Transcript water potential (y

TRANSPORT OVERVIEW
3 LEVELS OF TRANSPORT OCCUR IN
PLANTS:
1) UPTAKE AND RELEASE OF WATER AND
SOLUTES BY INDIVIDUAL CELLS
2) SHORT-DISTANCE CELL-TO-CELL
TRANSPORT AT THE LEVEL OF TISSUES
AND ORGANS
3) LONG-DISTANCE TRANSPORT OF SAP IN
XYLEM AND PHLOEM AT THE WHOLEPLANT LEVEL
TRANSPORT AND MEMBRANES
• THE PLASMA MEMBRANE’S SELECTIVE
PERMEABILITY CONTROLS THE
MOVEMENT OF SOLUTES BETWEEN A
PLANT CELL AND THE EXTRACELLULAR
FLUIDS.
• TRANSPORT PROTEINS MAY FACILITATE
DIFFUSION BY SERVING AS CARRIER
PROTEINS OR FORMING SELECTIVE
CHANNELS
PROTON PUMPS
• A PROTON PUMP HYDROLYZES ATP AND USES THE
ENERGY TO PUMP HYDROGEN IONS (H+) OUT OF THE CELL
– THIS PRODUCES A PROTON GRADIENT WITH A HIGHER
CONCENTRATION OUTSIDE OF THE CELL
– PRODUCES A MEMBRANE POTENTIAL, SINCE THE
INSIDE OF THE PLANT CELL IS NEGATIVE IN RELATION
TO THE OUTSIDE
• THIS MEMBRANE POTENTIAL AND THE STORED ENERGY OF
THE PROTON GRADIENT ARE USED BY THE PLANT TO
TRANSPORT MANY DIFFERENT MOLECULES
• POTASSIUM (K+) IONS ARE PULLED INTO THE CELLS
BECAUSE OF THE ELECTROCHEMICAL GRADIENT
• **THE INVOLVEMENT OF PROTON PUMPS IN THE
TRANSPORT PROCESSES OF PLANT CELLS IS A SPECIFIC
APPLICATION OF CHEMIOSMOSIS
SOLUTE TRANSPORT IN PLANT CELLS
WATER POTENTIAL
DRIVES TRANSPORT
• OSMOSIS RESULTS IN THE NET UPTAKE OR
LOSS OF WATER BY THE CELL AND DEPENDS
ON WHICH COMPONENT, THE CELL OR
EXTRACELLULAR FLUIDS, HAS THE HIGHEST
WATER POTENTIAL
• WATER POTENTIAL (Y) = THE FREE ENERGY
OF WATER THAT IS A CONSEQUENCE OF
SOLUTE CONCENTRATION AND APPLIED
PRESSURE; PHYSICAL PROPERTY PREDICTING
THE DIRECTION WATER WILL FLOW
WATER POTENTIAL
• MEGAPASCALS (Mpa)-units of water potential
pressure
• PURE WATER IN AN OPEN CONTAINER HAS A Y
OF ZERO Mpa (Y=O)
• ADDITION OF SOLUTES TO WATER LOWERS THE
Y INTO THE NEGATIVE RANGE
• INCREASED PRESSURE RAISES THE Y INTO THE
NEGATIVE RANGE
QUANTITATIVE WATER
POTENTIAL
• THE EFFECTS OF PRESSURE AND SOLUTE
CONCENTRATION ON WATER POTENTIAL
ARE REPRESENTED BY
Y =YP + YS
YP = PRESSURE POTENTIAL
YS = SOLUTE POTENTIAL OR OSMOTIC POTENTIAL
WATER POTENTIAL AND WATER MOVEMENT
WATER RELATIONS OF PLANT CELLS
AQUAPORINS
• AQUAPORINS ARE WATER-SPECIFIC
CHANNELS MADE UP OF TRANSPORT
PROTEINS, HAVE BEEN DISCOVERED
IN PLANT AND ANIMAL CELLS
• AQUAPORINS DO NOT ACTIVELY
TRANSPORT WATER, BUT RATHER
FACILITATE ITS DIFFUSION
TONOPLAST
• TONOPLAST = MEMBRANE SURROUNDING
THE LARGE CENTRAL VACUOLE FOUND IN
PLANT CELLS; IMPORTANT IN REGULATING
INTRACELLULAR CONDITIONS
• CONTAINS INTEGRAL TRANSPORT PROTEINS
THAT CONTROL THE MOVEMENT OF SOLUTES
BETWEEN THE CYTOSOL AND THE VACUOLE
• HAS A MEMBRANE POTENTIAL; PROTON
PUMPS IN THE TONOPLAST HELP THE PLASMA
MEMBRANE MAINTAIN A LOW H+
CONCENTRATION IN THE CYTOSOL BY
MOVING H+ INTO THE VACUOLE
PLASMODESMATA
• PLASMODESMATA CONNECT THE
CYTOSOLIC COMPARTMENTS OF
NEIGHBORING CELLS; THIS CYTOPLASMIC
CONTINUUM IS CALLED THE SYMPLAST
• THE WALLS OF ADJACENT CELLS ARE
CONNECTED TO FORM A CONTINUUM OF
CELL WALLS CALLED APOPLAST
COMPARTMENTS OF PLANT CELLS
THE SYMPLAST & APOPLAST BOTH LATERALLY TRANSPORT
BULK FLOW
• THIS TYPE OF TRANSPORT IS USUALLY ALONG THE
VERTICAL AXIS OF THE PLANT (UP & DOWN) FROM
THE ROOTS TO THE LEAVES AND VICE VERSA
• VASCULAR TISSUES ARE INVOLVED IN THIS TYPE OF
TRANSPORT AS DIFFUSION WOULD BE TOO SLOW
• BULK FLOW (MOVEMENT DUE TO PRESSURE
DIFFERENCES) MOVES WATER AND SOLUTES
THROUGH XYLEM VESSELS AND SIEVE TUBES
• TRANSPIRATION REDUCES PRESSURE IN THE LEAF
XYLEM; THIS CREATES A TENSION WHICH PULLS SAP
UP THROUGH THE XYLEM FROM THE ROOTS
ABSORPTION BY ROOTS
• WATER AND MINERALS ENTER PLANTS
THROUGH THE FOLLOWING PATHWAY
SOIL>>>>EPIDERMIS>>>ROOT
CORTEX>>>XYLEM
LATERAL TRANSPORT OF MINERALS AND WATER
IN ROOTS
VIDEO: TRANSPORT
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WATER AND MINERAL
ABSORPTION
SOIL
EPIDERMIS
-MOST ABSORPTION OCCURS NEAR ROOT TIPS
WHERE THE EPIDERMIS IS PERMEABLE TO WATER
-ROOT HAIRS, EXTENSIONS OF EPIDERMAL CELLS,
INCREASE THE SURFACE AREA AVAILABLE FOR
ABSORPTION
-MOST PLANTS FORM SYMBIOTIC RELATIONSHIPS
WITH FUNGI; THE “INFECTED” ROOTS FORM
MYCORRHIZAE, A STRUCTURE MADE FROM THE
PLANT ROOTS AND THE HYPHAE OF FUNGI.
WATER AND MINERALS ABSORBED BY THE
HYPHAE ARE TRANSFERRED TO THE HOST PLANT
EPIDERMIS
ROOT CORTEX:
-LATERAL TRANSPORT OF MINERALS AND
WATER THROUGH THE ROOT IS USUALLY
BY A COMBINATION OF APOPLASTIC AND
SYMPLASTIC ROUTES
-THE APOPLASTIC ROUTE EXPOSES
PARENCHYMAL CORTEX CELLS TO SOIL
SOLUTION
-THE SYMPLASTIC ROUTE MAKE
SELECTIVE MINERAL ABSORPTION
POSSIBLE
ENDODERMIS FUNCTIONS
ROOT CORTEX
XYLEM:
• ONLY MINERALS USING THE SYMPLASTIC
ROUTE MAY MOVE DIRECTLY INTO THE
VASCULAR TISSUES.
• MINERALS AND WATER PASSING THROUGH
APOPOLASTS ARE BLOCKED AT THE
ENDODERMIS BY A CASPERIAN STRIP (A RING
OF SUBERIN AROUND EACH CELL IN THE
ENDODERMIS) AND MUST ENTER AN
ENDODERMAL CELL
• WATER AND MINERALS ENTER INTO THE STELE
THROUGH THE CELLS OF THE ENDODERMIS
TRANSPORT OF XYLEM
SAP
• THE ASCENT OF XYLEM SAP DEPENDS MAINLY ON
TRANSPIRATION AND THE PHYSICAL PROPERTIES
OF WATER
• THE SHOOT DEPENDS UPON AN EFFICIENT
DELIVERY OF ITS WATER SUPPLY
– XYLEM SAP FLOWS UPWARD AT 15m PER HOUR OR FASTER
– XYLEM VESSELS ARE CLOSE TO EACH LEAF CELL, BECAUSE
VEINS BRANCH THROUGHOUT THE LEAVES
WATER TRANSPORTED UP FROM ROOTS MUST
REPLACE THAT LOST BY TRANSPIRATION
-TRANSPIRATION IS THE EVAPORATION OF WATER FROM THE
AERIAL PARTS OF A PLANT
-THE UPWARD FLOW OF XYLEM SAP ALSO PROVIDES
NUTRIENTS (MINERALS) TO THE SHOOT SYSTEM
ROOT PRESSURE
• WHEN TRANSPIRATION IS LOW, ACTIVE
TRANSPORT OF IONS INTO THE XYLEM
DECREASES THE STELE’S WATER POTENTIAL
AND CAUSES WATER FLOW INTO THE STELE.
THIS OSMOTIC WATER UPTAKE INCREASES
PRESSURE WHICH FORCES FLUID UP THE
XYLEM (= ROOT PRESSURE)
– ROOT PRESSURE CAUSES GUTTATION (EXUDATION
OF WATER DROPLETS AT LEAF MARGINS)
• ROOT PRESSURE IS NOT THE MAJOR
MECHANISM DRIVING THE ASCENT OF XYLEM
SAP
– CANNOT KEEP PACE WITH TRANSPIRATION
– CAN ONLY FORCE WATER UP A FEW METERS
PULLING XYLEM SAP
TRANSPIRATION PULLS XYLEM SAP
UPWARD, AND COHESION OF WATER
TRANSMITS THE UPWARD-PULL ALONG
THE ENTIRE LENGTH OF XYLEM
TRANSPIRATIONAL PULL
TRANSPIRATIONAL PULL
• TRANSPIRATIONAL PULL DEPENDS UPON THE CREATION OF
NEGATIVE PRESSURE
• GASEOUS WATER IN DAMP INTERCELLULAR LEAF SPACES
DIFFUSES INTO THE DRIER ATMOSPHERE THROUGH STOMATA
• THE LOST WATER VAPOR IS REPLACED BY EVAPORATION FROM
MESOPHYLL CELLS BORDERING THE AIRSPACES
• THE REMAINING WATER FILM, ADHERING TO THE HYDROPHILIC
CELL WALLS, RETREATS INTO THE CELL WALL PORES
• COHESION IN THIS SURFACE FILM OF WATER RESISTS AN
INCREASE IN THE SURFACE AREA OF THE FILM-A SURFACE
TENSION EFFECT
• THE WATER FILM FORMS A MENISCUS DUE TO THE NEGATIVE
PRESSURE CAUSED BY THE ADHESION AND COHESION
• THIS NEGATIVE PRESSURE PULLS WATER FROM THE XYLEM,
THROUGH THE MESOPHYLL, TOWARD THE SURFACE FILM ON
CELLS BORDERING THE STOMATA
MEASURING WATER POTENTIAL OF LEAVES
COHESION AND ADHESION
OF WATER
• THE TRANSPIRATIONAL PULL ON THE
XYLEM SAP IS TRANSMITTED TO THE SOIL
SOLUTION. COHESION OF WATER DUE TO
H+ BONDS ALLOWS FOR THE PULLING OF
WATER FROM THE TOP OF THE PLANT
WITHOUT BREAKING THE “CHAIN”
• THE ADHESION OF WATER (BY H+ BONDS)
TO THE HYDROPHILIC WALLS OF XYLEM
CELLS ALSO HELPS PULL AGAINST
GRAVITY
• THE SMALL DIAMETER OF VESSELS AND
TRACHEIDS IS IMPORTANT TO THE
ADHESION EFFECT
COHESION CON’T
• THE UPWARD PULL OF SAP CAUSES TENSION
(NEGATIVE PRESSURE) IN XYLEM, WHICH
DECREASES WATER POTENTIAL AND ALLOWS
PASSIVE FLOW OF WATER FROM SOIL INTO
STELE
• TRANSPIRATIONAL PULL CAN EXTEND DOWN
TO THE ROOTS ONLY THROUGH AN UNBROKEN
CHAIN OF WATER MOLECULES
• CAVITATION (FOMATION OF A WATER VAPOR
POCKET IN XYLEM) BREAKS THE CHAIN OF
WATER MOLECULES AND THE PULL IS STOPPED
ASCENT OF XYLEM SAP
CONTROL OF TRANSPIRATION
• TRANSPIRATION RESULTS IN A
TREMENDOUS WATER LOSS FROM THE
PLANT. THIS WATER IS REPLACED BY
THE UPWARD MOVEMENT OF WATER
THROUGH THE EXLEM. GUARD CELLS
SURROUNDING STOMATA BALANCE THE
REQUIREMENTS FOR PHOTOSYNTHESIS
WITH THE NEED TO CONSERVE WATER.
TRANSPIRATION VIDEO
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GUARD CELLS
• GUARD CELLS = CELLS THAT FLANK
STOMATA AND CONTROL STOMATAL
DIAMETER BY CHANGING SHAPE
• WHEN TURGID, GUARD CELLS “BUCKLE
DUE TO RADIALLY-ARRANGED
MICROFIBRILS AND STOMATA OPEN
• WHEN FLACCID, GUARD CELLS SAG AND
STOMATAL OPENINGS CLOSE
• THE CHANGE IN TURGOR PRESSURE THAT
REGULATES STOMATAL OPENING RESULTS
FROM REVERSIBLE UPTAKE AND LOSS OF K+
BY GUARD CELLS
– UPTAKE OF K+ DECREASES GUARD CELL WATER
POTENTIAL SO WATER IS TAKEN UP, CELLS BECOME
TURGID, AND STOMATA OPEN. THE TONOPLAST
PLAYS A ROLE AS MOST OF THE K+ AND WATER ARE
STORED IN THE VACUOLE
– THE INCREASE IN POSITIVE CHARGE IS COUNTERED
BY THE UPTAKE OF CHLORIDE (Cl-), EXPORT OF H+
IONS RELEASED FROM ORGANIC ACIDS, AND THE
NEGATIVE CHARGES ACQUIRED BY ORGANIC ACIDS
AS THEY LOSE THEIR PROTONS
– CLOSING OF THE STOMATA RESUTLS WHEN K+
EXITS THE GUARD CELLS AND CREATES AN
OSMOTIC LOSS OF WATER
STOMATAL CONTROL
VIDEO: STOMATA AND GUARD CELLS
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XEROPHYTES: ARID REGIONS
• XEROPHYTES, PLANTS ADAPTED TO ARID
CLIMATES, HAVE SOME OF THE FOLLOWING
EVOLUTIONARY ADAPTATIONS THAT REDUCE
TRANSPIRATION:
– SMALL, THICK LEAVES (REDUCED SURFACE
AREA:VOLUME, SO LESS WATER LOSS)
– A THICK CUTICLE
– STOMATA ARE DEPRESSIONS ON THE UNDERSIDE OF
LEAVES TO PROTECT FROM WATER LOSS DUE TO
DURING WINDS (LASERDISK)
– SOME SHED LEAVES IN THE DRIEST TIME OF THE
YEAR
– CACTI AND OTHERS STORE WATER IN STEMS DURING
THE WET SEASON
TRANSLOCATION OF
PHLOEM SAP
• TRANSLOCATION = THE TRANSPORT OF
THE PRODUCTS OF PHOTOSYNTHESIS BY
PHLOEM TO THE REST OF THE PLANT
• PHLOEM TRANSLOCATES ITS SAP FROM
SUGAR SOURCES TO SUGAR SINKS
• SUGAR SOURCE = ORGAN WHERE SUGAR
IS PRODUCES BY PHOTOSYNTHESIS OR BY
THE BREAKDOWN OF STARCH
• SUGAR SINK = ORGAN THAT CONSUMES
OR STORES SUGAR
PHLOEM LOADING AND
UNLOADING
• SUGAR PRODUCES AT A SOURCE MUST BE
LOADED INTO SIEVE-TUBE MEMBERS BEFORE
IT CAN BE TRANSLOCATED TO A SINK
• IN SOME PLANT SPECIES, THE SUGAR MAY
MOVE THROUGH THE SYMPLAST FROM
MESOPHYLL CELLS TO THE SIEVE MEMBERS
• IN OTHER SPECIES, THE SUGAR USES A
COMBINATION OF SYMPLASTIC AND
APOPLASTIC ROUTES
• SOME PLANTS HAVE TRANSFER CELLS. THESE
ARE MODIFIED COMPANION CELLS WHICH
HAVE NUMEROUS INGROWTHS OF THEIR
WALLS
VIDEO: TRANSLOCATION
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• IN PLANTS SUCH AS CORN, ACTIVE
TRANSPORT ACCUMULATES SUCROSE
IN SIEVE-TUBE MEMBERS TO TWO TO
THREE TIMES THE CONCENTRATION
IN MESOPHYLL CELLS
– PROTON PUMPS POWER THIS TRANSPORT
BY CREATING A H+ GRADIENT
– A MEMBRANE PROTEIN USES THE
POTENTIAL ENERGY STORED IN THE
GRADIENT TO DRIVE THE COTRANSPORT
OF SUCROSE BY COUPLING SUGAR
TRANSPORT TO THE DIFFUSION OF H+
BACK INTO THE CELL
LOADING OF SUCROSE INTO PHLOEM
PRESSURE FLOW
• PHLOEM SAP FLOWS UPO TO 1m PER HOUR, TOO FAST FOR JUST
DIFFUSION OR CYTOPLASMIC STREAMING
• THE FLOW IS BY A BULK FLOW (PRESSURE-FLOW) MECHANISM;
BUILDUP OF PRESSURE AT THE SOURCE AND RELEASE OF
PRESSRE AT THE SINK CAUSES SOURCE-TO-SINK FLOW
• AT THE SOURCE END, PHLOEM LOADING CAUSES HIGH SOLUTE
CONCENTRATIONS
– WATER POTENTIAL DECREASES, SO WATER FLOWS INTO
TUBES CREATING HYDROSTATIC PRESSURE
• AT THE SINK END, THE WATER POTENTIAL IS LOWER OUTSIDE
THE TUBE DUE TO THE UNLOADING OF SUGAR; OSMOTIC LOSS
OF WATER RELEASES HYDROSTATIC PRESSURE
– XYLEM VESSELS RECYCLE WATER FROM THE SINK TO THE
SOURCE
PRESSURE FLOW IN A SIEVE TUBE