Summary A plant is an integrated system A plant is an integrated system

which:which:1.1. Obtains water and nutrients Obtains water and nutrients

from the soil.from the soil.2.2. Transports them Transports them 3.3. Combines the HCombines the H22O with COO with CO22

to make sugar.to make sugar.4.4. Exports sugar to where it’s Exports sugar to where it’s

needed needed

Today, we’ll start to go over Today, we’ll start to go over how this occurshow this occurs

Transport in Plants – OutlineI.I. Plant water needsPlant water needsII.II. Transport of water and mineralsTransport of water and minerals

A. From Soil into RootsA. From Soil into RootsB. From Roots to leavesB. From Roots to leavesC. Stomata and transpirationC. Stomata and transpiration

Why do plants need so much water?Why do plants need so much water?The importance of water potential, pressure, The importance of water potential, pressure,

solutes and osmosis in moving water…solutes and osmosis in moving water…

Transport in Plants

1.1. Animals have Animals have circulatorycirculatory systems.systems.

2.2. Vascular plants have Vascular plants have one wayone way systems.systems.

Transport in Plants

•• One way systems: plants need One way systems: plants need a lota lot moremorewaterwater than same sized animals.than same sized animals.

•• A sunflower plant “drinks” and “perspires” 17 A sunflower plant “drinks” and “perspires” 17 times as much as a human, per unit of mass.times as much as a human, per unit of mass.

Transport of water and minerals in Plants

Water is good for plants:Water is good for plants:1.1. Used with CO2 in Used with CO2 in photosynthesisphotosynthesis to to

make “food”.make “food”.2.2. The “blood” of plants The “blood” of plants –– circulationcirculation

(used to move stuff around).(used to move stuff around).3.3. Evaporative Evaporative coolingcooling..4.4. Used for Used for turgorturgor pressurepressure to hold plant to hold plant

erect.erect.

Transport of water and minerals in Plants

Water (with minerals) Water (with minerals) --enters from the soil, enters from the soil, travels through xylem travels through xylem exits the leaves (through stoma).exits the leaves (through stoma).

What makes it move?What makes it move?

Transport of water and minerals in Plants

What makes it move?What makes it move?Water potential Water potential = the tendency of water = the tendency of water to move from one place to anotherto move from one place to another

across a membrane. across a membrane. pure waterpure water (would have (would have WP=0WP=0))

Transport of water and minerals in Plants

Water potential Water potential = the tendency water = the tendency water to moveto move

Water is usually a solutionWater is usually a solution•• potential pulls water. potential pulls water. •• ⊕⊕ potential pushes water. potential pushes water.

•• Thus, Thus, water flows toward more water flows toward more water potential.water potential.

Transport in Plants

Water potential (Water potential (ΨΨ PsiPsi) = ) = Pressure potential + Solute PotentialPressure potential + Solute Potential

ΨΨ==ΨΨpp++ΨΨss

Pressure potential,Pressure potential, ΨΨpp = hydraulic = hydraulic pressure. pressure.

(like air pressure in tires).(like air pressure in tires).

Transport in Plants

Water potential (Water potential (ΨΨ PsiPsi))

has 2 parts, has 2 parts, ΨΨ==ΨΨpp++ΨΨss

Pressure potential,Pressure potential, ΨΨpp = hydraulic = hydraulic pressure. pressure.

(like in a car’s brake line, or like air pressure in (like in a car’s brake line, or like air pressure in tires).tires).

Transport in Plants

•• Water potential (Water potential (ΨΨ) ) has 2 parts, has 2 parts, ΨΨ==ΨΨp+p+ΨΨss

Solute potential, Solute potential, ΨΨs =s = tendency of a tendency of a liquid to move across a membrane liquid to move across a membrane --

to the side with a higher concentration to the side with a higher concentration of dissolved solutes.of dissolved solutes.

Water flows toward more Water flows toward more solute solute potential (unless resisted by potential (unless resisted by ⊕⊕pressure potential).pressure potential).

Think of it like a tug of war –

The water is the ‘rope’

How water potential worksΨ=Ψp+Ψs

In the tube: In the tube:

ΨΨs = s = --0.40.4ΨΨp = 0p = 0ΨΨ = ?= ?

Beaker has distilled water Beaker has distilled water withwithΨΨ = 0= 0

Predictions?Predictions?

Fig 36.2

How water potential worksΨ=Ψp+Ψs

•• Difference in Difference in ΨΨ so so ––•• BeakerBeaker TubeTube•• Pressure potentialPressure potential

(from gravity on the (from gravity on the column of water) column of water) increasesincreases

•• Until it is equal and Until it is equal and opposite to the opposite to the osmotic pressure.osmotic pressure.

ΨΨp = 0.15p = 0.15

ΨΨs = s = --0.150.15

Water moves toward more osmotic

potential.

Fig 36.2

Water enters Water enters plant cells plant cells throughthroughosmosisosmosis

How water potential worksΨ=Ψp+Ψs

OrOrResistance to entry of Resistance to entry of

water water E.g. Cell walls (Here a E.g. Cell walls (Here a

piston)piston)

ΨΨs doesn’t changes doesn’t change

But water still leaks in But water still leaks in to raise to raise ΨΨp untilp until

balanced water balanced water potentialspotentials

Fig 36.2

Water potential

ΨΨ==ΨΨpp++ΨΨss

This is how:This is how:•• Plants get support (pressure)Plants get support (pressure)•• Water moves into and out of plant cellsWater moves into and out of plant cells

Turgor provides support and keeps plants from wilting

•• Water enters cell by Water enters cell by osmosisosmosis ––

•• ⊕⊕ pressure potential (pressure potential (turgorturgor pressurepressure) ) increases and balances the increases and balances the osmotic osmotic pressure (it is equal and opposite). pressure (it is equal and opposite).

•• Water stops moving Water stops moving -- the cell is turgid.the cell is turgid.

Summary of Water Potential•• Water potential Water potential = the tendency of a = the tendency of a

solution to solution to absorbabsorb or or releaserelease waterwater

•• Water flows towards? Water flows towards?

Or Or --

water potential wins water potential wins the tug of warthe tug of war

Summary of Water PotentialSolute potential, Solute potential, ΨΨs:s:

H20

K+

K+K+

K+

K+

K+

K+K+

K+

K+

Transport of water and minerals in Plants

•• OsmosisOsmosis has a major influence getting has a major influence getting water from the water from the soil to the root xylemsoil to the root xylem..

•• Pressure potentialPressure potential is responsible for is responsible for moving water through the moving water through the xylem to the xylem to the leaves (and air)leaves (and air). .

Transport in Plants

•• OsmosisOsmosis -- water movement water movement between living cells.between living cells.

•• Soil Soil root xylem,root xylem, water water passes through living cellspasses through living cells

Why?

From the soil to the root xylem

•• Water moves freely Water moves freely through cell walls and through cell walls and intercellular spaces, but,intercellular spaces, but,

•• CasparianCasparian stripsstripspreventing water and preventing water and ions flowions flow

•• Has to goes thru Has to goes thru cytoplasm of the cytoplasm of the endodermis cellsendodermis cells..

Fig 36.5

Movement of minerals into the cells is through active transport

•• Mineral ions move across membrane Mineral ions move across membrane transport transport proteins.proteins.

•• Active transportActive transport againstagainst a concentration a concentration gradient.gradient.

Fig 36.3

Transport in Plants

Mineral ion concentrations affects Mineral ion concentrations affects solute potentialsolute potential

Plants control: Plants control:

•• the concentration of mineral the concentration of mineral ions in living cells, henceions in living cells, hence

•• they control osmosis in rootsthey control osmosis in roots. .

2 Control Points with Transport Proteins

•• Cells near xylemCells near xylemcreate an osmotic create an osmotic gradient that moves gradient that moves water into the xylem. water into the xylem.

•• EndodermisEndodermis –– water from cortex water from cortex endodermisendodermis

Fig 36.4

Transport in PlantsMineral ions move out of the cell Mineral ions move out of the cell (active (active

transport)transport)Water potential is Water potential is more negative outsidemore negative outsideSo water moves out of the cell So water moves out of the cell (osmosis)(osmosis)

SO:SO:Minerals Minerals –– active/directactive/directWater follows passivelyWater follows passively

Transport in Plants•• Xylem Xylem -- movement is movement is

controlled by controlled by pressure pressure potentialpotential (hydraulic (hydraulic pressure).pressure).

•• Water and minerals are Water and minerals are pulledpulled (( pressure potential) pressure potential) through the xylem without through the xylem without expending energy. expending energy. How?How?

•• TranspirationTranspiration –– evaporation evaporation of water from leavesof water from leaves

•• TensionTension –– in the xylem sap in the xylem sap from transpirationfrom transpiration

•• CohesionCohesion –– in the xylem sap in the xylem sap along the plantalong the plant

How are water and minerals are pulled through the xylem?

It’s like sucking on a straw……

Water diffuses out of leaf.Water diffuses out of leaf.

Water evaporates off leaf cells Water evaporates off leaf cells to replace it.to replace it.

This pulls water from veins.This pulls water from veins.

This tension pulls the water This tension pulls the water column upcolumn up

Fig 36.8

•• In the xylem In the xylem -- movement is movement is controlled by controlled by pressure pressure potentialpotential (hydraulic (hydraulic pressure).pressure).

Because:Because:Dry air has very negative Dry air has very negative ΨΨ((ΨΨ = = --95 95 MPaMPa) ) Soil is between Soil is between ––0.01 to 0.01 to ––3 3 MPaMPa

It’s like sucking on a straw……

Transport of water and minerals in Plants

•• There is There is negative water negative water potentialpotential in stems.in stems.

•• What happens if you were to What happens if you were to cut the base of a stem?cut the base of a stem?

•• Break the cohesion in the Break the cohesion in the water columnwater column

Transport in Plants

•• With With high humidity high humidity wet soils wet soils

some plants will even have water some plants will even have water pushed outpushed out of the leaves of the leaves = = ““guttationguttation””

•• “root pressure”:“root pressure”: osmotic osmotic pressure due to higher solute pressure due to higher solute concentration in root xylem sap concentration in root xylem sap than in the soil.than in the soil.

Summary of Root to Leaf Water Movement•• Osmosis = motor for getting water from Osmosis = motor for getting water from

the soil into the root xylem.the soil into the root xylem.•• Water only moves through cell sap.Water only moves through cell sap.

•• Endodermis cells Endodermis cells controlcontrol the osmotic the osmotic gradient pulling water in (with energy and gradient pulling water in (with energy and transport proteins) transport proteins)

•• Xylem does the same: Xylem does the same: osmoticallyosmotically pulls pulls water into the root xylem.water into the root xylem.

Summary of Root to Leaf Water Movement

•• To get water to the top of To get water to the top of tall plants….tall plants….

•• It is pulled by the It is pulled by the evaporation of water from evaporation of water from the leavesthe leaves (like the pull (like the pull generated by sucking on a generated by sucking on a straw).straw).

Transpiration and the Stomata•• TranspirationTranspiration

(= evaporation of water from leaves) (= evaporation of water from leaves) pulls water and minerals up stems AND provides pulls water and minerals up stems AND provides evaporative cooling, butevaporative cooling, but

•• It results in It results in tremendous loss of watertremendous loss of water, which , which must be controlled.must be controlled.

Transpiration and the StomataPlants manage costs and benefits (CO2 in for Plants manage costs and benefits (CO2 in for

photosynthesis, H2O out) with:photosynthesis, H2O out) with:•• Epidermis Epidermis –– Flat cells covered by a Flat cells covered by a waxy waxy

waterproofwaterproof cuticlecuticle. .

•• StomataStomata –– Pores that let CO2 in and H2O out Pores that let CO2 in and H2O out when there’s not too much water stress.when there’s not too much water stress.These are highly regulated by the plantThese are highly regulated by the plant

How do stomata work?•• The The stomastoma (or pore):(or pore):surrounded by two surrounded by two guard cells guard cells

•• Guard cells Guard cells controlcontrol the opening the opening and closing of the stomaand closing of the stomathru…thru…changes in the guard cell water changes in the guard cell water

potentialpotential

•• Light cues most plants to open Light cues most plants to open stomatastomata

•• Active transport of Active transport of potassium ionspotassium ionsinto the guard cells. into the guard cells.

•• What happens to the water potential What happens to the water potential of the guard cells?of the guard cells?

•• What will happen to water?What will happen to water?

Mechanism of stomatal opening

Fig 36.11

•• H2O moves into the guard cells to H2O moves into the guard cells to maintain maintain osmoticosmotic balance.balance.

•• Stretching and turgidityStretching and turgidity of the of the guard cells….. guard cells…..

•• Stomata openStomata open..

•• (Closing is the reverse started by (Closing is the reverse started by passive diffusion of potassium out passive diffusion of potassium out of guard cells.)of guard cells.)

Mechanism of stomatal opening

Fig 36.11

•• StomataStomata typically typically open in the dayopen in the day(in response to light) and close at (in response to light) and close at night. night.

This provides CO2 for This provides CO2 for photosynthesis during the day, but photosynthesis during the day, but saves water at night.saves water at night.

Regulation of stomatal opening

•• A A low level of CO2low level of CO2 in the leaf in the leaf constrains photosynthesis and favors constrains photosynthesis and favors stomatalstomatal opening.opening.

•• If the plant is too dry:If the plant is too dry:mesophyllmesophyll cells release cells release abscisicabscisic

acidacid stomata to close.stomata to close.

Signals for stomatal opening