Post on 01-Jan-2016
description
http://bcs.whfreeman.com/thelifewire/content/chp00/00020.html
Chpt 35 : REVIEW SECONDARY GROWTH
Chpt 36: TRANSPORT IN PLANTS
http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/
Phloem loadingWater intakeOsmosisEtc.
Text Summary of Transport
http://www.wou.edu/~bledsoek/103materials/chapter_notes/103Ch42b.pdf
Transport sometimes over long distances
Xylem transports H2O & minerals from roots to shoots
Phloem transports sugars, etc. from where synth (source) to where needed (sink)
A. 3 levels of transport
1. Transport of water and solutes by individual cells, e.g., root hairs via plasmodesmata
2. Short-distance transport of substances from cell to cell at the levels of tissues and organs, e.g. loading sugar from photosynthetic leaf cells sieve tubes of phloem
3. Long-distance transport within xylem & phloem throughout whole plant
Minerals
H2O O2
O2
CO2 O2
H2OSugar
Light
A variety of physical processes are involved in the different levels of transport
Sugars are produced byphotosynthesis in the leaves.
5
Sugars are transported asphloem sap to roots and otherparts of the plant.
6
Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.
4
Transpiration, the loss of waterfrom leaves (mostly through
stomata), creates a force withinleaves that pulls xylem sap upward.
3
Water and minerals aretransported upward from
roots to shoots as xylem sap.
2
Roots absorb waterand dissolved minerals
from the soil.
1
Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.
7
CO2
3. Transpiration (loss H2O) through stomata creates force in leaves that pulls xylem sap up
Minerals
H2O
CO2 O2
H2O
4. Through stomata, leaves take in CO2 & expel O2 (CO2 for photosynth, O2 from cell resp)
Minerals
H2O
CO2 O2
H2O Sugar
Light
5. Sugar made by photosynth in leaves
6. Sugar transported as phloem sap to roots & other parts where needed
Minerals
H2O CO2
O2
CO2 O2
H2O Sugar
Light
7. Roots exchange gases with air spaces of soil (O2 used in catabolism of sugars)
B. Selective Permeability of Membranes
• Selective permeability of plant cell’s plasma membrane controls movement of solutes into & out of cell
• Specific transport proteins enable plant cells to maintain internal environment different from their surroundings
Review: passive and active transport compared
Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. No energy is required.
Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.
ATP
Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,
Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.
Gated channel
Most solutes cannot cx memb without help of transport proteins
B. Central Role of Proton Pumps Most impt active transport prot in plants is
the proton pump in plant cells
CYTOPLASM EXTRACELLULAR FLUID
ATP
H+
H+ H+
H+
H+
H+H+
H+
Proton pump generates membrane potentialand H+ gradient.
–
––
–
– +
+
+
+
+
- Creates a hydrogen ion gradient (a form of potential energy that can be harnessed to do work)- Contributes to a voltage known as a membrane potential (also potential energy)
Plants use energy stored in proton gradient & memb potential to drive transport of different solutes
+CYTOPLASM EXTRACELLULAR FLUID
Cations ( , for example) are driven into the cell by themembrane potential.
Transport protein
K+
K+
K+
K+
K+ K+
K+
K+
–
–
– +
+
1. Uptake of K+
–
–
+
+
H+
H+
H+
H+
H+
H+
H+H+
H+
H+
H+
H+
NO3–
NO 3 –
NO3
–
NO 3
–
NO3
–
NO3 –
–
–
– +
+
+
–
–
– +
+
+
NO3–
2. Cotransport couples downhill passage of (H+) with concommitant uphill passage of (NO3) this = active transport
H+of through a
cotransporter.
Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion
H+
H+
H+
H+
H+H+
H+
H+ H+
H+
SS
S
S
SPlant cells canalso accumulate a neutral solute,such as sucrose
( ), bycotransporting
down the
steep protongradient.
S
H+
–
–
–
+
+
+
–
–
++–
H+ H+S+
–
3. Uptake of sucrose with contransport of H+ moving down its conc gradient in uptake of sucrose by plant cells
C. Water Potential
To survive plants must balance water uptake & loss
Osmosis (diffusion of H2O acx selectively permeable memb)
responsible for net uptake or loss of water
• Water potential (Ψ) is a measurement that combines effects of solute conc & pressure. to determine the direction of H2O movement
• Water flows from regions of high water potential to regions of low water potential
• Solute contribution to water potential of a solution is proportional to the number of dissolved molecules
• Pressure contribution to water potential of a solution is the physical pressure on a solution (involves plant cell wall)
• Application of physical pressure increases Ψ
H2O
P = 0.23
S = 0.23
= 0 MPa = 0 MPa
(b)
H2O
P = 0.30
S = 0.23
= 0.07 MPa = 0 MPa
(c)
• If a flaccid cell is placed in an environment with a higher solute conc (hypertonic soln), the cell will lose water & plasmolyze (memb will shrink away from its cell wall)Flaccid = limp. A walled cell is flaccid in surroundings where there is no tendency for water to enter
Hypotonic solutionHypertonic solution
• If the same flaccid cell is placed in a soln with a solute concentration lower than that in the protoplast, the cell will gain water and become turgid (very firm) as cell wall pushes back against enlarging memb. A walled cell becomes turgid if it has a greater solute conc than its surroundings, resulting in entry of water.
• Loss of turgor (due to loss of water in environment) in plants causes wilting which can be reversed when the plant is watered.
Healthy plants are turgid most of the time.
D. Aquaporin and Water Transport
● Aquaporins = transport prots in memb that allow the passage of water
● Do not affect water potential
Plasmodesma
Plasma membrane
Cell wallCytosol
Vacuole
Ke
Symplast
Apoplast
Vacuolar membrane(tonoplast)
E. 3 Major Compartments Vacuolated Cells
Transport also regulated by compartmental structure of plant cells
1. cell wall (maintain cell shape) edge of space
2. plasma membrane (controls H2O in/out) & edge of protoplast (contents of cell less wall)
3. vacuole
• Plasma membrane
- Directly controls the traffic of molecules in/out protoplast
- Is a barrier between two major compartments, cell wall & cytosol
• 3rd major compartment vacuole = a large organelle that can occupy as much as 90% of more of the protoplast’s volume
• Vacuolar membrane regulates transport between the cytosol and the vacuole
Transport proteins inthe plasma membrane
regulate traffic ofmolecules betweenthe cytosol and the
cell wall.
Transport proteins inthe vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.
PlasmodesmaVacuolar membrane
Plasma membrane
Cell wall
Cytosol
Vacuole
Cell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.
Key
Symplast
Apoplast
ApoplastTransmembrane route
Symplastic route
Symplast
1. transmembrane route (out of memb-ax cell wall-into another cell)
2. symplastic route (cell memb to cell directly vis dermatoplasmata)
3. apoplastic route (stay outside cells)
F. Short lateral transport via one of 3 ways:Symplast = continuous cytosol, cell to cell via plasmodesmata Apoplast = continuum cell walls & extracellular spaces
Substances may transfer from one route to another.
Apoplastic route
These 3 ways from root hairs to vascular cylinder
Water & minerals travel short distances from root hairs to vascular cylinder of root via 3 lateral (not
up/down) routes
1. Transmemb: out of one cell, across a cell wall, & into another cell = repeated crossing plasma memb
2. Via symplast == continum of cytosol; only one crossing of plasma membrane & then cell-to-cell via plasmodermata
3. Along apoplast == continuem of cell wall; no entering protoplast
Can change from one route to another route
G. Bulk Flow for Long-Distance Transport is movement of fluid in the xylem & phloem
driven by pressure differences at opposite ends of the xylem vessels and sieve tubes
Diffusion OK for short distances, but too slow for long distances …. need bulk flow
Water & fluids move through tracheids & vessels of xylem & sieve tubes of phloem
Tracheids = tapered cells with pitsVessel elements = wider, long channels, end walls perforated for easy flow
DEADALIVE
Sieve tube member = cell with sieve plate
Companion Cell = nonconducting cntd via plasmodesmata
Phloem: loading of sugar = high + pressure at opposite end of sieve tube = mvt fluid
Xylem: negative pressure tension by transpiration from leaves pulls sap up from roots
Cytoplasm of sieve-tube members almost devoid of internal organelles & porous sieve plates = easier flow
Dead tracheids & vessel elements (porous end walls) have no cytoplasm to inhibit flow
Bulk flow due to pressure differences = way long-distance transport of phloem sap & active transport of sugar at cellular level maintains pressure difference
II. Roots absorb water & minerals from soil
enter the plant through the epidermis of roots, cx root cortex, pass into vascular cylinder & ultimately bulk flow to shoot system
Root hairs account for much surface area of roots
• 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 = increase surface area of roots2.5 mm
Lateral transport of minerals and water in roots
1
2
3
Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls.
Minerals and water that crossthe plasma membranes of roothairs enter the symplast.
As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove inward via the symplast.
Within the transverse & radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks thepassage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder.
Endodermal cells & also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water & minerals upward into shoot system.
Pathwaythroughsymplast
Plasmamembrane
Casparian strip
Apoplasticroute
Symplasticroute
Root hair
Epidermis Cortex EndodermisVascular cylinder
Vessels(xylem)
Casparian strip
Endodermal cell
4 5
2
1
Root hairs (extensions of epidermal cells) absorb
Passes freely apoplastic route into cortex Cells of epidermis & cortex take up = into symplast
Endodermis: innermost layer cells in root cortex, surrounds vascular cylinder & is last checkpoint for selective passage of minerals from cortex into vascular tissue
Minerals/water from roots into symplast of epidermis or cortex, continue via plasmodesmata of endodermal cells into vascular cylinder
Minerals/water from roots into apoplast meet Casparian strip (dead end & cannot enter vascular cylinder via apoplast) BUT can enter symplast and enter
Waxy belt in walls endodermal cells; impervious to water/minerals
• III. Water and minerals ascend from roots to shoots through the xylem
• Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant
• The transpired water must be replaced by water transported up from the roots
Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism is the major pressure driving flow of xylem sap up to shoot system
• Water is pulled upward by negative pressure in the xylem produced by transpiration through stomata
• Cohesion of water (H bonding) transmits upward pull along length of sylem to roots
IV. Stomata help regulate rate of transpiration
Leaves generally have broad surface areas & high surface-to-volume ratios that increase photosynthesis & increase water loss
● Plants lose a large amount of water by transpiration
● If lost water is not replaced by absorption through roots, plant will lose water & wilt
● Transpiration = evaporative cooling which lowers T of leaf to prevent denaturation of various enzs involved in photosynthesis & other metabolic processes
V. Nutrients translocated through phloem
Phloem sap (an aqueous solution, mostly sucrose) that translocated from source to sink
A sugar source : a plant organ that is a net producer of sugar, such as mature leaves
A sugar sink: an organ that is a net consumer or storer of sugar, such as a tuber or bulb
Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members.
In angiosperms sap moves through a sieve tube by bulk flow driven by positive pressure
Phloem loading may be via active transport; proton pumping & cotransport of sucrose & H+