Chapter 36—Transport in

Plants

I. Plant Transport MechanismsTransport occurs at the cellular, tissue, and whole plant levels

H2O & minerals

� transport in xylem� transpiration

� evaporation, adhesion &

cohesion � negative pressure (pull)

Sugars

� transport in phloem� bulk flow

� Calvin cycle in leaves loads sucrose into phloem

� positive pressure (push)

Gas exchange

� photosynthesis� CO2 in; O2 out� stomata

� respiration

� O2 in; CO2 out � roots exchange gases

within air spaces in soil

Why does over-watering kill a plant?

Cellular Transport

� solutes are moved into plant cells via active transport� central role of proton pumps

� Chemiosmosis—using ATP → transmembrane proton gradient → energy to do work (i.e. uptake of ions)

(Uses ATP to pump H+ out of cell = stored energy)

Figure 36.2, pg. 750

Movement of water in plantsWater uptake/loss in plant cells is based

on water potential(potential energy that causes water to move)

� osmosis through aquaporins

(passive transport proteins)� water flows from high potential to

low potential (towards more negative)

Ψ= Ψs + Ψp

s = solute potential

(adding solute lowers Ψ)

p = pressure potential

(increasing pressure raises Ψ)

Now, a plant example:

Figure 36.4, pg. 752

Where

does th

e

pre

ssure

com

e fro

m?

Short Distance Lateral

(cell to cell) Transport

3 plant compartments:

� cell wall

� cytoplasm/cytosol

� vacuole

Transmembrane route� repeated crossing of plasma membranes� slowest route but offers more control

Symplastic route� move from cell to cell within cytosol� use plasmodesmata

Apoplastic route

� move through connected cell wallwithout crossing cell membrane

� fastest route but never enter cell

Long Distance Transport

Bulk flow

• movement of fluid driven by pressure– flow in xylem tracheids & vessels

• negative pressure– transpiration creates negative pressure pulling xylem

sap upwards from roots

– flow in phloem sieve tubes• positive pressure

– loading of sugar from photosynthetic leaf cells generates high positive pressure pushing phloem sap through tube

II. Absorption of water and minerals

by rootsMineral uptake by root hairs

� dilute solution in soil� active transport pumps

� this concentrates solutes (~100x) in root

cells

Water uptake by root hairs� flow from high H2O

potential to low H2O potential

� creates root pressure

Route to xylem can be apoplastic or symplastic

Hooray

for root

hairs!

Endodermis � controls the route of water into the

root � cell layer surrounding vascular

cylinder of root

� lined with impervious Casparianstrip (waxy)

� forces fluid through selective cell membrane & into symplast

Mycorrhizae increase absorption

Symbiotic relationship between fungi & plant

� greatly increases surface area for absorption of water & minerals� increases transport to host plant

Ahh, symbiosis…

III. Ascent of Xylem “Sap”Transpiration pull generated by water leaving leaf (transpiration)

Rise of Water in a

Tree

Transpiration pull

� adhesion & cohesion� H bonding

(brings water & minerals up to shoot)

Water potential (mostly due to Ψp)

� high in soil → low in leaves

What is the source of energy for transpiration?

IV. Control of Transpiration by

Stomata

Photosynthesis-Transpiration Compromise—

� 600 g water transpired : 1 g CO2 assimilated into organic material

How Stomata Open and CloseIon mechanism

� uptake of K+ ions by guard cells�proton pumps (chemiosmosis)� water enters by osmosis (due to lower ψ)� guard cells become turgid

� loss of K+ ions by guard cells

� water leaves by osmosis (due to higher ψ)� guard cells become flaccid

How Stomata Open and CloseMicrofibril mechanism

� guard cells attached at tips� microfibrils in cell walls

� osmosis in → turgid cell → increase in length > increase in width→ stomate opens

� and vice versa

Cues for Stomatal Opening

• Light (blue light receptors)

• Depletion of CO2 (from photosynthesis)

• Circadian rhythms (24 hour cycles)

Cues for Stomatal Closing

• Water deficiency

• High temperatures

• Circadian rhythms (24 hour cycles)

V. Transport of Sugars in PhloemLoading of sucrose into phloem

� flow through symplast via plasmodesmata or apoplast� active cotransport of sucrose with H+ protons

� proton pumps (chemiosmosis)

Pressure Flow in aSieve Tube

Water potential gradient

� “source to sink” flow� direction of transport in phloem is variable

� sucrose flows into phloem sieve tube decreasing H2O potential

� osmosis of H2O from xylem vessels� increase in pressure due to

increase in H2O causes flow� higher pressure at source than

at sink

Figure 36.17, pg. 764