How Assimilate Move Through Pores SOURCE part of plant where assimilates (a.a. & sucrose) ENTER the phloem SINK part of plant where assimilates LEAVE the phloem a plant cell with a high concentration of sugars and other solutes, such as a leaf cell a plant cell with a low concentration of sugars sugars may be converted to starch for storage or used rapidly for energy or as building blocks of other carbohydrates Ex. Leaf source & Roots sink TRANSLOCATION Requires Energy Cell respiration in Companion cells at source provide ATP used to fuel active transport of sucrose INTO companion cell Increases concentration of sucrose in companion cell Allows sucrose to move down concentration gradient via diffusion INTO PHLOEM SIEVE ELEMENT With all this sucrose in companion cell and SE, water potential has DECREASED in these guys Water flows into companion cell and SE, moving down gradient At Sink: Sucrose diffuses OUT of phloem sieve element and DOWN a concentration gradient into a cell that is using sucrose Water potential gradient produced Water diffuses OUT of phloem sieve element Water is ADDED at the source Water is LOST at the sink Creates higher HYDROSTATIC PRESSURE inside phloem sieve element at the SOURCE than at the sink Phloem sap moves by MASS FLOW down pressure gradient, through phloem sieve elements through sieve pores From source to sink Comparing Xylem and Phloem Potometer A potometer is a device that measures the rate at which a plant draws up water Plant draws up water as it loses it by transpiration Measure the rate of transpiration based on rate water is lost Apparatus must be air tight and water tight…NO LEAKS Basic elements of a potometer are: A plant cutting A calibrated pipette to measure water loss A length of clear plastic tubing An air-tight seal between the plant and the water-filled tubing • The rate of transpiration is measured as the amount of water lost/ square meter/ minute • Water evaporates through the many stomata on the leaf surface • the rate of transpiration is directly related to the surface area • To arrive at the rate of transpiration, calculate the leaf surface area of each plant Micrographs of Plant Stem Click here Diagrams Complete each labeled figure on an ENTIRE page of notebook paper. Use colored pencils AND label everything! P. 121 fig. 7.1 P. 122 fig. 7.2 P. 127 fig. 7.7 P. 129 fig. 7.9 P. 130 fig. 7.11 P. 132 fig. 7.14 P. 136 fig. 7.19 P. 137 fig. 7.20 Quiz question #2: write these on the corresponding line and then answer question Line 1: Light intensity Line 2: Wind Line 3: Humidity
