Lectures 15-19

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Published on 20 Apr 2013
School
University of Waterloo
Department
Biology
Course
BIOL120
Professor
LEC 15
Plant Transport
- symplastic transport = Molecules move thru ‘living’ space (interior of cells
bounded by plasma mem)
o Diffusion = move across mem
o Facilitated trans. = use transport protein
o Active trans. = transport protein + ATP gradient
o Endo/exo cytosis = enter/exit by making vesicle
o Osmosis = conc. gradient
- apoplastic Transport = molecules move thru space around protoplast
(external to plasma mem)
- need semi permeable barrier for movement across membranes
Water Movement Principles
- water = living cell solvent
- movement by 3 processes
o Bulk Flow
Water molecules move in mass in response to potential energy
differences
o Diffusion
Spontaneous movement of water down a conc. Gradient
(higher to lower)
o Osmosis
Movement of water/solvent across selectively permeable
membrane
Lower solute conc to higher (higher water conc to lower)
Turgor Pressure Contributes to Cell Stiffness
- Cell was counters turgor pressure
- For plant cells to grow, water must increase & cell walls must expand
Why is Turgor Pressure Important to Maintain?
- driving force for cell expansion
- provides support for cells and tissues.
- drives opening of stomata & phloem transport.
- tells plant about soil water status
Water Potential (Ψ)
- Refers to measurement predicting which way water will flow b/w plant cell
and surroundings or between different parts of a plant (ex roots & leaves)
- Defined as combination of osmotic potential and the pressure potential (the
effect of cell wall pressure)
- Measured in units represented by Greek letter psi (Ψ)
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Water Potential
- Pressure potential always + (cell wall can’t exert negative pressure)
- Osmotic potential always zero or -
- Most living plant cells have water potential of negative or zero
- If water potential is negative, cell has more capacity to take up water
Hypotonic Solutions
- solution surrounding cells has a lower conc. of solutes
- Water flows into cell but cell wall prevents rupture
Isotonic Solutions
- solution surrounding cells has same conc. of solutes
- Water at equilibrium but cell not turgid
Hypertonic Solutions
- The solution surrounding cells has a higher conc. of solutes
- Water flows out of cells and plasma mem shrinks away from wall
(plasmolysis)
- Symplastic connections can be compromised and cause cell death
Transport in Plants
Bidirectional process:
o From roots to shoot & from leaves to the rest of the plant
o Mediated by vascular system made up of two complex tissues, xylem
and phloem
Overview of Water & Solute Transport
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Transpiration
- Serves two important functions
o Cools leaves heated by sunlight
o Pulls water and water-soluble minerals up from the root
- Water evaporated thru stomata and stems by transpiration
- design of xylem tissues and polar water facilitate transport of water upward
thru plant
Water Characteristics
- Adhesion: attraction between different kinds of molecules (cellulose adhere
to water, binds)
- Cohesion: the attraction between molecules of the same kind (water binds to
others w/ h-bonds)
- Tension: negative pressure on water or solutions (created by evap from
stomata causing a pull on water column that’s transmitted down. Tree trunk
shrinks in trans.)
- Tension-Cohesion Theory is used to explain water transport in the xylem
Cavitation
- Greater tension increases risk of breakage of water column
- Formation of air bubbles/ice crystals can break water column
- Breakage occurs less in tracheids than vessel elements because of anatomical
differences
Relative Water Potential
o Becomes more and more negative as water moves from soil to leaves
Water uptake from the soil
- Root hairs take up water and minerals from soil
- Roots compete directly with soil particles for water
Water movement through the root
- apoplasm = water flowing between cells
- symplasm = water flowing through the cytoplasm
- apoplasm/symplasm until it reaches endodermis, where symplastic route is
used
Root Pressure
- Negative water potential of root cells causes enough water uptake to
generate root pressure
- Removal of stem does not perturb flow of water from roots
- Root pressure is not sufficient to move water up plant more than a few feet
- Root pressure is lowest during the day when transpiration rates are highest
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Document Summary

Plant transport symplastic transport = molecules move thru living" space (interior of cells bounded by plasma mem: diffusion = move across mem, facilitated trans. = transport protein + atp gradient: endo/exo cytosis = enter/exit by making vesicle, osmosis = conc. gradient. Apoplastic transport = molecules move thru space around protoplast (external to plasma mem) Need semi permeable barrier for movement across membranes. Movement by 3 processes: bulk flow. Water molecules move in mass in response to potential energy differences: diffusion. Spontaneous movement of water down a conc. Movement of water/solvent across selectively permeable membrane. Lower solute conc to higher (higher water conc to lower) For plant cells to grow, water must increase & cell walls must expand. Drives opening of stomata & phloem transport. tells plant about soil water status. Refers to measurement predicting which way water will flow b/w plant cell and surroundings or between different parts of a plant (ex roots & leaves)

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