Water and Plant Cells/ Water Balance of a Plant
 

Terms - Diffusion, Osmosis, Facilitative Diffusion, Active Transport, Adhesion, Cohesion, Transpiration
                                               (learn the definitions for these terms)

Diffusion – Movement of molecules from a high concentration to a low concentration.  (passive)

Water moves from regions where water potential is relatively positive to regions where water potential is relatively more negative. (passive)

Osmosis – Movement of water from a high concentration to a low concentration through a selective permeable membrane. (passive)

Active Transport – Movement of molecules across membrane using transport systems and biological energy ATP. (active); transport system makes it selective. This movement may be towards or against osmotic gradient

Facilitative Diffusion – Movement of molecules across membranes using transport systems; however energy is not required. (passive); transport system makes it selective.

Adhesion - water molecules stick to other materials.

Cohesion – water molecules stick to other water molecules.

Transpiration – evaporation of water from stoma of leaves when guard cells are turgid.
                               potassium ions are pumped in actively; water moves in by osmosis
 

Review:

 vascular tissue - xylem moves water, dissolved minerals and hormones.
                                          phloem moves photosynthates

 uptake of water occurs at tips of roots - root hairs

  Apoplast and Symplast
 

                                              " See handouts from lecture"

  Cortex

   Endodermis with Casparian Strips  (selective absorption – forces ions to move into  
   symplast via active transport which is selective.
 

                                                " See handouts from lecture"

   Two types of tracheary elements in xylem: Tracheids and Vessel Elements

           Pits
                                                " See handouts from lecture"
 

   Root Pressure -  Osmotic pressure formed when water moves into vessel elements.

      Transpirational Pull - Adhesive and Cohesive Forces  (passive force)

      Two factors - difference in water vapor concentration and diffusional resistance

      Roll of Stomatal Control
 
                                               " See handouts from lecture"

      Transpiration rates  =  g/dm2/day

                                        Pine                4.6
                                        Oak               12
                                        Corn              20
                                        CAM plants  1
 
 
 
 

Cavitation – the breaking of the water column in a tracheid chain.
 

Review the cause of:
Early wood vessels
Late wood vessels
 
 
 
 
 
 

Translocation  (movement of photosynthates) in Phloem Tissue  (sieve tube elements and sieve cells)
 

                                                             Table 10.1  in Text
 

                                                  " See handouts from lecture"
 
 
 

Companion cells aid the sieve elements – numerous plasmodesmata

 Phloem Sap is collected and analyzed

                                                " See handouts from lecture"

                                                             Table 10.2 in Text
 

Sugars are translocated in nonreducing form – sucrose

Bulk Flow (Pressure Flow Hypothesis )
 

                                                 " See handouts from lecture"
 
 

 
Source to  Sink  (involves active and passive transport mechanisms)

                                                " See handouts from lecture"

                                            Study figures in text
 
Rates of movement exceeds rate of diffusion -  1 m/hr-1

Bidirectional movement

Phloem Loading -  from Chloroplasts to Sieve Elements
 

Movement via apoplast or symplast or both?
 
 
 

Sieve Element Loading use a Sucrose – Symport System

                                                      " See handouts from lecture"
 

Phloem Loading – Specific and Selective

                                 Symplastic or Apoplastic
 
 

Phloem unloading – symplastic or apoplastic
 

                                                       " See handouts from lecture"