Chapter 36 Transport in Plants

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Chapter 36Transport in Plants
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• For vascular plants the evolutionary movement
  onto land involved the differentiation of the
  plant body into roots and shoots
• Vascular tissue transports nutrients throughout a
  plant this transport may occur over long
  distances

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Transport Scale/Distance

• Transport in vascular plants occurs on three
  scales
• Transport of water and solutes by individual
  cells, such as root hairs
• Short-distance transport of substances from cell
  to cell at the levels of tissues and organs
• Long-distance transport within xylem and phloem
  at the level of the whole plant

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Overview of Transport in Plants
CO2
O2
Light
H2O
Sugar
Sugars are transported as phloem sap to
roots and other parts of the plant.
O2
H2O
CO2
Minerals
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Selective Permeability of Membranes

• The selective permeability of a the plasma
  membrane controls the movement of solutes into
  and out of the cell
• Specific transport proteins are involved in
  movement of solutes

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Proton Pumps

• Proton pumps create a hydrogen ion gradient that
  is a form of potential energy that can be
  harnessed to do work
• Contribute to a voltage known as a membrane
  potential

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• Plant cells use energy stored in the proton
  gradient and membrane potential to drive the
  transport of many different solutes

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Cotransport

• In cotransport a transport protein couples the
  passage of one solute to the passage of another

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Sucrose uptake

• The cotransport is also responsible for the
  uptake of the sugar sucrose by plant cells

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Happy 200th Birthday Mr. Darwin
Feb 12th 1809
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Water Potential

• To survive plants must balance water uptake and
  loss
• Remember Osmosis is the passive transport of
  water across a membrane
• Water potential is a measurement that combines
  the effects of solute concentration and physical
  pressure (due the presence of the plant cell
  wall) and determines the direction of movement of
  water
• Water flows from regions of high water potential
  to regions of low water potential

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Plasmolysis

• If a flaccid cell (not firm) is placed in an
  environment with a higher solute concentration
  (Hypertonic)
• The cell will lose water and become plasmolyzed

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Turgidity

• If the same flaccid cell is placed in a solution
  with a lower solute concentration (Hypotonic)
• The cell will gain water and become turgid
• Healthy plant cells are turgid most of the time

Turgidity helps support nonwoody plant parts
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Wilting

• Turgor loss in plants causes wilting which can be
  reversed when the plant is watered

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Aquaporins

• Water molecules are small enough to move across
  the lipid bilayer of the plasma membrane, but
  they also move through specific channels for the
  passive diffusion of water across the
  membraneAquaporins
• Aquaporins dont effect the direction of water
  flow, but rather the rate of diffusion.

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Three Major Compartments of Vacuolated Plant Cells

• Transport is also regulated by the compartmental
  structure of plant cells (3 compartments)
• Cell Wall
• The plasma membrane controls the traffic of
  molecules into and out of the protoplast and is
  the barrier between the cell wall and the cytosol
• Cytosol
• Vacuole

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Vacuole

• The vacuole is a large organelle that can occupy
  as much as 90 of more of the protoplasts volume
  
• The vacuolar membrane
• Regulates transport between the cytosol and the
  vacuole

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Cell-to-cell continuity

• The cell walls and cytosol are continuous from
  cell to cell in most plant tissues
• The cytoplasmic continuum is called the symplast
• The cell wall continuum is called the apoplast

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Short Distance Transport of Water/Solutes in
Tissues and Organs

• Water and minerals can travel through a plant by
  one of three routes
• Out of one cell, across a cell wall, and into
  another cell (transmembrane route)
• Via the symplast (symplastic route)
• Along the apoplast (apoplastic route)

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Bulk Flow in Long-Distance Transport

• In bulk flow movement of fluid in the xylem and
  phloem is driven by pressure differences at
  opposite ends of the xylem vessels and sieve
  tubes
• Transpiration (evaporation of water from a leaf)
  reduces pressure in the leaf xylem. This creates
  a tension (negative pressure) that pulls material
  up the xylem
• In phloem, hydrostatic pressure (pressure exerted
  upon the tube from the surrounding tissue) is
  generated at one end of a sieve tube, which
  forces sap to the other end of the tube

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Roots absorb water and minerals from the soil

• Water and mineral salts from the soil enter the
  plant through the epidermis of roots and
  ultimately flow to the shoot system
• Soil solution?Root Hair Epidermis?Root Cortex
  ?Root Xylem
• Root Hairs
• Much of the absorption of water and minerals
  occurs near root tips, where the epidermis is
  permeable to water and where root hairs are
  located
• Root hairs account for much of the surface area
  of roots

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Root growth
Movie
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Lateral transport of minerals and water in roots
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Mycorrhizae

• Most plants form mutually beneficial
  relationships with fungi, which facilitate the
  absorption of water and minerals from the soil
• Roots and fungi form mycorrhizae, symbiotic
  structures consisting of plant roots united with
  fungal hyphae

White mycelium of the fungus around this pine
root provides a vast surface area for absorption
of water and minerals from the soil.
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The Endodermis

• Is the innermost layer of cells in the root
  cortex
• Surrounds the vascular cylinder and functions as
  the last checkpoint for the selective passage of
  minerals from the cortex into the vascular tissue
• Water can cross the cortex via the symplast or
  apoplast
• The waxy Casparian strip of the endodermal wall
  blocks apoplastic transfer (but not symplastic)
  of water and minerals from the cortex to the
  vascular cylinder

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Ascent of Xylem Sap

• Plants lose an enormous amount of water through
  transpiration and the transpired water must be
  replaced by water transported up from the roots
• Xylem sap rises to heights of more than 100 m in
  the tallest plants

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Pushing Xylem Sap Root Pressure

• At night, when transpiration is very low, root
  cells continue pumping mineral ions into the
  xylem of the vascular cylinder, which lowers
  water potential
• Water flows in from the root cortex generating a
  positive pressure that forces fluid up the xylem.
  This is upward push is called root pressure

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Root Pressure-Gluttation

• Root pressure sometimes results in guttation,
  (the exudation of water droplets on tips of grass
  blades or the leaf margins of some small,
  herbaceous dicots in the morning). More water
  enters the leaves than is transpired, and the
  excess is forced out of the leaf.

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Pulling Xylem Sap

• The Transpiration-Cohesion-Tension Mechanism
• Transpirational Pull
• Water vapor in the airspaces of a leaf diffuses
  down its water potential gradient and exits the
  leaf via stomata
• Transpiration produces negative pressure
  (tension) in the leaf which exerts a pulling
  force on water in the xylem, pulling water into
  the leaf

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Cohesion and Adhesion in the Ascent of Xylem Sap

• The transpirational pull on xylem sap
• Is transmitted all the way from the leaves to the
  root tips and even into the soil solution
• It is facilitated by the cohesion and adhesion
  properties of water

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Transpiration Control

• Stomata help regulate the rate of transpiration
• Leaves generally have broad surface areas and
  high surface-to-volume ratios
• These characteristics (1) increase photosynthesis
  (2) Increase water loss through stomata

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Effects of Transpiration on Wilting and Leaf
Temperature

• Plants lose a large amount of water by
  transpiration. If the lost water is not replaced
  by absorption through the roots the plant will
  lose water and wilt
• Transpiration also results in evaporative
  coolingwhich can lower the temperature of a leaf
  and prevent the denaturation of various enzymes
  involved in photosynthesis and other metabolic
  processes

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• About 90 of the water a plant loses escapes
  through stomata
• Each stoma is flanked by guard cells which
  control the diameter of the stoma by changing
  shape

Guard Cells
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• Changes in turgor pressure that open and close
  stomata result primarily from the reversible
  uptake and loss of potassium ions by the guard
  cells

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Xerophyte Adaptations That Reduce Transpiration

• Xerophytes are plants adapted to arid climates
• They have various leaf modifications that reduce
  the rate of transpiration
• The stomata of xerophytes
• Are concentrated on the lower leaf surface
• Are often located in depressions that shelter the
  pores from the dry wind

Stomata in recessed crypts of Oleander plant
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Translocation of Phloem Sap

• Organic nutrients are translocated through the
  phloem (translocation is the transport of organic
  nutrients in the plant)
• Phloem sap
• Is an aqueous solution that is mostly sucrose
• Travels from a sugar source to a sugar sink
• A sugar source is a plant organ that is a net
  producer of sugar, such as mature leaves
• A sugar sink is an organ that is a net consumer
  or storer of sugar, such as a tuber or bulb

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Seasonal Changes in Translocation

• A storage organ such as a tuber or bulb may be a
  sugar sink in summer as it stockpiles
  carbohydrates.
• After breaking dormancy in the spring the storage
  organ may become a source as its stored starch is
  broken down to sugar and carried away in phloem
  to the growing buds of the shoot system

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Phloem loading

• Sugar from mesophyll leaf cells must be loaded
  into sieve-tube members before being exported to
  sinks
• Depending upon the species, sugar moves by
  symplastic and apoplastic pathways

In many plants phloem loading requires active
transport. Proton pumping and cotransport of
sucrose and H enable the cells to accumulate
sucrose.