Transportation

       Transport is a very important action for living beings. So today we’ll study about Transportation..

          •   Water and minerals move upward through the xylem

          •   Dissolved sugars and hormones are transported in the phloem

          •    Methods of water and solute movement

          •    Basic principles underlying movement of water across membranes, phenomena of diffusion,

               imbibition, osmosis, mass flow and concept of water potential.

          •    Diffusion ; evaporation as an example

           •    Osmosis as a special kind of diffusion taking place when two solution of different concentrations       are  separated by a selectively permeable membrane which allows passage of water molecules but not of solute molecules.

               

           

                  

                   

               •    Imbibition as adsorption of water molecules to hydrophilic substances such as proteins, agar             etc,

               •    Mass flow as a method of movement of water and dissolved solutes in bulk (not in the form of

                   molecules) due to pressure gradient or gravity .

 Concept of water potential:

                   •    Any system containing water has a water potential. Water potential is related to kinetic energy of

                      water molecules, affected by factors like pressure, dissolved substances, hydrophilic substances,

                      etc.

                   •    Water potential is denoted by  Ψ  and is measured by the units of pressure (atm, Pa, M Pa).

                                               

                   •    Pure water at atmospheric pressure has the highest water potential.

                   •    Arbitrarily, the water potential of pure water at atmospheric pressure is considered as zero.

                   •    When solutes dissolve in water, the water potential is decreased and it becomes negative.

                   •    Therefore, the water potential of most naturally occurring aqueous systems is negative.

                   •    As the solute concentration increases, the water potential decreases.

                   •    Therefore, the water potential is inversely proportional to solute concentration of the system.

                   •    The component of water potential decreased due to the solute concentration is called solute

                      potential and is denoted by  Ψ s.

                                          

   

                   •    When pressure is increased the kinetic energy of water molecules are also increased, thereby

                      increasing the water potential of the system.

                   •    Therefore, water potential is directly proportional to pressure of the system.

                   •    The component of water potential increased due to pressure is called pressure potential and is

                               

                      denoted by  Ψ p.

                   •    Always water moves from a system of high water potential to a system of low water  potential.

                 •   Water potential of a cell

                   •    Cell is a system that contains water. Therefore, it has a water potential.

                   •    The protoplast (all the components – cell membrane, protoplasm and the vacuole) is an aqueous

                      system that contains solutes and hydrophilic substances. Therefore, it has a solute potential which

                                            

                      is negative and denoted as  Ψ s.

                   •    If the cell is in a turgid condition protoplast exert a pressure on cell wall which is called turgor

                      pressure (Tp), whereas, the cell wall exert an equal and opposite pressure on the protoplast which

                      is called wall pressure (Wp). Therefore, the cell contents are under a high pressure condition

                      which increases the water potential. This component of water potential is called the pressure potential and denoted as  Ψ p.

                                           

                                         •    Therefore, the water potential of the cell  Ψ w is given as in the following equation.

                                                        

                   •    Ψ w =  Ψ s +  Ψ p

                               

                          

                 •   Entry of water into vacuolated cells, turgor and plasmolysis

                   •    Discuss how solute potential, pressure potential and water potential changes when a

                      cell is   placed in water and in hypertonic solutions.

                  

                   •    Water moves in and out of cells according to water potential gradients. This happens when

                      tissues are immersed in solutions.

                   

                   •    Turgor or turgidity of a cell is the rigidity it gains by absorption of water and

                      developing a  positive pressure potential. Pressure in the cell vacuoles presses

                      cytoplasm onto the cell wall. Cell wall is elastic to a certain degree and the cell inflates.

                   •    Plasmolysis is the contraction of the cytoplasm away from cell wall due to loss of water.

                   •    Demonstrate plasmolysis using  Rhoeo  epidermal tissue  under microscope.

                   •    Incipient plasmolysis is a state when the cell is neither turgid nor flacid. It is the instant at

                      which the cell begins plasmolysis.

                   

 Carryout experiments to determine solute potentials of epidermal peels of  Rhoeo  epidermis

                      (plasmolysis method)

                   •    Carryout experiments to determine water potential of  Colocasia   petioles

                   •    Use tables to calculate solute potentials of sucrose solutions of different

                      molarities.

               •    Structure of plant root

                   Describe the structure of primary roots of monocotyledonous and dicotyledonous

                   plants .

               y      Transportation of water in plant roots.

                   Roots absorb water when the water potential of root cells is lower than that of  soil

                   solution. Cellulose cell walls and intercellular spaces in roots are totally permeable to water  and

                   solutes. Suberinized and lignified cell walls are not permeable to water. Water and solutes pass

                   through the cortex into xylem in different paths given below.Membrane of endodermal cells and

                   cortical cells act as a selectively permeable membrane.

               •    Transportation of water within a plant .