Resource Acquisition and Transport in Vascular

Plants

Green algae (ancestors of land plants) live in water.

Mosses (early plants) live in very moist environments, very low – non vascular

Land plants acquire resources: Above ground –

carbon dioxide and sunlight

Below ground – water and minerals

Minerals

H2O

H2O

CO2 O2

Sugar

Light

Land plants compete for these resources.

Natural selection favored the plants with efficient systems for long-distance transportation of Water Minerals Products of

photosynthesis

Minerals

H2O

H2O

CO2 O2

Sugar

Light

Plants have to balance Acquisition of light and CO2 Evaporative loss of water

Various arrangement of leaves make it possible to maximize light and CO2 uptake and minimize water loss.

Leaves that are self shaded undergo programmed cell death and fall off because their photosynthetic output is less than their metabolic needs.

Roots undergo modifications to increase intake of water and minerals.

Roots associate with fungi to increase surface area (mycorrhizae)

Early association with land plants made colonization of land plants possible.

Transport occurs: Short distance: passive and active transport Long distance: bulk flow

Cell membranes are fluid mosaic model made of

phospholipid bilayer

proteins

Most nutrients cannot diffuse across phospholipid bilayer

Active transport: cell must expend ATP (energy)

Transport proteins are involved in active transport of nutrients

Proton pump: energy from ATP pump protons, H+ out of the cells inside of the membrane had –ve charge, outside

has +ve charge: membrane potential

CYTOPLASM

ATP

EXTRACELLULAR FLUID

Proton pumpgenerates mem-brane potentialand gradient.

Proton pump facilitates cation uptake

CYTOPLASM EXTRACELLULAR FLUID

Cations ( , forexample) aredriven into the cellby the membranepotential.

Transport protein

Membrane potential and cation uptake

Proton pump facilitates cotransport of anions with H+

Cell accumulatesanions ( , for example) by coupling their transport to; theinward diffusionof through a cotransporter.

Cotransport of anions

Proton pump facilitates transport of neutral solutes with H+

Plant cells canalso accumulatea neutral solute, such as sucrose( ), bycotransporting down thesteep protongradient.

Cotransport of a neutral solute

Osmosis (diffusion of water) across semipermeable membrane.

Concentration of water determines direction of water flow

Water flow is affected by rigid cell walls which exerts pressure on plasma membrane.

Water potential (): The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure; units megapascals (MPa).

Water potential refers to water’s potential energy – water’s capacity to perform work when it moves region of high water potential to a region of low water potential.

= s + p

Where:

= water potential

s = solute potential/ osmotic potential

p = pressure potential

Free water has highest water potential. When bound to solutes water potential goes down.

Pressure potential can be positive or negative.

Usually cells are under positive water potential. Usually cell contents press against cell wall and cell wall presses against protoplast – turgor pressure.

Water potential and water movement in an artificial model.

a) In absence of pressure s determines net movement’

Addition ofsolutes

0.1 Msolution

Purewater

H2O

= 0 MPa P = –0.23 MPa

P = 0S = –0.23

b) Positive pressure can raise by increasing p.

Applyingphysicalpressure

H2O

= 0 MPa P = –0 MPa

P = 0S = –0.23

c) Raising on the right causes movement to the left.

Applyingphysicalpressure

H2O

= 0 MPa P = –0.07 MPa

P = 0.30S = –0.23

d) –ve pressure reduces p, causes net movement to the left by reducing .

Negativepressure

H2O

P = –0.23 MPa

P = 0.30S = –0.23

P = –0.30 MPa

P = –0.30S = –0.23

Initial flaccid cell undergoes plasmolysis when placed in an environment with high solute concentration.

It becomes turgid when placed in pure water.

P = –0.9 MPa

P = 0

S = –0.9

0.4 M sucrose solution:P = –0.7 MPa

P = 0

S = –0.7

Initial flaccid cell:

P = 0 MPa

P = 0

S = 0

Distilled water:

What happens when you forget to water a plant? What happens when you water it?

Water transport is aided by transport proteins – aquaporins.

Transmembrane route

Key

Symplast

Apoplast

Symplastic route

Transport routes between cells

Apoplastic route

Apoplast

Symplast

Three major pathways of transport: Apoplastic route Symplastic route Transmembrane route

Bulk flow requires more efficient transport than diffusion and active transport.

Transport of water and minerals into the xylem: pushing and pulling

Pushing up the xylem sap: root pressure

Casparian strip

Endodermal cellPathway alongapoplast

Pathway throughsymplast

Casparian strip

Plasmamembrane

Apoplasticroute

Symplasticroute

Roothair

Vessels(xylem)

Cortex

EndodermisEpidermis Vascular cylinder

When too much enters plants give out excess water through leaves – guttation

Transpiration: loss of water vapor through leaves and other aerial parts of plants. A single corn plant transpires 60L of water in the growing season.

Transpiration pull

LE 36-12

Upperepidermis

MesophyllAir

space

Cuticle

Lowerepidermis

Cuticle CO2 O2 CO2Xylem

O2

Stoma

Evaporation

EvaporationWater film

Airspace

Cytoplasm

Cell wall

Vacuole

Air-waterinterface

High rate of transpiration

Low rate of transpiration

= –10.00 MPa = –0.15 MPa

Cell wall

Airspace

Cohesion and adhesion causes ascent of sap

Xylemsap

Mesophyllcells

Stoma

Watermolecule

AtmosphereTranspiration

Xylemcells

Adhesion Cellwall

Cohesion,byhydrogenbonding

Cohesion andadhesion inthe xylem

Watermolecule

Wat

er p

ote

nti

al g

rad

ien

t

Roothair

Soilparticle

WaterWater uptakefrom soil

Trunk xylem = –0.8 Mpa

Root xylem = –0.6 MPa

Leaf (air spaces)= –7.0 MPa

Outside air = –100.0 MPa

Leaf (cell walls) = –1.0 MPa

Soil = –0.3 MPa

Stomata regulates rate of transpiration: opening and closing of stomata are regulated by transport of K+.

Cells turgid/Stoma open

Role of potassium in stomatal opening and closing

Cells flaccid/Stoma closed

H2O

H2O

H2OH2O

H2O H2O

H2O

H2O

H2O

H2O

K+

Cells turgid/Stoma open Cells flaccid/Stoma closed

Adpatations in desert plants Reduced life cycle Reduced period of leaf production Thicker cuticle Bristles to reflect the heat Reduced leaves, photosynthetic stems Growing underground Taking carbon dioxide at night

Cuticle Upper epidermal tissue

Lower epidermaltissue

Trichomes(“hairs”)

Stomata 100 µm

Reduced leaves, photosynthetic stems

Movement of sugars from source to sink

Translocation: transport of photosynthetic products for use and storage by phloem tissue.

Sugar source: plant organ that is the net producer of sugar (leaves)

Sugar sink: net consumer of depository sugar (growing tips, roots, buds, stems, fruits)

Positive pressure bulk flow in sieve tubes (phloem).

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

Thinning helps prevent excess demand on sugar source (pruning in agriculture)