TOPIC  6:  PLANT  STRUCTURES  

Learning  outcomes  

•  Iden=fy  plant  structure  •  Describe  the  func=ons  for  each  plant  structure  •  Explain  the  transporta=on  of  water  and  nutrients  in  xylem  •  Explain  the  transporta=on  of  nutrients  in  phloem  

Plant  Organs  

•  Flowering  plants  have  two  major  components  to  their  structure.  

1.  A  root  system;  extends  below  ground  

2.  A  shoot  system;  composed  of  the  stem,  leaves,  and  reproduc=ve  organs.  

•  At  the  end  of  the  root  and  shoot  system  is  a  terminal  bud  from  which  ver=cal  growth,  called  primary  growth,  occurs.  

Content  

1.  Leaf:  Structure,  func=on  and  adapta=on  2.  Stems:  Structure,  func=on  and  adapta=on  

3.  Roots:  Structure,  func=on  and  adapta=on  4.  Transporta=on  

1.  Xylem  2.  phloem  

     1.  Leaf  

�  plants  make  carbohydrates,  occurs  in  the  leaves  via  photosynthesis.      

�  To  conduct  photosynthesis,  leaves  need  solar  energy,  water,  and  carbon  dioxide.  

�  Photosynthe=c  leaves  share  similar  structural  components.  �  The  blade,  the  wide  part  of  the  

leaf  �  The  pe=ole,  the  stalk  

connec=ng  leaf  to  stem.  

The  blade:  

� the  wide  part  of  the  leaf  � Have  maximum  surface  area  for  the  collec=on  of  energy  &  absorb  CO2  

The  pe5ole:  

�   the  stalk  connec=ng  leaf  to  stem.  

�   receive  H2O  from  the  root  by  way  of  vascular  =ssue  that  terminates  in  the  leaves  

•  There  is  tremendous  diversity  in  leaf  structure  between  plant  species.      

•  In  some  plant  species,  leaves  may  serve  addi=onal  func=ons,  such  as  storage.  

•  Some  plants  are  deciduous  (they  drop  their  leaves  during  certain  seasons.)  

Leaves  adapta5ons  

•  Modified  as  tendrils-­‐  that  allow  the  plant  to  aZach  to  objects.  

•  The  leaves  of  cactus  are  spines  that  reduce  H2O  loss  &  protect  the  plant  from    browsing  animals.  

•  The  leaves  as  traps  for  catching  insects.  

Organiza5on  of  Leaf    

•  The  top  and  boZom  of  a  typical  eudicot  leaf  is  composed  of  epidermis  

–  The  epidermis  o[en  has  hairs  or  glands.  

–  Stomata  are  located  on  the  lower  epidermis.  

•  The  interior  of  the  leaf  is  composed  of  photosynthe=c  mesophyll  cells.  

–  The  spongy  mesophyll  is  arranged  randomly  to  increase  surface  area  for  gas  exchange.  

–  The  palisade  mesophyll  is  comprised  of  elongated,  ver=cally-­‐oriented  cells.  Contain  the  most  chloroplast.  

2.  Stems  

•  The  stem  is  the  main  axis  of  the  plant.  

•  Stems  can  produce  side  (lateral)  branches  from  lateral  (axillary)  buds.  

•    Nodes  are  the  points  where  leaves  aZach  to  stems.  

•  An  internode  is  the  region  between  nodes.  

•  The  stem  also  contains  the  vascular  5ssue  that  transports  water  and  nutrients  to  leaves  (  to  support  photosynthesis).  

•  In  some  plant  species,  stems  may  also  carry  out  photosynthesis  or  serve  as  a  storage  organ  (e.g.  tuber)  

Sweet potato- tuber plant

Monocotyledon  Versus  Dicotyledon  Plants    

Vascular  5ssue:  

�  The  arrangement  of  the  vascular  =ssue  differs  between  monocots  and  eudicots.  

�  Plants  have  two  types  of  vascular  =ssue.  1.  The  xylem  transports  water  and  minerals.  

2.  The  phloem  transports  organic  nutrients.  

�  The  vascular  =ssues  serve  as  a  type  of  circulatory  system  for  plants.  

Monocot  vs.  Dicot    Vascular  Tissue  Arrangement    

Scattered Forming a ring

   3.  Roots  •  Roots  system  supports  

the  plant  by  anchor  plants  to  the  soil.  

•  Roots  also  absorb  water  and  nutrients  from  the  soil.  

•  The  surface  area  of  roots  is  greatly  increased  by  the  produc=on  of  root  hairs.      

Root  Hair  

•  There  are  different  types  of  root  systems.  

1.  Some  plants  have  a  single  taproot.  2.  Grasses  have  fibrous  root  systems.  

3.  Some  plants  have  prop  roots;  a  type  of  adven55ous  root  for  support.  

Root  system  

•  For  perennial  plants,  the  roots  act  as  a  storage  order  that  allows  the  shoot  system  to  regrow  each  year.      

grow and bloom over the spring and summer, die back every autumn and winter, and then return in the spring from their root-stock

TRANSPORTATION  OF  WATER  &  MINERALS  :  XYLEM  

Xylem:  Tracheids  and  Vessel  Members  

•  Water  and  dissolved  mineral  ions  flow  through  conduc=ng  tubes  of  xylem    

•  Interconnected,  perforated  walls  of  tracheids  and  vessel  members  (dead  cells)  form  the  tubes  

1.  The  water  and  nutrients  taken  up  by  roots  and  root  hairs  through  soil  water.  

2.  H2O  enter  the  root  by  osmosis  

3.  H2O  +  nutrients  are  transported  to  leaves  via  the  interconnected  vessel  elements  of  the  xylem.  

4.  H2O  +  nutrients  transported  to  leaves  

1.  Root  Pressure  

•  The  water  and  nutrients  taken  up  by  roots  and  root  hairs  through  soil  water.  

•  This  movement  is  provided  in  part  by  root  pressure,  a  posi5ve  pressure  created  when  water  enters  the  root  by  osmosis.      

•  Soil  hypotonic  à  root  hypertonic   Root pressure

2.  Transpira5on  

•  Transpira=on  –  Evapora=on  of  water  from  plant  parts  (mainly  though  stomata)  into  air    

–  pulls  water  upward  through  xylem  by  causing  con=nuous  nega=ve  pressure  (tension)  from  leaves  to  roots  

•  root  hypotonic  !  leaves  hypertonic  

3.  Cohesion-­‐Tension  Theory  

•  The  cohesion-­‐tension  model  explains  how  water  travels  up  the  xylem  to  leaves.  

•  leaves  have  numerous  openings  called  stomata.  

•  When  these  stomata  are  open,  water  evaporates  from  the  interior  of  the  leaf  to  the  outside  air,  a  process  called  transpira=on.  

Cohesion-­‐Tension  Theory  1.  As  plant  leaves  transpire  water,  a  

tension  is  created  that  pulls  water  from  roots  to  leaves.  

2.  This  tension  is  maintained  because  water  molecules  display  an  aZrac=on  to  one  another  called  cohesion.  

3.  Hydrogen  bonds  among  water  molecules  resist  rupturing  (cohesion)  so  water  is  pulled  upward  as  a  con=nuous  fluid  column  

4.  Water  also  adheres  to  the  xylem  elements  in  a  process  called  adhesion.    

5.  Hydrogen  bonds  break  and  water  molecules  diffuse  into  the  air  during  transpira=on  

Cohesion & adhesion

Transpiration

Root pressure created when water enter root cells via osmosis

Water rises through xylem vessels because :

1. Cohesion: Water molecules are attracted to each other 2. Adhesion: Water molecules form hydrogen bonds with the xylem cell wall

Transpiration Evaporation of water from stomata creating a tension that pulls the water column from root to leaf

** Because of cohesion, new water molecules is drawn from the xylem which is replaced by water from the roots

Opening  and  Closing  of  Stomata  

�  The  opening  and  closing  of  the  leaf  stomata  is  controlled  by  turgor  pressure  within  the  guard  cells.  

�  As  water  enters  the  guard  cells,  these  cells  swell,  opening  the  stomata.  

�  As  water  exits  the  guard  cells,  the  loss  of  turgor  causes  the  stomata  to  close.      

Water  Conserva5on  

•  Cu5cle    – Waxy  covering  that  protects  all  plant  parts  exposed  to  surroundings  

– Helps  the  plant  conserve  water  

Water  Conserva5on  •  Environmental  signals  cause  stomata  

to  open  and  close    

•  Closed  stomata  limit  water  loss  (but  prevent  gas  exchange  for  photosynthesis  and  aerobic  respira=on)  

•  Some  plant  like  CAM  plant  adapt  for  water  conserva=on  and  photosynthesis  by  opening  their  stomata  at  night  to  allow  CO2  for  photosynthesis  and  close  stomata  during  the  day  to  prevent  dehydra=on.  

The pineapple is an example of a CAM plant.

CAM= Crassulacean acid metabolism

TRANSPORTATION  OF  ORGANIC  NUTRIENT:  PHLOEM  

Phloem:  Sieve-­‐Tube  Members  

perforated end plate of sieve-tube Cell (sieve plate)

one of a series of living cells that abut, end to end, and form a sieve tube

companion cell (in the background, pressed tightly against sieve tube)

Phloem:  Sieve-­‐Tube  Members  

Organic  Nutrients  in  the  Phloem  

•  As  mature  leaves  photosynthesize,  phloem  load  sucrose.    

•  Phloem  is  considered  source  of  sugar  (Source  cell).  

•  The  phloem  transported  sugar  to  =ssues  that  require  sugars,  called  sink  =ssues  (Sink  cell).  

•  Sugar  are  unloaded  at  sink  region  (ac=vely  growing  or  storage  parts  of  the  plant  )  

Pressure  flow  theory  

•  Phloem    

–  Translocate  photosynthe=c  products  down  the  gradient  of  pressure  and  solute  concentra5on    

•  Transloca5on  Process  –  Distributes  sucrose  and  other  

organic  compounds  throughout  the  plant  

–  An  energy-­‐requiring  process  –  Can  be  elaborate  by  

Pressure-­‐flow  theory    

Translocation

1.  Source  cells:  produce  glucose/  organic  molecules    (by  photosynthesis)    

2.  Glucose  is  converted  to  sucrose  for  transport  

3.  Companion  cell  loads  sucrose  4.  Water  follows  from  xylem  by  

osmosis  5.  Sap  volume  and  pressure  

increased    6.  Sap  flow  within  phloem  7.  Unload  the  organic  molecules  by  

the  companion  cell  8.  Sucrose  is  stored  in  sink  cell/

=ssues  (as  starch)  9.  Water  diffuses  into  xylem  10. Water  recycles  as  part  of  

transpira=on  to  re  supply  the  sucrose  loading    

(1)

(2) (3)

(4)

(5)

(7) (8) (9)

(10)

Pressure  flow  theory  

(6)

glucose

https://www.youtube.com/watch?v=MxwI63rQubU

https://www.youtube.com/watch?v=60SgZgK3Gss