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Transport in plants
Transport in plants occurs on 3 levels
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Transport of materials into individual cells-Transport at cellular level
Cell to cell transport- Lateral transport
Long distance transport
I. Transport at cellular level
 Transport at cellular level depends on the selective permeability of membranes
A. Passive Transport: diffusion down a concentration gradient
 Much of the diffusion is facilitated. For example, the membrane of most plant
cells have potassium channels that allow potassium ions to pass, but not similar
ions , such as sodium
 Selective channels are usually gated and regulated; that is, certain environmental
stimuli can cause the channels to open or close. For example, regulation of K+
gates in the membranes of guard cells functions in the opening and closing of
stomata
B. Active Transport: movement against a concentration gradient
Proton Pump and Membrane Voltage
 Proteins pump H+ out of plant cell (ATP consumed)
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H+ out leaves inside negative: Membrane potential (voltage - battery)
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H+ gradient and Membrane voltage provide energy for taking up and
concentrating soil minerals. For example, membrane potential helps drive of K+
into root cells
5.
C. Water Potential and Osmosis
 H2O moves by osmosis in the hypotonic →hypertonic direction
 In the case of a plant cell, the cell wall adds a second factor affecting osmosis.
The combined effect of the solute concentration and pressure are incorporated
into a single measurement called water potential, abbreviated by a Greek letter psi
ψ
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The combined effects of solute concentration and pressure on water potential are
incorporated into the following equation (ψ= ψp + ψs) where ψp is the pressure
potential and ψs is the solute potential
Plant biologists measure ψ in units of pressure called megapascals (MPa)
Water moves from regions of high water potential to regions of low water
potential
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The water potential of pure water is defined as zero megapascals
The addition of solutes lowers the water potential. For example, 0.1 molar
solution of any solute has a water potential of -0.23 MPa.
In the following figure, identical cells, flaccid, are placed in two different
solutions, one a relatively concentrated sugar and the other distilled water
In (a) the cell initially has a greater water potential than its surroundings. The cell
loses water and plazmolyzes
In (b) the cell initially has a lower water potential than its surroundings. Water
enters the cell by osmosis. The cell begins to swell and push against the wall,
producing a turgor pressure. When ψp and ψs are equal in magnitude and thus
ψ=0, there is no further net movement of water and the cell is called turgid.
Tonoplast
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Membrane that surrounds central vacuole
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Regulates molecular transport between the cytoplasm and the central vacuole
Has proton pumps that expel H+ from the cytosol into the vacuole- maintains a
low cytosolic concentration of H+
Aquaporins
• 1990’s scientists discover water pores in membranes that facilitate the diffusion of
water
• Water still diffuses through lipid bilayer
• Aquaporins speed up this diffusion
II. Lateral Transport
Water and minerals move across the root cortex to the stele by the combination of the
symplast and apoplast routes
The symplast is the continuum of cytosol linked by plasmodesmata
The apoplast is the continuum of cell walls
Long Distance Transport
1. Root Pressure: Pushing
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The endodermis of the root prevents the leakage of minerals back out of the stele.
The accumulation of minerals in the stele lowers water potential there
Water flows from the root cortex, generating a positive pressure that forces xylem
sap (water and minerals) up the xylem
Root pressure cause guttation, the exudation of water droplets that can be seen in
the morning on tips of grass blades or the leaf margin of herbaceous (non-woody)
dicots
B. Transpiration: Pulling
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Loss of water vapor (transpiration) lowers water potential in the lea by producing
a negative pressure (tension). This low water potential draws up water from the
xylem
The adhesive and cohesive of water help transpiration to pull water up the narrow
xylem vessels and tracheides
V. Translocation of phloem sap
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Translocation is the transport of Phloem sap (mostly sucrose) in the phloem from
site of synthesis (source) to site of use (sink).
Source: leaves; Sinks: fruits, seeds, roots
Translocation through the phloem depends on pressure gradients between source
and sink regions.
1. Loading of sugars into the sieve tube at the source reduces the water potential
inside the sieve-tube members. This cause the tube to take up water surrounding
tissues by osmosis
2. This absorption of water generates a hydrostatic pressure that forces the sap to
flow a long the tube
3. The pressure gradient in the tube reinforced by the uploading of sugar and the
consequent loss of water from the tube at the sink
4. Xylem recycles water from sink to source
IV. Control of Transpiration
 Guard cells regulate water loss
Factors contribute to stomatal opening
 Light stimulates guard cells to accumulate K+ and become turgid
 Low CO2
 Internal clock located in the guard cells-Daily rhythm of opening and closing
Factors contribute to stomatal closing
 When the plant is suffering a water deficiency, a hormone called abscisic acid
signals guard cells to close stomata
 High temperature and eexcessive transpiration
Mechanism - opening
 Cytoplasmic increase in K+
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Osmotic entry of H2O
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Cells swell - cell walls strain and torsion out - stoma opens