04 Transport System in Multicellular Plants
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04 Transport System in Multicellular Plants

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    04 Transport System in Multicellular Plants 04 Transport System in Multicellular Plants Presentation Transcript

    • Transport system inmulticellular plants ALBIO9700/2006JK
    • Need for transport systems  Plants have thin, flat leaves which present large surface area to the sun  Relatively easy for CO2 and O2 to diffuse into and out of the leaves, reaching and leaving every cell quickly enough so that there is no need for a transport system for these gases  Plants have two transport systems:  For carrying mainly water and inorganic ions from roots to the parts above the ground  For carrying substances made by photosynthesis from the leaves to the other areas  However, fluids don’t move as rapidly as blood does in a mammal, nor is there an obvious pump such as the heart  Neither plant transport system carries O2 and CO2 ALBIO9700/2006JK
    • Distribution of xylem and phloem tissuein roots of dicotyledonous plants ALBIO9700/2006JK
    • Distribution of xylem and phloem tissuein stems of dicotyledonous plants ALBIO9700/2006JK
    • Distribution of xylem and phloem tissuein stems of dicotyledonous plantsSclerenchyma:Plant tissue whose function is to strengthen and support, composedof thick-walled cells that are heavily lignified (toughened).Parenchyma:Plant tissue composed of loosely packed, more or less spherical cells,with thin cellulose walls. Although parenchyma often has nospecialized function, it is usually present in large amounts, forming apacking or ground tissue.Collenchyma:Plant tissue composed of relatively elongated cells with thickened cellwalls, in particular at the corners where adjacent cells meet.It is a supporting and strengthening tissue found in non-woodyplants, mainly in the stems and leaves. ALBIO9700/2006JK
    • Distribution of xylem and phloem tissuein leaves of dicotyledonous plants ALBIO9700/2006JK
    • Distribution of xylem and phloem tissuein leaves of dicotyledonous plants ALBIO9700/2006JK
    • Transpiration  The loss of water vapour by diffusion down a water potential gradient from a plant to its environment  Mostly takes place through the stomata on the leaves  Transport of water  Water from soil – root hair  Root – xylem tissue  Xylem tissue – stem  Stem - leaves ALBIO9700/2006JK
    • From soil to root hair  Water moves into the root hairs down a water potential gradient through a partially permeable membrane into the cytoplasm and vacuole of the root hair cell  Very fine root hairs provides a large surface area in contact with soil water, increasing the rate of water absorbed  Micorrhizas  associations formed by fungi located in or on roots which serve a similar function to root hairs  act like a mass of fine roots which absorb nutrients, especially phosphate  Some plants growing on poor soils are unable to survive without these fungi  The fungi receive organic nutrients from the plant ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • From root hair to xylem  Water moves down the water potential gradient across the root (water potential inside xylem vessels < water potential in root hairs)  Two possible routes through the cortex:  Apoplast pathway – water seeps across the root from cell wall to cell wall without entering cytoplasm of cortical cells  Symplast pathway – water moves into the cytoplasm or vacuole of cortical cell and into adjacent cells through plasmodesmata  Apoplast pathway barred at stele (endodermis have a thick, waterproof, waxy band of suberin in cell walls – Casparian strip) due to impenetrable barrier to water  Only way to cross the endodermis is through the cytoplasm of the cells  Suberin deposits become more extensive as endodermal cells get older except in passage cells (gives plant control over inorganic ions and may help with generation of root pressure)  Once across the endodermis, water continues to move down water potential gradient across the pericycle and towards the xylem vessels ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • From xylem to leaf  Water constantly moves out of xylem vessels down a water potential gradient either into the mesophyll cells or along their cell walls as water evaporates from the cell walls  The removal of water from top of xylem vessels reduces the hydrostatic pressure (pressure exerted by a liquid)  Pressure difference causes water to move up the xylem vessels (water up a straw)  Xylem vessels have strong, lignified walls to stop from collapsing due to tension ALBIO9700/2006JK
    •  Movement of water up through xylem vessels is by mass flow (all water molecules move together as a body of liquid) Helped by water molecules attracted to each other (cohesion) and to the lignin in the walls of xylem vessels (adhesion) If an air bubble forms in the continuous column of water, column breaks and difference in pressure cannot be transmitted through the vessel (air lock) Adaptive features:  Small diameter of xylem vessels – prevents breaks  Pits in vessel walls  Allow water to move out (bypass air lock)  Air bubbles cannot pass through pits  Allows water to move out of xylem vessels to surrounding living cells ALBIO9700/2006JK
    • ALBIO9700/2006JK
    •  Root pressure  Plants may also increase pressure difference by raising the water pressure at the base of the vessels  By active secretion of solutes (active transport) into water in xylem vessels in root  Solutes lowers water potential, draws in water and increases water pressure  Water transport is a passive process fuelled by transpiration from the leaves ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • From leaf to atmosphere -transpiration  Air inside leaf (spaces around mesophyll) is usually saturated with water vapour from water around mesophyll cell walls  If there is a water potential gradient between the air inside leaf and outside, water vapour will diffuse out through small pores (stomata) – transpiration  Increase in the water potential gradient between the air spaces in the leaf and the air outside will increase rate of transpiration  Transpiration cools leaves ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Factors that affect transpiration rate  Temperature, wind speed, light intensity or humidity  Rate of water vapour leaving leaves vs. Rate of water taken up by stem  High proportion of water taken up is lost by transpiration  Rate at which transpiration is happening directly affects the rate of water uptake  Potometer  Completely water-tight and airtight (no leakage of water and no air bubbles break the continuous water column)  Submerge in water  Cut the end of the stem with a slanting cut  Position of the meniscus at set time intervals is recorded. Plot a graph of distance moved against time. Compare rates of water uptake under different conditions. ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Xylem vessel element  Tissue functions in support and transport  Angiosperms (flowering plants except conifers) contain several different types of cell:  Vessel elements and tracheids are involved in transport of water  Fibres are elongated cells with lignified walls that help support the plant (dead cells)  Parenchyma cells are ‘standard’ plant cells except they don’t usually have chloroplasts (shapes vary but often isodiametric) ALBIO9700/2006JK
    •  Xylem vessels  Vessels are made up of many elongated vessel elements arranged end to end  Lignin is laid down in the cell wall  As it builds up, cell dies and an empty space (lumen) is left  Lignin not laid down at plasmodesmata areas leaving ‘gaps’ called pits (not open pores; crossed by permeable, unthickened cellulose cell wall)  The end walls of neighbouring vessel elements break down completely to form a continuous non-living tube (xylem vessel) ALBIO9700/2006JK
    •  Tracheids  Dead cells with lignified walls but without open ends (don’t form vessels)  Elongated cells with tapering ends  They have pits in the walls so water can pass from one tracheid to the next  Main conducting tissue only in ‘primitive’ plants i.e. ferns and conifers  In the root, water which has crossed the cortex, endodermis and pericycle moves into the xylem vessels through the pits in their walls and then moves up the vessels towards the leaves ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Xerophytes  Plants that live in places where water is in short supply requiring adaptations to reduce the rate of transpiration  The marram grass leaves can roll up, exposing a tough, waterproof cuticle to the air outside the leaf, while the stomata open into the enclosed, humid space in the middle of the ‘roll’. Hairs trap a layer of moist air close to the leaf surface (reducing diffusion gradient)  Opuntia is a cactus stems that store water. Leaves are reduced to spines, which reduce surface area for transpiration  Sitka spruce have leaves in the form of needles (reducing surface area available for water loss). Covered in a layer of waterproof wax and have sunken stomata  Phlomis italica have ‘trichomes’ that act as a physical barrier to the loss of water  The cardon has swollen, succulent stems that store water and photosynthesise. The stems are coated with wax and leaves are extremely small ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Plant adaptations to arid conditions Adaptation Examples Needle-like leaves, bipinnate Mulga (acacias), she-oaks (casuarinas), leaves, phyllodes hakeas, Xanthorrhoea (grass trees) Photosynthetic stems She-oaks (Casuarina) Woody fruits Banksias, hakeas Waxy leaves Saltbush (Atriplex) Ephemeral growth Ephemeral plants e.g. paper daisies (Helipterum), yellow tops (Senecio) Partially deciduous Eucalypts Leaf curling Hummock grass (Triodia) Sunken stomata Hakeas Water storage Baobab tree, parakeelyas Hanging leaves Eucalypts (sclerophylls) Hairy or shiny leaves Banksias, paper flowers (Thomasia) Water-directing leaves and stem Mulga (acacia), Xanthorrhoea (grass trees) ALBIO9700/2006JK
    • Mulga (acacias) ALBIO9700/2006JK
    • Casuarinas ALBIO9700/2006JK
    • Hakeas ALBIO9700/2006JK
    • Banksias ALBIO9700/2006JK
    • Banksia fruits ALBIO9700/2006JK
    • SaltbushGrass trees ALBIO9700/2006JK
    • Yellow topsPaper daisies ALBIO9700/2006JK
    • Eucalypts ALBIO9700/2006JK
    • Hummock grass ALBIO9700/2006JK
    • Baobab treeParakeelyas ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Translocation  Transport of soluble organic substances within a plant, for example sugars (assimilates)  Transported in sieve elements which are found in phloem tissue along with several other types of cells including companion cells (parenchyma and fibres)  Phloem sap moves by mass flow  To create the pressure differences needed for mass flow in phloem, the plant has to use energy (active process) ALBIO9700/2006JK
    •  The pressure difference is produced by active loading of sucrose into the sieve elements (in the photosynthesising leaf) This decreases the water potential in the sap inside sieve element Water follows sucrose into the sieve element (osmosis – down water potential gradient) At other points, sucrose is removed and water follows by osmosis In the leaf water moves into sieve tube, in the root water moves out ALBIO9700/2006JK
    •  Pressure difference created causing water flow from the high pressure area to the low pressure area, taking with it any solutes Source: any area of a plant in which sucrose is loaded into the phloem Sink: any area where sucrose is taken out of the phloem Sap flows both upwards and downwards in phloem (xylem always upwards) Can only flow one way in any particular sieve tube at any one time ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Loading of sucrose into phloem  Sucrose in solution moves from mesophyll cell to the phloem tissue  May move by the symplast pathway (moving from cell to cell via plasmodesmata) or apoplast pathway (travelling along cell walls)  Companion cells and sieve elements work in tandem  Sucrose is loaded into a companion cell by active transport  H+ are moved out of the companion cells using ATP  Large excess of H+ outside  Can move back into cell down concentration gradient through protein which acts as carrier for both H+ and sucrose  Sucrose molecules are carried through this co-transporter molecule into companion cell against concentration gradient for sucrose  Sucrose molecules can then move from the companion cell into the sieve tube (through plasmodesmata) ALBIO9700/2006JK
    • Unloading of sucrose from phloem  Occurs into any tissue which requires sucrose but mechanism still unknown  Probably by diffusion  In tissue, enzymes convert sucrose into something else (e.g. invertase hydrolyses sucrose to glucose and fructose)  This decreases its concentration and maintains concentration gradient ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Evidence for the mechanism of phloem transport  Phloem protein is not present in living, active phloem tissue  The rate of transport in phloem is about 10,000 times faster than diffusion  Considerable evidence for the active loading of sucrose into sieve elements in sources such as leaves:  Phloem sap always has a relatively high pH (around 8) – expected if H+ is being actively transported out of the cell  There is a difference in electrical potential across the plasma membrane (-150mV) – consistent with excess of H+ outside the cell compared with inside  ATP is present in phloem sieve elements in large amounts – expected as it is required for active transport of H+ out of cell ALBIO9700/2006JK
    • Sieve elements  Sieve tubes are made up of many elongated sieve elements, joined end to end vertically to form a continuous column  A living cell (cellulose cell wall, plasma membrane and cytoplasm containing ER and mitochondria)  Amount of cytoplasm is very small and only forms a thin layer lining the inside of the wall of the cell  No nucleus and ribosomes  Sieve plate: made up of end walls of 2 meeting sieve elements, perforated by large pores  In living phloem, pores are always open, presenting little barrier to the free flow of liquids through them  Contents of phloem sieve tubes (phloem sap) ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Companion cells  Each sieve element has one companion cell lying beside it  Cellulose cell wall, plasma membrane, cytoplasm, small vacuole and nucleus  Number of mitochondria and ribosomes is larger than normal (metabolically very active)  Numerous plasmodesmata pass through their cell walls (direct contact between cytoplasms of companion cell and sieve element ALBIO9700/2006JK
    • ALBIO9700/2006JK
    • Difference between sieve elements and xylem vessels  Xylem vessels are dead, translocation through phloem sieve tubes involves active loading of sucrose at sources requiring living cells  Xylem vessels have lignified cell walls, whereas phloem tubes do not (lignin in cell walls kills the cell)  Water flow through dead xylem vessels unimpeded and strong walls support the plant  The end walls of xylem elements disappear completely, whereas those of phloem sieve elements form sieve plates (probably supporting structures/allows phloem to seal up rapidly if damaged/prevents entry of microorganisms which feed on the nutritious sap or cause disease) ALBIO9700/2006JK