Chapter 36 Transport in Vascular Plants

A. Physical Forces

H2O

CO2

O2

lightsuga

r

H2O

CO2

O2

minerals

A. Physical Forces

H2O and minerals transport in

sugars transport in

gas exchange

xylem moves water

because oftranspiration

evaporation, cohesion and adhesion

phloem bulk flow

major substances transported are:

transport occurs on cellular

from

transport of into root hairs

short-distance transport from

loading of from photosynthetic leaves into phloem sieve tubes

long-distance transport transport in

throughout whole plant

three scales

environment into plant cells

H2O and solutes

cell to cell

sugar

xylem and phloem

membranes selective permeability

diffusion, passive transport, active transport

phospholipid bilayer, protein channels

solutes are moved into plant cells by

active transport protein

in cell membrane

mechanism that uses the energy stored in a concentration gradient to drive cellular work

Cellular Transport

proton pumps

use to pump against the concentration gradient the cell

sets up a separation of across a membrane

active transport

chemiosmosis –

ATP H+ (hydrogen) ionsout of

membrane potential –opposite charge

The Proton Pump

both the proton pump and membrane potential have which is used to drive the transport of many different solutes

stored energy

water uptake and loss must beWater Potential

water moves by

add which affects osmosis

water potential, , takes both and into account measured in

balanced

osmosis

cell walls physical pressure

Ψ solute (dissolved substances) concentration physical pressure

megapascals, MPa (or bars)

Ψ = ΨS + ΨPwhere: Ψ = water potential

ΨS = solute potential (osmotic potential)

ΨP = pressure potential the ΨS of pure water is

Purewater

= 0 MPa

zero

adding solute the water potential (because there is less free water molecules less capacity to do work) and ΨS is

Addition ofsolutes

0.1 Msolution

Purewater

= 0 MPa = –0.23 MPa

P = 0S = –0.23

H2O

lowers

negative

ΨP can be relative to atmospheric pressure

Applyingphysicalpressure

= 0 MPa = 0 MPa

P = 0.23S = –0.23

Applyingphysicalpressure

= 0 MPa = 0.07 MPa

P = 0.30S = –0.23

H2O

H2O

Purewater

Purewater

positive or negative

water under (pulling) gives pressure eg) water in xylem

Negativepressure

= –0.23 MPa

P = 0S = –0.23

= –0.30 MPa

P = –0.30S = 0

H2O

Purewater

tension negative

water gives pressure eg) turgor pressure

water always moves from areas of to areas of

water moves through the phospholipids bilayer and through transport proteins called

cells will be or depending on the environment

plasmolyzed turgid

pushing out positive

high Ψlow Ψ

aquaporins

plasmolyzed turgid

loss of turgor causes wilting

plant cells are compartmentalized Short-Distance Transport

cell wall

cell membrane – cytosol

vacuole

Plasmodesma

Plasma membrane

Cell wall

Cytosol

Vacuole

Vacuolar membrane(tonoplast)

transport routes for water and solutes transmembrane route

repeated of plasma membrane

Transmembrane route

crossing

symplast route

Key

Symplast

Symplastic route

Symplast

Transmembrane route

movement within plasmodesmata junctions connect cytosol of

neighboring cells

cytosol

Key

Symplast

Apoplast

Apoplastic route

Apoplast

Symplast

Transmembrane route

Symplastic route

apoplast route movement through the continuum of

from cell to cell no cell membranes are crossed

cell walls

which is the movement of fluid driven by

Long-Distance Transport

lack of some organelles in phloem cells and the complete lack of cytoplasm in xylem cells makes them very efficient tubes for transport

flow in xylem tracheids and vessels

creates which xylem sap upwards from roots

loading of sugar from photosynthetic leaf cells generates high positive pressure which pushes phloem sap through sieve tubes

bulk flowpressure

transpiration negative pressurepulls

much of the absorption of takes place at the root tips

B. Roots

root hairs extensions of walls are huge amount of

water and minerals

epidermal cells

hydrophilic

surface area

soil solution moves into

flows through solution moves into

of root cells

water moves from Ψ in soil to Ψ in root active transport concentrates certain molecules in the root cells

eg) K+ ions

apoplast

walls into cortex

symplast

highlow

mycorrhizae symbiotic structures plant roots with fungus greatly increases

surface area for water and mineral absorption

greatly increases volume of soil reached by plant

endodermis layer surrounding vascular

cylinder of root lined with impervious

forces solution through selective cell membrane and into symplast

also prevents leakage of xylem sap back into soil

solution in endodermis and parenchyma cells is discharged into cell walls (apoplast) by

this allows the solution to then move to the xylem cells

Casparian strip

active and passive transport

Casparian strip

Endodermal cellPathway alongapoplast

Pathway throughsymplast

Casparian stripPlasmamembrane

Apoplasticroute

Symplasticroute

Roothair

Vessels(xylem)

CortexEndodermisEpidermis Vascular cylinder

root pressure C. Ascent of Xylem Sap

in xylem of roots the Ψ water flows causing

pressure of xylem sap

accounts for of ascent of sap

mineral ions lowers

in root pressure

positive

upward push

very small part

generated by powered Ψ in leaf is than Ψ in water vapour leaves the leaf through the

stomata (transpiration) water

transpiration pull

Ψ is in roots and in leaves, moves water plant

adhesion, cohesion, hydrogen bonding

leafsolar

higher atmosphere

pulled uphigh low

up

Xylemsap

Mesophyll cells

Stoma

Water molecule

AtmosphereTranspiration

Xylemcells

Adhesion

Cell wall

Cohesion, by hydrogenbonding

Cohesion andadhesion inthe xylem

Water molecule

Wat

er p

ote

nti

al g

rad

ien

t

Root hair

Soil particle

Water

Water uptake from soil

Trunk xylem Ψ = –0.8 Mpa

Root xylem Ψ = –0.6 MPa

Leaf Ψ(air spaces) = –7.0 MPa

Outside air Ψ = –100.0 MPa

Leaf Ψ(cell walls) = –1.0 MPa

Soil Ψ = –0.3 MPa

photosynthesis and transpiration D. Stomata

compromise in and out but also

out leaf transpires more than its weight in a day xylem sap can flow at 75 cm/min

O2, H2O

CO2

CO2 O2 H2O

H2O evaporation takes place even with

drought will cause wilting transpiration causes

of the leaves

closed stomata

evaporative cooling

regulation of stomata microfibril mechanism

guard cells attached at tips

contain microfibrils in cell walls guard cells elongate and bow out when turgid guard cells shorten and become less bowed when

flaccid Cells turgid/Stoma open

Radially orientedcellulose microfibrils

Vacuole

Cell wall

Guard cell

Cells flaccid/Stoma closed

ion mechanism proton pumps are used to move

into guard cells (stored in vacuoles) Ψ in cells than surrounding cells

H2O moves

of K+ ions causes H2O to move of guard cells

become and

Cells turgid/Stoma open Cells flaccid/Stoma closedH2O

H2O

H2OH2O

H2O

H2O

H2O

H2O H2OH2O

K+

guard cells become and

K+ ions

lowerin

turgid

open

loss out

flaccid close

other cues light

blue-light receptors in plasma membrane triggers ATP-powered proton pumps causing K+ uptake

stomata open

depletion of CO2 CO2 in air spaces in mesophyll is used

for photosynthesis depletion causes stomata to open

circadian rhythm automatic 24-hour cycle

stomata open in day, close at night

xerophytes plants adapted for

small, thick leaves

adapted to water loss

reflective leaves hairy leaves stomata in pores on underside of leaves alternative photosynthetic pathway

(CAM)

arid regions

reduce

is the transport of organic nutrients

E. Organic Nutrients

water phloem contains:

sugar (sucrose) (30% by weight) minerals amino acids hormones

translocationsap

sieve tubes carry sap from to

sap flow rate can be as high as 1 m/hr

sugars are loaded into the flow through via active of sucrose into

phloem cells with H+ ions in proton pump

sugar source (leaves) sugar sink (growing roots, buds, stems and fruit)variable direction of flow

phloem

symplast plasmodesmata

cotransport

Mesophyll cell

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Companion(transfer) cell

Mesophyll cellBundle-sheath cell

Phloemparenchyma cell

Sieve-tubemember

Protonpump

Low H+ concentration

Sucrose

High H+ concentrationCotransporter

Key

Apoplast

Symplast

pressure flow Ψ in is than in the

xylem at because of the that takes place

H2O diffuses from xylem

is generated which causes the through phloem sieve tubes

Ψ in is than in the xylem at because of the from the phloem

H2O diffuses from phloem

phloem lowersugar source sugar loading

into phloem

positive pressuresap to move

phloem highersugar sinks

back into xylem

sugar being removed

Vessel(xylem)

Sieve tube(phloem)

Sucrose Source cell(leaf)

H2O

H2O

Sucrose

Sink cell(storageroot)

H2O

Pre

ssu

refl

ow

Tra

nsp

irat

ion

stre

am

low Ψhigh Ψ

low Ψhigh Ψ