Plant  transport:  Xylem  and  Phloem    Chapter  4  pp.  47  -­‐54  

Lecture  Outline:  Evolu:on  of  vascular  :ssue  The  xylem  transports  water  and  nutrients  The  phloem  transports  sugars  and  other  organic  compounds  Xylem  and  phloem  structure  and  func:on  How  do  roots  take  up  water  and  nutrients  from  the  soil?  Cohesion-­‐tension  mechanism  and  transpira:on  relate  to  the  physical  proper:es  of  water  The  func:on  of  stomata  in  transpira:on  Sugar  transport  in  the  phloem  The  pressure-­‐flow  hypothesis  describes  transport  of  sugar  in  plants    

Bloodroot    Sanguinaria  canadensis  

Plant  classifica:on  and  vascular  :ssue  

nonvascular  plants  

seedless  vascular  plants  

gymnosperms  (cone-­‐bearing  seed  plants)  

angiosperms  (flowering    seed  plants)  

H2O and minerals

O2

CO2

CO2 O2

H2O

Light

Sugar

Tracheids

100 µm

Xylem  

Vessel element

Pits

Vessel Tracheids

Tracheid

Long-­‐distance  transport  in  the  phloem  and  xylem  

Phloem    

Sieve-tube element (left) and companion cell: cross section

Sieve-tube elements: longitudinal view Sieve plate with pores

Transpira/on  is  the  evapora:ve  loss  of  water  

from  the  leaves  of  a  plant.  

  H2O and minerals

H2O

But  first,  water  and  minerals  must  travel  from  the  soil  through  the  dermal  and  ground  :ssue  into  the  vascular  cylinder  (xylem/phloem)  of  a  root…  

eudicot  root  cross  sec:on  

100 µm

Epidermis

Cortex

Endodermis

Vascular cylinder

Pericycle

Xylem

Phloem

Dermal Ground Vascular

Key to labels

Cross  sec:on  of  a  eudicot  root  

Pathway along apoplast  

Casparian strip  

Endodermal cell  

Pathway through symplast  

Casparian strip  

Plasma membrane  

Apoplastic route  

Symplastic route  

Root hair  

Epidermis  

Cortex  

Endodermis  

Xylem  

Vascular cylinder  

Transport  of  water  and  mineral  nutrients  from  root  hairs  to  the  xylem  

Transpira:on  is  the  movement  of  water  against  gravity,  from  the  soil  to  the  leaves,  without  using  any  energy…  HOW  do  plants  do  this?  

Transpira:on  depends  on:  1.  the  evapora:on  of  H2O  

from  the  leaves  pulls  water  upwards  from  the  roots  (tension)  

2.  the  physical  proper:es  of  water  (cohesion)   hTp://photo.accuweather.com  

Xylem sap

Mesophyll cells Stoma Stoma

Water molecule

Transpiration Atmosphere

Adhesion by hydrogen bonding

Cell wall

Xylem cells

Cohesion and adhesion in the xylem

Cohesion by hydrogen bonding

Water molecule

Root hair Soil particle

Water Water uptake from soil

The  ascent  of  xylem  sap  (water  and  minerals)  

Water  has  cohesive  behavior  

– H

H

O —

—

+ +

+

–

–

H2O  =  water  

The  ascent  of  xylem  sap  depends  

on  hydrogen  bonds  (-­‐-­‐-­‐)  

between  water  molecules  

Xylem sap

Stomata function in transpiration

Water molecule

Transpiration Atmosphere

Adhesion by hydrogen bonding Cell

wall Xylem cells

Cohesion and adhesion in the xylem

Cohesion by hydrogen bonding

Water molecule

Root hair Soil particle

Water Water uptake from soil

The  ascent  of  xylem  sap  (water  and  minerals)  

K+

H2O

H2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Guard cells turgid

Guard cells flaccid

Osmosis  of  water  causes  stomata  (leaf  pores)  to  open  and  close  

Water  flows  into  the  cells,  stoma  opens  

Water  flows  out  of  the  cells,  stoma  closes  

Xylem sap

Water molecule

Transpiration Atmosphere

Adhesion by hydrogen bonding Cell

wall Xylem cells

Cohesion and adhesion in the xylem

Cohesion by hydrogen bonding

Water molecule

Root hair Soil particle

Water Water uptake from soil

The  cohesion-­‐tension  theory  

explains  the  ascent  of  xylem  sap  (water  and  minerals)  

The  Pressure-­‐flow  hypothesis  describes  

how  sugars  are  transported  from  sources  to  sinks  via  

the  phloem    (this  does  require  

energy!)   H2O and minerals

O2

CO2

CO2 O2

H2O

Light

Sugar

Source  to  sink  

Daucus  carota  roots  are  sources  in  the  springHme,  and  shoot  is  the  sink  

Daucus  carota  leaves  are  sources  in  the  summer  Hme,  and  flowers/fruits  and  developing  organs  are  sinks    

Starch  -­‐  how  plants  store  sugar  

Chloroplast

Starch

starch  molecules  are  simply  glucose  molecules  linked  together  (similar  to  cellulose)  

Sieve-tube element

Sieve plate

Nucleus of companion cells

Sieve-tube elements: longitudinal view Sieve plate with pores (SEM)

10 µm

Structure  of  the  phloem  

4  

Fig.  36-­‐20  

3  

2  

1  

1  

2  

3  

Vessel  (xylem)  

Sieve  tube  (phloem)  

Source  cell  (leaf)  

Load  sugar  into  the  phloem  at  the  source  (requires  ENERGY!)  

Water  from  the  xylem  flows  into  the  phloem  (OSMOSIS)  ,  crea:ng  pressure  

 Unload  sugar  at  the  sink  (requires  ENERGY!)  Water  from  the  phloem  flows  back  into  the  xylem  (OSMOSIS)  –  water  is  recycled,  crea:ng  a  constant  circuit!  

Sink  cell  (storage  root)  

Sugar  

H2O  

H2O  

 Bulk  flo

w  

H2O  

Sugar  

                       Bu

lk  flow

 

Pressure-­‐flow  hypothesis    

Lecture  Review,  Chap  4  •  Do  any  plants  lack  vascular  :ssue?    If  so,  give  an  example  of  a  

kind  of  plant  that  lacks  this  par:cular  :ssue  type.    •  Relate  structure  to  func:on  in  sieve-­‐tube  cells,  vessel  elements,  

and  tracheid  cells.  •  How  are  water  and  mineral  nutrients  from  the  soil  transferred  

into  the  vascular  cylinder  of  a  root?  •  Define  transpira:on.      •  Describe  the  cohesion-­‐tension  mechanism  and  relate  it  to  the  

func:on  of  the  stomata  and  the  proper:es  of  water.  •  Why  do  water  molecules  s:ck  together?    How  does  this  relate  to  

why  plants  can  move  water  against  the  force  of  gravity  without  using  any  energy?  

•  Trace  the  path  of  sugar  in  the  phloem  from  source  to  sink.    What  is  the  name  of  the  hypothesis  to  describe  this  flow?