Ch. 36 Transport in Plants by yurtgc548


									Ch. 36 Transport in Plants
   Occurs at three levels:
       Uptake by cell
       Cell to cell
       Long distance from root to leaves and
Cellular uptake
   Passive transport-movement across
    membranes slow without:
       Transport proteins-in cell membrane.
              May bind to molecule and transport.(carrier protein)
              May create passageway (channel protein)
Active transport
   Moving solutes against conc. Gradient.
       Uses carrier proteins and energy (ATP)
       Ex. Proton pump-uses energy released to
        move Hydrogen ions to establish high
        conc. (remember electron
Water potential
   Water moves by osmosis
   Moves from hypotonic to hypertonic
   In plants, cell wall has an effect as well
    as solute conc. Combined, they equal
    water potential.
Water potential basics

   Movement from high to low.
   Called water potential because moving
    water can do work-potential energy.
   Water potential of pure water open in
    atmosphere =zero.
   Solutes lower water potential.
   Increase in physical pressure increases
    water potential.
Water potential cont.
   As solute increases, water potential
    decreases. As pressure increases, so
    does water potential.
   Explains water movement across
   See p.751 and 752
Transport across membranes
   Water is polar; inside cell membrane is
   Selective channels called aquaporins aid in
    passive transport of water across membrane.
   Cell walls of plant cells allow continuous
    movement across.
Absorption by root
   Root hairs-grow from epidermal cells near tip;
   Soil sticks to hairs; water and minerals stick
    to soil.
   Mycorrhizae-fungus fibers; symbiotic
    relationship with root hairs.
   Soil solution moves into epidermal walls
    (hydrophilic) and moves into root cortex.
   Connections between cytoplasm area of plant cells
    allows continuous flow. (symplast)
Lateral transport
   Movement of materials from outer to inner
   Transmembrane movement-across plasma
    membrane after plasma membrane.
   Apoplastic-movement through cell walls.
   Symplastic-movement through cytoplasm
    connected by plasmodesmata.
Root structure and water
   Water moves through cortex
   Reaches inner layer of cortex known as
    endodermis. Protects inner area of root
    known as stele.
   Stele contains xylem and phloem
   Endodermal cells have waxy layer called
    Casparin strip.
   Water moving through symplast goes
    into endodermis.
   Water moving apoplastically has barrier.
   Water must move across endodermis
    plasma membrane and then in
   Water moves into tracheids and vessel
    element cells of xylem.
   Loss of water vapor through leaves and
    aerial parts of plants.
   Water moves out through pores called
   Stomata are connected to air spaces in
    spongy mesophyll of leaf tissue.
   Water found in air spaces because
    space in contact with moist cells.
Root pressure
   Xylem contains minerals as well as water.
   Transpiration low at night; minerals still
    pumped in causing decrease in water
   Water moves in creating a pressure or push.
   Root pressure causes guttation-water
    droplets on leaf margins.
   Root pressure NOT major force moving water
Transpiration-Cohesion Pull
   Water molecules exhibit hydrogen bonding.
   Hydrogen bonding results in cohesion and
   Transpiration causes negative pressure in
    xylem; water sticks together.
   Air dry--- water exits stomata—water from
    cells moves into air space as replacement---
    pulls on water in xylem.
Transpiration-Cohesion Cont.
   Water sticks to hydrophilic cell walls of
    tracheids and vessel elements.
   Water chain must remain continuous.
   Bulk flow of water due to solar energy;
    no energy of plant used.
Controlling transpiration
   Stomata open to allow CO2 in. Water
    can escape.
   Oxygen from photosynthesis leaves
    through stomata.
   Stoma surrounded by two guard cells.
   Guard cells change shape to control size
    of opening.
Guard cell function
   Guard cells take in or lose water due to
    K+ conc.
   As this ion conc. increases, water
    moves in; guard cells become
    turgid;cells are thicker along outer edge
    so they bow. (p.760)
   Stomata typically open during day and
    closed at night.
Guard cells cont.

   K+ can be stimulated by light; depletion
    of CO2 can cause stoma to open;
    internal clock-circadian rhythms-can
    control opening/closing.
   High temp. can cause closing of
    stomata along with high transpiration.
   Abscisic acid made due to low water
    can cause guard cells to shrink.
Translocation-transport of
food in plants
   Occurs in phloem.
   Phloem sap may move up or down.
   Movement of food from “source” (high
    conc.) to “sink” (low conc.)
   Fruit and areas of plant growth are
   Cells of phloem tissue called sieve
Translocation cont.
   Phloem also has companion cells-have
    nucleus and organelles-help control what
    occurs in sieve tube cells.
   Food moves from source to sink due to
    pressure flow.
   High solute conc. causes water to move into
    sieve tube; creates pressure.
   As solutes leave sink, water follows which
    causes loss of pressure.
   Sugar movement into sieve tubes and
    companion cells accomplished by active
    transport; mechanism too fast by simple
   Sucrose too big to cross membranes on own.
   In spring, roots of trees and other perennials
    are source and stem/leaf cells coming out of
    dormancy are sink area. Sap moves up.

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