Chapter 25: Transport of Water and Nutrients in Plants AP Biology

Chapter 25: Transport of Water and Nutrients in Plants

AP Biology

• The success of plants depends on their ability to gather and conserve resources from their environment

• The transport of materials is central to the integrated functioning of the whole plant

© 2011 Pearson Education, Inc.

Plants require nutrients and water

Figure 36.2-3

H2Oand minerals

O2

CO2

CO2O2

H2O

Light

Sugar

Adaptations for acquiring resources were key steps in the evolution of vascular plants

• The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water

• Early nonvascular land plants lived in shallow water and had aerial shoots

• Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transportThe evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis

• Xylem transports water and minerals from roots to shoots• Phloem transports photosynthetic products from sources to sinks


• Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss


• Ion exchange makes nutrients available to plants

• Soil organisms contribute to plant nutrition– N-fixing bacteria (Rhizobium)– Mycrorrhizae (symbiotic association with fungi)


Plants Acquire Mineral Nutrients from Soil

• Differences in Water potential govern the direction of water movement (see next slides for review)

• Water and ions move across the root cell plasma membrane– Membrane proteins required!! (need to move across hydrophobic

membrane and often against gradient)• Aquaporins• Ion channels and proton pumps• Ion exchange makes nutrients available to plants


Water and Solutes are Transported in the Xylem by Transpiration-Cohesion-Tension

Water Potential and Transport of Water Across Plasma Membranes

• To survive, plants must balance water uptake and loss

• Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure


• Water potential is a measurement that combines the effects of solute concentration and pressure

• Water potential determines the direction of movement of water• Water flows from regions of higher water potential to regions of

lower water potential• Potential refers to water’s capacity to perform work• Water potential is abbreviated as Ψ and measured in a unit of

pressure called the megapascal (MPa)• Ψ = 0 MPa for pure water at sea level and at room temperature• Water moves in the direction from higher water potential to

lower water potential


How Solutes and Pressure Affect Water Potential

• Both pressure and solute concentration affect water potential

• This is expressed by the water potential equation: Ψ ΨS ΨP

• The solute potential (ΨS) of a solution is directly proportional to its molarity

• Solute potential is also called osmotic potential


• Pressure potential (ΨP) is the physical pressure on a solution

• Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast

• The protoplast is the living part of the cell, which also includes the plasma membrane


Figure 36.8

Solutes have a negative effect on by bindingwater molecules.

Pure water at equilibrium

H2O

Adding solutes to theright arm makes lowerthere, resulting in netmovement of water tothe right arm:

H2O

Pure water

Membrane Solutes

Positive pressure has a positive effect on by pushing water.


H2O

H2O

Positivepressure

Applying positivepressure to the right armmakes higher there,resulting in net movementof water to the left arm:

Solutes and positivepressure have opposingeffects on watermovement.


H2O

H2O

Positivepressure

Solutes

In this example, the effectof adding solutes isoffset by positivepressure, resulting in nonet movement of water:

Negative pressure(tension) has a negativeeffect on by pullingwater.


H2O

H2O

Negativepressure

Applying negativepressure to the right armmakes lower there,resulting in net movementof water to the right arm:

• Water potential affects uptake and loss of water by plant cells

• If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis

• Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall


Water Movement Across Plant Cell Membranes

Aquaporins: Facilitating Diffusion of Water

• Aquaporins are transport proteins in the cell membrane that allow the passage of water

• These affect the rate of water movement across the membrane


• Differences in Water potential govern the direction of water movement (see next slides for review)

• Water and ions move across the root cell plasma membrane– Membrane proteins required!! (need to move across hydrophobic

membrane and often against gradient)• Aquaporins• Ion channels and proton pumps• Ion exchange makes nutrients available to plants


Water and Solutes are Transported in the Xylem by Transpiration-Cohesion-Tension

Short-Distance Transport of Solutes Across Plasma Membranes

• Plasma membrane permeability controls short-distance movement of substances

• Both active and passive transport occur in plants• In plants, membrane potential is established

through pumping H by proton pumps– Plant cells use the energy of H gradients to cotransport other

solutes by active transport– In animals, membrane potential is established through pumping

Na by sodium-potassium pumps

• Plant cell membranes have ion channels that allow only certain ions to pass


Absorption of Water and Minerals by Root Cells

• Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water

• Root hairs account for much of the surface area of roots

• After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

• The concentration of essential minerals is greater in the roots than soil because of active transport


Different mechanisms transport substances over short or long distances


•Three transport routes for water and solutes are:–The apoplastic route, through cell walls and extracellular spaces–The symplastic route, through the cytosol (plasmodesmata)–The transmembrane route, across cell membrane

–This requires the use of transport proteins to allow passage of anything but gases

–Allows for selectivity!!! (only point in plant transport where selectivity can occur)

Figure 36.6

Cell wall

Cytosol

Plasmodesma

Plasma membrane

Apoplastic route

Symplastic route

Transmembrane route

Key

Apoplast

Symplast

Water and ions pass to the xylem by way of the Apoplast and symplast:• The apoplast consists of everything external to the plasma

membrane• It includes cell walls, extracellular spaces, and the interior of vessel

elements and tracheids• The symplast consists of the cytosol of the living cells in a plant, as

well as the plasmodesmataWater can cross the cortex via the symplast or apoplast

• The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder

• Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder


Pathway alongapoplast

Casparian stripEndodermal

cell

Pathwaythroughsymplast

Plasmamembrane Casparian strip

Apoplasticroute

Symplasticroute

Roothair

Epidermis Endodermis

Vessels(xylem)

Vascular cylinder(stele)

Cortex

Figure 36.10

• The endodermis is the innermost layer of cells in the root cortex

• It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue

Transport of Water and Minerals into the Xylem



Animation: Transport in Roots Right-click slide / select “Play”

• Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure

• Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem

• Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm


Transpiration drives the transport of water and minerals from roots to shoots via the xylem

Pulling Xylem Sap: The Cohesion-Tension Hypothesis

• According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots

• Xylem sap is normally under negative pressure, or tension


Transpirational Pull• Water vapor in the airspaces of a leaf diffuses

down its water potential gradient and exits the leaf via stomata

• As water evaporates, the air-water interface retreats further into the mesophyll cell walls

• The surface tension of water creates a negative pressure potential

• This negative pressure pulls water in the xylem into the leaf

• The transpirational pull on xylem sap is transmitted from leaves to roots


Outside air 100.0 MPa

7.0 MPa

1.0 MPa

0.8 MPa

0.6 MPa

0.3 MPa

Leaf (air spaces)

Leaf (cell walls)

Trunk xylem

Trunk xylem

Soil

Wa

ter

po

ten

tia

l g

rad

ien

t

Xylem sap

Mesophyll cells

Stoma

Water moleculeAtmosphereTranspiration

Xylemcells

Adhesion byhydrogen bonding

Cell wall

Cohesion andadhesion inthe xylem

Cohesion byhydrogen bonding

Water molecule

Root hair

Soil particle

WaterWater uptakefrom soil

Figure 36.13

Adhesion and Cohesion in the Ascent of Xylem Sap

• Water molecules are attracted to cellulose in xylem cell walls through adhesion

• Adhesion of water molecules to xylem cell walls helps offset the force of gravity

• Water molecules are attracted to each other through cohesion

• Cohesion makes it possible to pull a column of xylem sap



Animation: Water TransportRight-click slide / select “Play”


Animation: TranspirationRight-click slide / select “Play”

Xylem Sap Ascent by Bulk Flow: A Review

• The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism

• Bulk flow is driven by a water potential difference at opposite ends of xylem tissue

• Bulk flow is driven by evaporation and does not require energy from the plant; like photosynthesis it is solar powered


• Bulk flow differs from diffusion– It is driven by differences in pressure potential,

not solute potential

– It occurs in hollow dead cells, not across the membranes of living cells

– It moves the entire solution, not just water or solutes

– It is much faster


Bulk flow is how solutes are transported through xylem

The rate of transpiration is regulated by stomata• Leaves generally have broad surface areas and high

surface-to-volume ratios• These characteristics increase photosynthesis and

increase water loss through stomata• Guard cells help balance water conservation with gas

exchange for photosynthesis


Stomata: Major Pathways for Water Loss

• About 95% of the water a plant loses escapes through stomata

• Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape

• Stomatal density is under genetic and environmental control


Mechanisms of Stomatal Opening and Closing

• Changes in turgor pressure open and close stomata

– When turgid, guard cells bow outward and the pore between them opens

– When flaccid, guard cells become less bowed and the pore closes

• This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells


Figure 36.15

Radially orientedcellulose microfibrils

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Cellwall

VacuoleGuard cell

(a) Changes in guard cell shape and stomatal openingand closing (surface view)

(b) Role of potassium in stomatal opening and closing

K

H2OH2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Stimuli for Stomatal Opening and Closing• Generally, stomata open during the day and close at night to

minimize water loss• Stomatal opening at dawn is triggered by

– Light– CO2 depletion– An internal “clock” in guard cells

• All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

• Drought, high temperature, and wind can cause stomata to close during the daytime

• The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata


Effects of Transpiration on Wilting and Leaf Temperature

• Plants lose a large amount of water by transpiration

• If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt

• Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes


Adaptations That Reduce Evaporative Water Loss

• Xerophytes are plants adapted to arid climates• Some desert plants complete their life cycle during

the rainy season• Others have leaf modifications that reduce the rate

of transpiration• Some plants use a specialized form of

photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night


Ocotillo(leafless)

Ocotillo afterheavy rain

Oleander leaf cross section

Oleanderflowers

Ocotillo leaves

Old man cactus

Cuticle Upper epidermal tissue

Trichomes(“hairs”)

Crypt Stoma Lower epidermaltissue

10

0

m

Figure 36.16

Sugars are transported from sources to sinks via the phloem (Phloem Pressure Flow)

• The products of photosynthesis are transported through phloem by the process of translocation


Movement from Sugar Sources to Sugar Sinks

• In angiosperms, sieve-tube elements are the conduits for translocation

• Phloem sap is an aqueous solution that is high in sucrose

• It travels from a sugar source to a sugar sink• A sugar source is an organ that is a net producer

of sugar, such as mature leaves• A sugar sink is an organ that is a net consumer or

storer of sugar, such as a tuber or bulb


• A storage organ can be both a sugar sink in summer and sugar source in winter

• Sugar must be loaded into sieve-tube elements before being exported to sinks

• Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways

• Companion cells enhance solute movement between the apoplast and symplast


Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms

• Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow In many plants, phloem loading requires active transport

• Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose

• At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water


The Pressure Flow Model

Figure 36.18

Loading of sugar

Uptake of water

Unloading of sugar

Water recycled

Source cell(leaf)Vessel

(xylem)

Sieve tube

(phloem)

SucroseH2O

H2O

H2OSucrose

Sink cell(storageroot)

Bu

lk f

low

by

ne

ga

tiv

e p

res

su

re

Bu

lk f

low

by

po

sit

ive

pre

ss

ure

2

1

3

4

2

1

34

Figure 36.17

Mesophyll cell

Key

Apoplast

Symplast

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Mesophyll cellBundle-sheath cell

Phloemparenchyma cell

Companion(transfer) cell

Sieve-tubeelement

High H concentration Cotransporter

Protonpump

Sucrose

Low H concentration

H

H H

S

SATP

(a) (b)


Animation: Translocation of Phloem Sap in Summer Right-click slide / select “Play”


Animation: Translocation of Phloem Sap in Spring Right-click slide / select “Play”

Documents

Chapter 25: Transport of Water and Nutrients in Plants AP Biology