Plant Transport

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Plant Transport
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• 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.

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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).

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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)

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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).

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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

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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.

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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.

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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

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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

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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

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Lateral transport Part I

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

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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

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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)

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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

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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.

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

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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.

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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

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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.

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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. 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.

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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.

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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.

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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.

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• The Control of Transpiration

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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

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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

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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.

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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.

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Translocation of Phloem Sap

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

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• 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.

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• 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).

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