+

Mrs. Valdes

AP Biology

Chapter 36: Resource

Acquisition and Transport

in Vascular Plants

+Overview: Underground PlantsSuccess of plants depends

on their ability to gather & conserve resources from environment

Diffusion, active transport, and bulk flow work together to transfer water, minerals, and sugars

“Stone Plants” live mostly subterranean with two leaf tips full of clear jelly that acts as lenses to channel light underground

+Fig. 36-2-1

H2O

H2Oand minerals

+Fig. 36-2-2

H2O

H2Oand minerals

CO2 O2

O2

CO2

+Fig. 36-2-3

H2O

H2Oand minerals

CO2 O2

O2

CO2

SugarLight

+Concept 36.1: Land plants acquire resources both above and below groundAlgal ancestors of land plants absorbed water,

minerals, and CO2 directly from surrounding waterEvolution of xylem and phloem in land plants long-

distance transport of water, minerals, and products of photosynthesis

Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss Stems: conduits for water

and nutrients; supporting structures for leaves

Phyllotaxy: arrangement of leaves on stem; specific to each species

+Light absorption affected by leaf area index

total upper leaf surface of plant divided by surface area of land on which it grows

affected by light absorption

+Root Architecture and Acquisition of Water and Minerals

Soil: resource mined by root systemTaproot systems: anchor plants; characteristic of most trees

Roots + hyphae of soil fungi form symbiotic associations called mycorrhizaeMutualisms with fungi helped plants colonize

land

+Concept 36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow Transport begins with

absorption of resources by plant cells

Movement of substances into/out of cells regulated by selective permeability

Diffusion across a membrane = passive

Pumping of solutes across a membrane = active AKA needs energy

Most solutes pass through transport proteins in cell membrane Proton pump: most important transport protein for active transport plant cells create hydrogen ion gradient AKA form of potential energy

that can be harnessed to DO WORK contribute to voltage known as membrane potential

+Plant cells use energy stored in proton gradient and membrane potential to drive transport of many different solutes

Cotransport: transport protein couples diffusion of one solute to active transport of another

“Coattail” effect of cotransport: responsible for uptake of sugar sucrose by plant cells

+Diffusion of Water (Osmosis)Osmosis: determines net

uptake/water loss by cell; affected by solute concentration and pressure

Water potential: measurement that combines effects of solute concentration and pressure Determines direction of movement

of water (in/out) Water flows from regions of

higher water potential to regions lower water potential

Symbol:Ψ Units: measure pressure;

megapascals (MPa) Ψ = 0 MPa for pure water at sea

level and room temperature

+How Solutes and Pressure Affect Water Potential• REMEMBER: Pressure and

solute concentration affect water potential

• Solute potential (ΨS): proportional to number of dissolved molecules• also called osmotic potential

• Pressure potential (ΨP): physical pressure on a solution

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

+Measuring Water Potential• Water moves from higher water potential lower water potential

• Addition of solutes reduces water potential

• Physical pressure increases water potential

• Negative pressure decreases water potential

+

ψ = −0.23 MPa

Fig. 36-8a

(a)

0.1 Msolution

Purewater

H2O

ψP = 0

ψS = 0ψP = 0ψS = −0.23

ψ = 0 MPa

+Fig. 36-8b

(b)Positivepressure

H2O

ψP = 0.23

ψS = −0.23

ψP = 0

ψS = 0ψ = 0 MPa ψ = 0 MPa

+Fig. 36-8c

ψP =  ψS = −0.23

(c)

Increasedpositivepressure

H2O

ψ = 0.07 MPa

ψP = 0

ψS = 0ψ = 0 MPa

0.30

+Fig. 36-8d

(d)

Negativepressure(tension)

H2O

ψP = −0.30ψS =

ψP =ψS = −0.23ψ = −0.30 MPaψ = −0.23 MPa

0 0

•Water potential affects uptake and loss of water by plant cells•Flaccid cell: placed in environment with higher solute concentration• cell will lose water and undergo plasmolysis

•If same flaccid cell placed in solution with lower solute concentration• cell will gain water and become turgid

•Turgor loss in plants causes wilting• can be reversed when the plant is watered

+Aquaporins: Facilitating Diffusion of WaterAquaporins: transport proteins in cell membrane that allow passage of water rate of water movement likely regulated by

phosphorylation of aquaporin proteins

+Three Major Pathways of Transport Transport also regulated by compartmental structure of

plant cells1: Plasma membrane directly controls traffic of molecules into/out of the protoplast2: Plasma membrane is barrier between two major compartments, the cell wall and the cytosol3: Vacuole, large organelle that occupies as much as 90% or more of protoplast’s volume

Vacuolar membrane regulates transport between cytosol and vacuole

In most plant tissues, cell wall and cytosol continuous from cell to cell

Symplast: cytoplasmic continuum Plasmodesmata: cytoplasm of

neighboring cells connected by channels

Apoplast: continuum of cell walls and extracellular spaces

+Water and minerals can travel through plant

by three routes: Transmembrane route: out of one cell, across a cell wall,

and into another cell Symplastic route: via continuum of cytosol Apoplastic route: via cell walls and extracellular spaces

+Bulk Flow in Long-Distance TransportBulk flow: movement of a fluid driven by pressure; required for efficient long distance transport of fluid

Water and solutes move together through:tracheids and vessel elements

of xylem sieve-tube elements of phloem

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

+Concept 36.3: Water and minerals are transported from roots to shoots

Plants can move large volume of water from their roots to shoots

Most water and mineral absorption occurs near root tips, where epidermis is permeable to water and root hairs are locatedRoot hairs account for much of

surface area of rootsAfter soil solution enters roots,

extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

+• Endodermis: innermost layer of cells in root cortex• Surrounds vascular cylinder • Last checkpoint for selective passage of minerals from

cortex into vascular tissue• Water can cross the

cortex via the symplast or apoplast

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

Transport of Water and Minerals into Xylem

+Bulk Flow Driven by Negative Pressure in the Xylem

Transpiration: evaporation of water from a plant’s surface; Plants lose large volume of water

Xylem sap: Water replaced by bulk flow of water and minerals; from steles of roots to stems and leaves

Is sap mainly pushed up from the roots, or pulled up by the leaves?

+Pushing Xylem Sap: Root PressureAt night, transpiration very low, root cells continue

pumping mineral ions into xylem of vascular cylinder, THUS! Lowering water potential

Positive root pressure relatively weak and is minor mechanism of xylem bulk flow

Water flows in from the root cortex, generating root pressure sometimes results in guttation

exudation of water droplets on tips or edges of leaves

+Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

Water pulled upward by negative pressure in xylem Transpiration Pull:

Water vapor in airspaces of leaf diffuses down water potential gradient exits leaf via stomata

Transpiration produces negative pressure(tension) in leaf exerts pulling force on water in xylem RESULT: pulling water into leaf

+Cohesion and Adhesion in Ascent of Xylem Sap

• Transpirational pull on xylem sap transmitted from leaves to root tips and even into soil solution!• facilitated by:• cohesion of water

molecules to each other • adhesion of water

molecules to cell walls• Drought stress OR freezing

can cause cavitation• formation of water vapor

pocket by a break in chain of water molecules

+Fig. 36-15a

WatermoleculeRoothair

Soilparticle

WaterWater uptakefrom soil

+Fig. 36-15b

Adhesionby hydrogenbondingCell

wallXylemcells

Cohesionby hydrogenbonding

Cohesion andadhesion inthe xylem

+Fig. 36-15c

Xylemsap

Mesophyllcells

Stoma

Watermolecule

AtmosphereTranspiration

+ Xylem Sap Ascent by Bulk FlowKnow…1- Movement of xylem sap against gravity is maintained by transpiration-cohesion-tension mechanism2- Transpiration lowers water potential leaves which generates negative pressure (tension) that pulls water UP through xylem3- NO energy cost to bulk flow of xylem sap

+Concept 36.4: Stomata help regulate rate of transpiration

Leaves generally have broad surface areas and high surface-to-volume ratios

THIS increases photosynthesis and increases water loss through stomata

About 95% of water a plant loses escapes through stomata

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

+Mechanisms of Stomatal Opening and Closing

Changes in turgor pressure open and close stomata

Result from reversible uptake/loss of potassium ions by guard cells

+Fig. 36-17a

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

Radially orientedcellulose microfibrils

Cellwall

VacuoleGuard cell

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

+Fig. 36-17b

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

(b) Role of potassium in stomatal opening and closing

H2O

H2OH2O

H2OH2O

H2O H2O

H2O

H2O

H2O

K+

+Stimuli for Stomatal Opening and ClosingGenerally, stomata open during day and close

at night to minimize water lossStomatal opening at dawn triggered by:

light, CO2 depletion Internal “clock” in guard cells

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

+Effects of Transpiration on Wilting and Leaf Temperature

Plants lose A LOT of water by transpiration

Lose H2O and H2O not replace by H2O transport plant wilt

Transpiration also results in evaporative cooling: can lower temperature of

leaf prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

+Adaptations That Reduce Evaporative Water LossXerophytes: plants

adapted to arid climateshave leaf modifications

that reduce rate of transpiration

crassulacean acid metabolism (CAM): form of photosynthesis where stomatal gas exchange occurs at night

+Fig. 36-18 Ocotillo (leafless)

Oleander leaf cross section and flowersCuticle

Upper epidermal tissue

Ocotillo leaves

Trichomes(“hairs”)

Crypt StomataLower epidermaltissue

100 µ

m

Ocotillo after heavy rain

Old man cactus

+Concept 36.5: Sugars transported from leaves and other sources to sites of use or storage Translocation: process of transporting products of

photosynthesis through phloem Phloem sap : aqueous solution high in sucrose

travels from sugar source to sugar sink sugar source: organ that is a net producer of sugar

Ex: mature leaves sugar sink: organ that is a net consumer

or storer of sugar, such as a tuber or bulb Storage organ can be both sugar sink in

summer and sugar source in winter Sugar MUST BE loaded into sieve-tube

elements before being exposed to sinks Depending on species, sugar move by:

symplastic both symplastic and apoplastic pathways

Transfer cells: modified companion cells that enhance solute movement between apoplast and symplast

+Fig. 36-19a

Key

Apoplast

Symplast

Mesophyll cellCell walls

(apoplast)Plasma membrane

Plasmodesmata

Companion(transfer) cell

Sieve-tubeelement

Mesophyll cell

Bundle-sheath cell

Phloemparenchyma cell

+In many plants, phloem loading requires active transport

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

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

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

In studying angiosperms, researchers concluded sap moves through sieve tube by bulk flow driven by positive pressure

Pressure flow hypothesis explains why phloem sap always flows from source to sink

+Concept 36.6: Symplast is highly dynamic• Symplast: living tissue

responsible for dynamic changes in plant transport processes

• Plasmodesmata can change in permeability in response to:• turgor pressure • cytoplasmic calcium

levels• cytoplasmic pH

• Plant viruses can cause plasmodesmata to dilate

• Mutations that change communication within symplast can lead to changes in development

+Electrical Signaling in the PhloemPhloem allows for rapid electrical communication between VERY separated organs

Phloem is “superhighway” for systemic transport of macromolecules and viruses

Systemic communication helps integrate functions of whole plant

+You should now be able to:1. Describe how proton pumps function in transport of materials

across membranes2. Define the following terms: osmosis, water potential, flaccid,

turgor pressure, turgid3. Explain how aquaporins affect the rate of water transport

across membranes4. Describe three routes available for short-distance transport in

plants5. Relate structure to function in sieve-tube cells, vessel cells, and

tracheid cells

6. Explain how the endodermis functions as a selective barrier between the root cortex and vascular cylinder

7. Define and explain guttation

8. Explain this statement: “The ascent of xylem sap is ultimately solar powered”

9. Describe the role of stomata and discuss factors that might affect their density and behavior

10. Trace the path of phloem sap from sugar source to sugar sink; describe sugar loading and unloading