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Plant Transport

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Water loss due to Transpiration Plants lose a great deal of water due to transpiration. Basically the open stomata allow for evaporation of water from the leaves. – PowerPoint PPT presentation

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Title: Plant Transport


1
Plant Transport
2
  • There are three levels at which transport in
    vascular plants occurs
  • movement into and out of individual cells,
  • cell to cell transport over short distances
    (e.g., within a particular tissue), and
  • (3) long-distance transport of water, minerals,
    sucrose and other substances via the vascular
    tissue.

3
Review of transport at the cellular level Part I
  • Diffusion (Simple) - the movement of substances
    from an area of higher concentration to an area
    of lower concentration (or down a concentration)
    gradient across a selectively permeable membrane
    (pore size and polarity are important in this
    type of movement).

4
Review of transport at the cellular level Part II
  • Osmosis is the movement of water across a
    selectively permeable membrane in response to the
    relative concentrations of solutes on the inside
    and outside of the membrane. The external
    environment of cells may be isotonic (same solute
    concentration), hypertonic (higher solute
    concentration), or hypotonic (lower solute
    concentration)

5
Review of transport at the cellular level Part
III
  • Facilitated Diffusion the movement of molecules
    with a concentration gradient but using a
    membrane protein which functions as a channel or
    as a carrier (facilitated diffusion). This type
    of transport does not require energy (nor do the
    others described above).

6
Review of transport at the cellular level Part IV
  • Active transport involves the movement of
    substances against a concentration gradient and
    it requires an energy expenditure

7
Short-distance Travel of Solutes
  • Instead of Na, H is pumped by proton pumps
    across the cell membrane. For example, the
    membrane potential is achieved by the presence of
    a greater concentration of H outside of the
    plant cell membrane compared to the H
    concentration inside the membrane.
  • In addition, H is cotransported with other ions
    and neutral substances in plants instead of Na
  • Like animal cells, gated ion channels are also
    present in plant cell membranes that open and
    close in response to chemical, pressure or
    voltage stimuli.

8
Short-distance Transport of Water Across Plasma
Membranes Part I
  • The physical property that predicts the direction
    in which water will flow in plants is called
    water potential. This adds the effect of
    physical pressure to solute concentration.
  • Please read pp. 768-771.

9
Short-distance Transport of Water Across Plasma
Membranes Part II
  • solute physical pressure water potential.
  • The abbreviation for water potential is the Greek
    letter psi (y) with units of pressure called a
    megapascal (MPa). Pure water open to the air
    under standard conditions (sea level and room
    temp.) is 0 MPa, inside a living plant cell the
    internal pressure is _at_ 0.5 MPa. Potential
    refers to waters potential energy (waters
    capacity to do work when it moves from a higher y
    to a lower y). Water will move across a membrane
    from the solution of higher water potential to
    the solution with the lower water potential.
  • The y equation is y yS yP
  • Where yS solute potential or osmotic
    potential and yP the pressure potential

10
Short-distance Transport of Water Across Plasma
Membranes Part III
  • The yS of a solution is proportional to the
    number of dissolved solutes and thus the yS of
    pure water 0. The physical pressure on a
    solute is yP this value can be positive or
    negative

11
Short-distance Transport of Water Across Plasma
Membranes Part IV
  • Another factor in water transport in plant (and
    animal) cells is the presence of water transport
    proteins in the cell membrane. These aquaporins
    do not affect the water potential gradient or the
    direction of water flow instead they affect the
    rate of water flow

12
Lateral transport Part I
  • Short distance travel between cells is called
    lateral transport. There are three routes for
    this type of transport

13
Short distance travel Part II
  • Symplastic transport involves the movement of
    substances across one plasma membrane and then
    the substances move from cell to cell via
    plasmodesmata

14
Short distance travel Part III
  • Apoplastic transport involves the movement of
    substances along an apoplast (an extracellular
    pathway consisting of the cell walls, the
    extra-cellular spaces, and interior of dead
    cells)

15
Short distance travel Part IV
  • Transmembrane transport involves the movement of
    substances from one cell, across the cell wall
    and then into an adjacent cell. It requires
    repeated crossings of plasma membranes. It also
    means that materials are moving through the
    symplast and the apoplast

16
Bulk flow
  • is the long distance movement of water and
    solutes through vascular tissue that is driven by
    pressure. Hydrostatic pressure is involved in
    the movement of phloem sap while negative
    pressure is involved in the movement of the fluid
    in xylem.

17
  • Three specific Transport mechanisms
  • in plants
  • Absorption of water and minerals by roots
  • Transport of Xylem Sap
  • Translocation of Phloem Sap

18
Absorption of water and minerals by roots
  • This occurs when water and minerals tagging along
    with the water cross the epidermis of the root,
    then cross the root cortex, then pass into the
    stele (see chapter 35), and finally flow up the
    xylem.

19
Absorption of water
  • occurs primarily at the root tips and in
    particular across the root hairs. Soil solution
    flows into the hydrophilic walls of epidermal
    cells, then passes along the apoplast into the
    cortex of the root. As it moves along the
    apoplast, epidermal and cortex cells take up
    water and some solutes into the symplast.
  • Mycorrhizae facilitate greatly (through increased
    surface area) the movement of water and minerals
    into roots

20
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21
Transport of Xylem Sap
  • (figure 36.13). The movement of water from the
    roots through the xylem of the plant to the
    uppermost part of the shoot system is influenced
    by root pressure, transpirational pull, and
    cohesion and adhesion of water molecules.

22
Water loss due to Transpiration
  • Plants lose a great deal of water due to
    transpiration. Basically the open stomata allow
    for evaporation of water from the leaves. An
    average sized maple tree can lose more than 200 L
    of water per hour during summer months.
    Obviously then there must be a continuous flow of
    water from the roots to the leaves for
    replacement.

23
Guttation
  • There is an upward push of water and minerals
    from the roots upward during the night when
    transpiration is greatly reduced in some smaller
    plants.
  • Root cells use energy to pump mineral ions into
    the xylem, leakage of these ions out of the stele
    is prevented by the endodermis surrounding the
    stele.
  • The accumulation of mineral in the stele lowers
    water potential there. Water flows in from the
    root cortex, generating a positive pressure that
    forces fluid up the xylem.
  • The upward push is root pressure. Root pressure
    causes a phenomenon known as guttation. Because
    root pressure is pushing water up and because
    transpiration is reduced, more water enters the
    leaves than is transpired, and thus the excess is
    forces out as guttation fluid. Overall root
    pressure is one of the more minor factors
    involved in the movement of water and minerals
    upward in a plant.

24
Transpiration continued
  • Most of the water that is moved within the plant
    is pulled upward from the roots instead of pushed
    from the roots. The mechanism that accounts for
    this movement is the Transpiration-Cohesion-Tensio
    n Mechanism.

25
More about Transpiration
  • Plants must gain CO2 in order to undergo
    photosynthesis. This is accomplished by the
    opening of the stomata and the internal spaces in
    the leaves that allow CO2 to be brought into
    close proximity to the cells where photosynthesis
    takes place. But because the air is in generally
    drier, the gaseous water diffuses down its
    concentration gradient and exits the leaf via the
    stomata, a process called transpiration.
  • Transpiration depends on the generation of a
    negative pressure (tension) in the leaf due to
    the unique properties of water. Mesophyll cells
    are coated by a thin film of water but this water
    is constantly lost to the drier air in the spaces
    between the cells. The adhesion of water to the
    wall of cells and surface tension cause the
    surface of the water film to form a meniscus the
    water is being pulled on by the adhesive and
    cohesive forces. The water film has a negative
    pressure and the more concave the meniscus, the
    more negative the pressure which is the pulling
    force that draws water out of the leaf xylem,
    through the mesophyll, and toward the cells and
    surface film bordering the air spaces near
    stomata.

26
  • The Control of Transpiration

27
Control of Transpiration Part I
  • The guard cells of the stomata help a plant
    balance the need to conserve water with the
    requirement for CO2 by controlling the size of
    the stomata

28
Control of Transpiration Part II
  • One way to evaluate how effectively a plant uses
    water is to calculate its transpiration-to-photosy
    nthesis ratio. This is the amount of water lost
    per gram of CO2 assimilated onto organic material
    by photosynthesis. This ratio is about 600 g
    water lost1g of carbohydrate that is
    incorporated into plant tissue. But C4 plants
    are more efficient with a 3001 ratio or less.
  • The CI-340 Handheld Photosynthesis System
    features a new design concept and compact
    solid-state structure. The entire system the
    display, key pad, computer, data memory, CO2 /
    H2O gas analyzers, flow control system and
    battery are contained in a single, hand-held
    chassis. Everything required to measure
    photosynthesis, transpiration, stomatal
    conductance, PAR and internal CO2 is conveniently
    included in one easy to operate instrument

29
Transpiration Part III
  • Transpiration also supplies minerals that are
    transported upward in the xylem with the water
    and helps the leaves to thermoregulate. But if
    transpiration exceeds the delivery of water by
    the xylem, then the plant will wilt. However,
    plants are able to exert some control over the
    rate of transpiration by changing the size of the
    stomata opening. How does this happen? When
    guard cells take in water via osmosis, they
    become more turgid and they swell. But the cell
    walls of guard cells (in most dicots) are not
    uniformly thick and thus they buckle outward when
    they are turgid. This opens the stomata, but as
    water loss increases, the guard cells become less
    turgid and more flaccid, closing the opening. K
    ions are involved in these changes in turgor
    pressure.

30
Transpiration Part IV
  • There are three factors that affect the opening
    and closing of stomata Light/dark cycles. Light
    signals them to open by stimulating the
    accumulation of potassium (causing the guard
    cells to become turgid).
  • The depletion of CO2 also stimulates the opening
    of the stomata.
  • Thirdly, an internal clock that controls
    circadian rhythms also is involved in the opening
    of the stomata.
  • Environmental stressors can cause stomata to
    close e.g., water deficiency and loss of turgor
    pressure. The closing of stomata is also under
    hormonal control (abscisic acid). High
    temperatures can also result in the closing of
    the stomata.

31
Translocation of Phloem Sap
  • Phloem contains sieve tube members that function
    in the movement of sucrose containing phloem sap
    by a process called translocation.

32
  • Phloem containing the products of photosynthesis
    must also be transported throughout the plant,
    but unlike the movement of xylem this movement is
    variable so it can move in different directions
    than does the xylem sap. It does however, move
    from a sugar source to a sugar sink. The sugar
    source is the plant organ that is a net producer
    of sugar by either photosynthesis or the
    breakdown of the storage compound of plants,
    starch. The sugar sink is the organ that is a
    net consumer of store or sugar (e.g., growing
    roots, buds, stems and fruits.
  • Phloem sap differs greatly from xylem sap. It is
    composed primarily of sugar (as high as 30).
    Other possible components of phloem sap include
    minerals, amino acids, and hormones.
  • Sugar is produced in the mesophyll cells of the
    leaves and is then loaded into sieve-tube
    members. Some plants use only symplastic
    pathways others use both symplastic and
    apoplastic pathways
  • Phloem unloading at the sugar sink is a highly
    variable process (varying by species and by type
    of organ) but basically is the result of the
    lower sugar concentration in the tissues of the
    sugar sink compared to the higher concentration
    in the phloem.

33
  • Like the movement of water in the xylem, bulk
    flow is involved in the movement of phloem sap.
    This bulk flow is pressure driven
  • The pressure flow hypothesis explains why phloem
    sap always flows from source to sink in
    angiosperms, but much is unknown about the
    movement of phloem sap (especially in other
    vascular plants), but research is ongoing.
    Moreover, it appears that the rate of
    photosynthesis does not influence yield but
    instead research shows that the ability to
    transport sugars is the determining factor in
    yield. Thus there is current research in this
    area in order to discover ways to increase
    agricultural crop yields (e.g., genetically
    engineering higher yield crops).

34
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