Transport in Plants

AP BiologyCh. 36

Ms. Haut

Physical forces drive the transport of materials in plants over a range of distances

• Transport in vascular plants occurs on three scales:– Transport of water and solutes by individual cells, such as

root hairs– Short-distance transport of substances from cell to cell at the

levels of tissues and organs– Long-distance transport within xylem

and phloem at the level of the whole plant• A variety of physical processes are

involved in the different types of transport

Minerals

H2O

H2O

CO2 O2

Sugar

Light

CO2

O2

Transport at Cellular Level

Relies on selective permeability of membranes

• Transport proteins– Facilitated diffusion– Selective channels (K+ channels)

• Aquaporins—water-specific protein channels that facilitate water diffusion across plasma membrane

Transport at Cellular Level• Proton pumps

– create a hydrogen ion gradient that is a form of potential energy

– contribute to a voltage known as a membrane potential

CYTOPLASM

ATP

EXTRACELLULAR FLUID

Proton pumpgenerates mem-brane potentialand gradient.

Transport at Cellular Level• Plant cells use energy stored in the proton gradient

and membrane potential to drive the transport of many different solutes

CYTOPLASM EXTRACELLULAR FLUID

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

Transport proteinMembrane potential and cation uptake

Transport at Cellular Level• In the mechanism called cotransport, a transport

protein couples the passage of one solute to the passage of another

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

Cotransport of anions

Effects of Differences in Water Potential

• To survive, plants must balance water uptake and loss

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

Effects of Differences in Water Potential

• 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

How Solutes and Pressure Affect Water Potential

• Both pressure and solute concentration affect water potential– The addition of solutes reduces water potential

• The solute potential of a solution is proportional to the number of dissolved molecules

• Pressure potential is the physical pressure on a solution

= P + S

Differences in Water Potential

Drive Water Transport in Plant Cells

= P + S

Three Major Compartments of Vacuolated Plant Cells

• Transport is also regulated by the compartmental structure of plant cells

• The plasma membrane directly controls the traffic of molecules into and out of the protoplast

• The plasma membrane is a barrier between two major compartments, the cell wall and the cytosol

Cell compartments

Plasmodesma

Plasma membrane

Cell wallCytosol

Vacuole

Key

SymplastApoplast

Vacuolar membrane(tonoplast)

• The third major compartment in most mature plant cells is the vacuole, a large organelle that occupies as much as 90% or more of the protoplast’s volume

• The vacuolar membrane regulates transport between the cytosol and the vacuole

Cell compartments

Plasmodesma

Plasma membrane

Cell wallCytosol

Vacuole

Key

SymplastApoplast

Vacuolar membrane(tonoplast)

• In most plant tissues, the cell walls and cytosol are continuous from cell to cell

• The cytoplasmic continuum is called the symplast

• The apoplast is the continuum of cell walls and extracellular spaces

Transmembrane route

Key

SymplastApoplast

Symplastic route

Transport routes between cellsApoplastic route

Apoplast

Symplast

Lateral Transport of Minerals and Water

Casparian strip—waxy material (suberin) that creates selectivity (only minerals already in symplast can enter stele)

The Roles of Root Hairs, Mycorrhizae, and Cortical Cells

• Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and root hairs are located

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

Mycorrhizae• Most plants form mutually

beneficial relationships with fungi, which facilitate absorption of water and minerals from the soil

• Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae

2.5 mm

Pushing Xylem Sap: Root Pressure

• At night, when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential

• Water flows in from the root cortex, generating root pressure

• Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicots

Transportation of Xylem Sap (Water): Transpiration-Cohesion Theory

•Water evaporates from leaves through stomata—creates a low pressure at top of water column

•Water replaced by water from xylem—water in areas of high pressure move to areas of low pressure

Strong cohesion of water with the pressure difference helps to pull the entire water column up from roots to rest of plant

Transpirational Pull• Water is pulled upward by

negative pressure in the xylem• Water vapor in the airspaces of

a leaf diffuses down its water potential gradient and exits the leaf via stomata

• Transpiration produces negative pressure (tension) in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf

Cohesion and Adhesion in the Ascent of Xylem Sap

• The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution

• Transpirational pull is facilitated by cohesion and adhesion

XylemsapMesophyllcellsStoma

Watermolecule

AtmosphereTranspiration

Xylemcells

Adhesion Cellwall

Cohesion,byhydrogenbonding

Cohesion andadhesion inthe xylem

Watermolecule

Wat

er p

oten

tial g

radi

ent

RoothairSoilparticleWater

Water 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

Guardcell

K+

K+

K+

K+

During the day, K+ is pumped into the guard cells

•Opening and closing is regulated by turgor pressure•Stoma of most plants open during the day and closed during the night

H2O flows into cells by osmosis

H2O

H2O

Turgor pressure increases and guard cells expand, opening the pore

At night K+ pumped out of cells

K+

K+

K+

K+

Turgor pressure decreases and guard cells shrink, closing the pore H2O flows out of cells by osmosis

H2OH2O

Organic nutrients are translocated through the phloem

• Translocation is the transport of organic nutrients in a plant

Movement from Sugar Sources to Sugar Sinks

• Phloem sap is an aqueous solution that is mostly 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

Translocation

• Sugar must be loaded into sieve-tube members before being exposed to sinks

• In many plant species, sugar moves by symplastic and apoplastic pathways

Vessel(xylem)

Sieve tube(phloem)

Sucrose

Source cell(leaf)

H2O

H2O

Sucrose

Sink cell(storageroot)

H2O

Pres

sure

flow

Tra n

s pir a

ti on

stre

am

Transportation of Food: Pressure-flow Hypothesis

•Sugars are made in photosynthetic cells and pumped by active transport into sieve tubes•Concentration of dissolved substances increases in the sieve tube and water flows in by osmosis•Pressure builds up at the source end of the sieve tube

Water flows in

At the source end of the sieve tube:

Transportation of Food: Pressure-flow Hypothesis

•Sugars are pumped out

•Water leaves the sieve tube by osmosis

•Pressure drops at the sink end of the sieve tube

Water flows in

At the sink end of the sieve tube:

Water flows out

•Difference in pressure causes sugars to move from source to sink