Transport in Plants

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Transport in Plants
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What is the tallest tree on the planet?

• Seems like it would require a pump, like you and
  I have, but a much larger one to transport
  substances from roots to leaves. Trees as we know
  do not have any pumps of that nauture. So how
  do they do it?

• Sequoia sempervirens - The coastal redwood (115m
  379 feet)

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Maybe Plants Push Xylem Sap Root Pressure

• 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
• 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 leaves it (transpired),
  and the excess is forced out of the leaf.

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Plant transport mechanisms solve a fundamental
biological problem

• The need to acquire materials from the
  environment and distribute them throughout the
  entire plant body

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Activity1

• Clear nail polish
• Leaves
• Activity 2
• Flaccid carrot and cucumber slices
• Bowl
• dH2O, bottled water, tap water
• Salt

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Precursor 1 Water chemistry and characteristics

• Polarity
• H-bonds (Strong or weak? Can you draw and
  H-bond between 2 or more water molecules?)
• Consequences include Cohesion, Adhesion, Surface
  Tensionetc (properties of water)

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Activity 3 A mini-experiment/demonstration

• Indirect and relative measure of H-bond strength
  (as well as cohesion andadhesion)
• Glass slides
• Plastic cups
• Water
• Pennies
• Masking Tape (Thumbs)

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Precursor 2. Selective Permeability of Membranes

• The selective permeability of a the plasma
  membrane controls the movement of solutes into
  and out of the cell AND the role of
• Specific transport proteins are involved in
  movement of solutes (and water too!)
• Passive Transport Diffusion, Facilitated
  Diffusion, Osmosis (Differences?)
• Active Transport (Features of?)

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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
• They contribute to a voltage known as a membrane
  potential (Plant cytoplasm is (-) compared to
  extracellular fluid)
• Consequences include
• Fac diffusion of other cations
• Cotransport symport and antiport (secondary
  active transport)

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Membrane potential and cation uptake

• Plant cells use the proton gradient and membrane
  potential to drive the transport of many
  different solutes (e.g. cation () uptake
  opposites attract)

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Cotransport (symport)

• In cotransport a transport protein (known as a
  symport) couples the passage of one solute to the
  passage of another in the same direction

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Cotransport (Antiport)

• Energy released as a molecule (e.g.H) diffuses
  back into the cell and powers the active
  transport of a second molecule (ex. Ca or Na)
  out of the cell

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Sucrose uptake

• The cotransport is also responsible for the
  uptake of the sugar sucrose (a neutral solute) by
  plant cells

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An important membrane protein side note
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Water Potential

• To survive plants must balance water uptake and
  loss
• Water potential is a measurement that combines
  the effects of solute concentration and physical
  pressure (due the presence of the plant cell
  wall) It is a measurement of the FREE amount of
  water molecules and the direction of movement of
  water (i.e. waters potential to do work).
• Water flows from regions of high water potential
  (areas of more free water molecules) to regions
  of low water potential (less free water
  molecules)

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Ex. of water doing work on an organismal level
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Which has the greatest water concentration?

• A or B A or B
• Water potential is essentially not much different

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Getting a little technical - The water potential
equation. Dont freak out! Think Poseidon!
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By convention, plant physiologists measure water
potential in units of pressure called megapascals
(MPa). Note bars is acceptableFor a baseline,
the water potential for pure water at 1ATM is
expressed as having 0 Mpa or 0 bars
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Breaking it down.
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Contd
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(No Transcript)
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Consider this (U-tube Examples AP Loves them)

• An artificial model

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Contd Addition of Solute example
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Contd Positive Pressure Example
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Contd A negative pressure example
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Connection to plants
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AP will not be thrilled however if that was your
response to an Explain what happens prompt

• So whats a better answer?

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AP Explain what happens prompt possible answers

• 1 star The cell gains water
• 2 stars Since water moves from high water
  potential to low water potential, it will enter
  the cell.
• 3 stars (include the data if provided) Since
  the water potential for the cell is -0.7 bars and
  the surrounding environment has a water potential
  of 0 bars, water moves into the cell.
• 4 stars (include consequences ?) Since the
  water potential for the cell is -0.7 bars and the
  surrounding environment has a water potential of
  0 bars, water moves into the cell making it
  turgid.

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Contd Produce 4 star answer for scenario B(At
home, not now )

• Note The original cell has a starting water
  potential of -0.7 bars

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Compare each situation with respect to the
cytoplasms water potential and the surrounding
environments water potential

• cell env. cell env. cell
  env.
• Water Pot
• Bonus info, free of charge What could you say
  about each situations
  cell env cell env cell env
• Water concentration?
• Solute Concentration?
• Osmotic potential?

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Collaborative Review/Study Break

• On mini-poster paper
• 1. Explain the role(s) of a gradient of protons
  in moving substances across a plant cells plasma
  membrane
• 2. How do symports and antiports differ? Give an
  example of key substances each mechanism
  transports.
• 3. What is water potential and discuss why it
  is important with respect to plant cells

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CHECK YOUR VEGETABLE AND YOUR FRUIT!!

• Evaluate your slices
• Explain what has happened to them to a classmate
  (or to a teacher)

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Next Step How do roots take in 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 and through the shoot system
  (xylem tissue) by bulk flow and active transport
  respectively.
• Bulk flow the group movement of molecules in
  response to a difference in pressure between two
  locations (see more later)
• Soil solution?Root Hair Epidermis?Root Cortex
  ?Root Xylem

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Contd

• 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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A mutulaistic symbiotic relationship. and a
surface area multiplier
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Plant Cell Structure- more info for understanding
transport

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

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Water travels to the root xylem by one of three
pathways

• 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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Lateral transport of minerals and water in roots
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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 (the loss of water vapor through
  the stomata) 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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Pulling Xylem Sap

• The Transpiration-Cohesion-Tension Theory
• Transpirational Pull
• Water transport begins as water evaporates from
  the walls of the mesophyll cells inside the
  leaves and into the intercellular spaces
• Driven by the

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Cohesion and Adhesion in the Ascent of Xylem Sap

• The transpirational pull on xylem sap
• Solar Powered
• Bulk Flow (pressure differences created by water
  potential differences)
• 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
• Narrow diameter of xylem

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Contd

• Transpiration produces negative pressure
  (tension) in the leaf which exerts a pulling
  force on water in the xylem, pulling water into
  the leaf
• This water vapor escape through the stomata

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• The Transpiration Dance
• and
• Transpiration animations
• https//www.youtube.com/watch?vU4rzLhz4HHk

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Stomata and Transpiration Control

• Stomata help regulate the rate of transpiration
• Leaves generally have broad surface areas and
  high surface-to-volume ratios. Good and bad
• ? increase photosynthesis
• ? Increase water loss through stomata

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Check your nail-polished spinach leaves

• Tear your leaf as to produce a lip of dried
  nail polish
• Peel off as large a section of the dried nail
  polish only
• Microscopic observation reveals imprint of the
  organization of the leaf surface specifically
  stomata (guard cell) arrangement

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Stomata contd

• About 90 of the water a plant loses escapes
  through stomata (lenticel, cuticle other 10)
• Each stoma is flanked by guard cells which
  control the diameter of the stoma by changing
  shape

Guard Cells
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Shape changes due to multiple factors including

• Changes in turgor pressure that open and close
  stomata result primarily from the reversible
  uptake and loss of potassium ions (K) by the
  guard cells
• Creates water potential differences

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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
• Possess thicker waxy cuticles
• Sunken stomata
• Trichomes (hair)

Stomata in recessed crypts of Oleander plant
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Second Major Plant Tranport Event -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 sucrose solution
• 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 or a
  leaf too!

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Translocation of phloem sap contd The
pressure-flow hypothesis
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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.
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Answers to first study break sesssion

• 1. After an H gradient is established (by
  pumping protons out of the cell) the resulting
  inward flow of H down its concentration
  gradient provides energy to actively transport
  other substances into the cell
• 2. In symport, two substances move in the same
  direction through a cell membrane in antiport
  two substances cross the cell membrane in
  opposite directions
• Sample AP FR and Key