2006-2007

Resource Acquisition & Transport in

PlantsChapter 36

Adaptations for Acquiring Resources Land plants inhabit 2 worlds:• Above ground – shoots acquire

sunlight and CO2

• Below ground – roots acquire water and minerals

Early land plants absorbed water, minerals and CO2 directly from or near a water source

As land plants increased in number, there was competition for resources

Plants developed specialized tissues to allow them to be successful in areas further from water

Shoot Architecture & Light Capture

Leaf size and structure accounts for much of the outward diversity in plant form

Largest leaves are found in tropical species, smallest in dry or cold environments

Photosynthesis is affected by leaf arrangement, orientation and height

Root Architecture and Acquisition of H2O and Minerals

The evolution of root branching enabled land plants to more effectively acquire water and nutrients from different types of soil while providing strong anchorage

The evolution of mutualistic associations between roots and fungi called mycorrhizae allowed plant to live in poorly developed soils

Pathways of Transport in Plants Apoplast - everything external to the plasma

membranes of living cells

• Cell walls

• Extracellular spaces

• Interior of vessel elements and tracheids

Symplast – entire mass of cytosol of the living cells in a plant

• Includes plasmodesmata and cytoplasmic channels that interconnect them

Transport in Plants

H2O & minerals• transport in xylem

• transpirationo evaporation, adhesion & cohesion

o negative pressure

Sugars• transport in phloem

• bulk flowo Calvin cycle in leaves loads sucrose into phloem

o positive pressure

Gas exchange• photosynthesis

o CO2 in; O2 out

o stomates

• respiration

o O2 in; CO2 out

o roots exchange gases within air spaces in soil

Ascent of Xylem Fluid

Transpiration pull generated by leaf

Water & mineral absorption

Water absorption from soil

o osmosis

o aquaporins Mineral absorption

• active transport

• proton pumps

o active transport of H+

H2O

root hair

aquaporin

proton pumps

Mineral Absorption

Proton pumps

• active transport of H+ ions out of cell

o chemiosmosis

o H+ gradient

• creates membranepotential

o difference in charge

o drives cation uptake

• creates gradient

o cotransport of othersolutes against theirgradient

Water Flow Through Roots

Porous cell wall

• water can flow through cell wall route & not enter cells

• plant needs to force water into cells

Casparian strip

Controlling the Route of Water in Root

Endodermis

• cell layer surrounding vascular cylinder of root

• lined with impermeable Casparian strip

• forces fluid through selective cell membrane ofiltered & forced into xylem cells

Root Anatomy

dicot monocot

Mycorrhizae Increase Absorption

Symbiotic relationship between fungi & plant• symbiotic fungi greatly increases surface area for

absorption of water & minerals

• increases volume of soil reached by plant

• increases transport to host plant

Mycorrhizae

Transport of Sugars in Phloem

Loading of sucrose into phloem• flow through cells via plasmodesmata

• proton pumpso cotransport of sucrose into cells down

proton gradient

can flow 1m/hr

Pressure Flow in Phloem

Mass flow hypothesis• “source to sink” flow

o direction of transport in phloem is dependent on plant’s needs

phloem loading• active transport of sucrose

into phloem

• increased sucrose concentration decreases H2O potential

water flows in from xylem cells• increase in pressure due to

increase in H2O causes flow

Chloroplasts

Epidermal cell

NucleusGuard cell

Thickened innercell wall (rigid)

Stoma open Stoma closed

H2O

water moves into guard cells

H2O H2O H2O

H2O H2O

H2O

H2O

H2O H2O H2O H2O

Control of Stomates

Uptake of K+ ions by guard cells• proton pumps

• water enters by osmosis

• guard cells become turgid

Loss of K+ ions by guard cells• water leaves

by osmosis

• guard cells become flaccid

K+

K+

K+

K+

K+ K+

K+ K+

K+ K+K+K+

water moves out of guard cells

Control of Transpiration Balancing stomate function

• always a compromise between photosynthesis & transpiration

o leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis