Biological Foundations 112, Lecture 10


Water Movement in the Plant

Chapter 32, pages 692-699


I.   Water Movement in Plants

     A.   Terminology dealing with the movement of water

            1.   Diffusion

                  a.   The even dispersion of one substance into another

                  b.   Occurs when water passes through a semipermeable membrane

                  c.   Occurs when a solute dissolves in a solvent

            2.   Forces between molecules

                  a.   Cohesion

                        (1)  Clumping of like molecules together when  the molecules are not
                               compressed

                        (2)  These molecules clump because they are polar (contains positive and
                               negative regions)

                        (3)  Opposite charges attract and like charges repel

                        (4)  Hydrogen bonds - a positive region of one molecule is attracted to the
                                                             negative charge of another like molecule

                        (5)  Examples:

                               (a)  Water is polar and evaporates more slowly

                               (b)  Gasoline is nonpolar and evaporates easily

                  b.   Adhesion

                        (1)  Clumping of unlike molecules

                        (2)  Only occurs between polar molecules

                        (3)  Water and cellulose strongly attracted to each other

                        (4)  Reduces water's mobility and thus its Water Potential

            3.   Osmosis - diffusion of water across a differentially permeable membrane

            4.   Turgor Pressure - the hydrostatic pressure within a cell due to osmosis

            5.   Plasmolysis - a greatly reduced turgor pressure to the point that the protoplast pulls
                                         away from the cell wall

            6.   Hypoosmotic - a solution that has a lower solute concentration than the cells that                                                      the solution surrounds

            7.   Hyperosmotic - a solution that has a higher solute concentration than the cells that
                                             the solution surrounds

            8.   Halophyte - plants that can tolerate high salt concentrations

            9.   Xerophyte - plants that can tolerate dry conditions

          10.   Mesophyte - plants that must have moderate moisture conditions

     B.   Passive and Active Transport

            1.   Passive Transport

                  a.   Definition - solutes that diffuse down their gradient

                  b.   No direct expenditure of metabolic energy by the cell occurs

                  c.   Very slow process

                  d.   Process speeded up by:

                        (1)  Transport Proteins

                               (a)  Carrier proteins bind selectively to a solute

                               (b)  Proteins carry the solute molecule across the membrane and release it
                                     on the other side

                               (c)  Requires a shape change by the carrier protein for this to happen

                        (2)  Selective Channels

                               (a)  A passageway or canal through the membrane

                               (b)  Each channel is designed for only a particular ion and are thus selective 
                                      for that ion only

                               (c)  Potassium channels allow only potassium to pass through

                               (d)  Some channels are gated, using environmental stimuli to open the gate

            2.   Active Transport

                  a.   Definition - pumping solutes across a membrane against the solute's gradient

                  b.   Proton Pump - example in plant cells

                        (1)  Hydrolyzes ATP, using energy released to pump hydrogen ions (H+) out of
                              the cell

                        (2)  Creates a higher concentration of H+ ions on the outside of the cell and thus
                               a membrane potential which is a form of stored energy

                        (3)  Plants use this energy is used to drive the transport of many solutes

                        (4)  Example - Potassium Transport

                               (a)  Potassium ions are positively charged

                               (b)  Inside cell is negatively charged

                               (c)  Potassium is diffusing down its electrochemical gradient

                        (5)  Example - Nitrate (NO3-) Transport

                               (a)  Nitrate is negatively charged

                               (b)  Carriers allow nitrate and H+ to enter together - called Cotransport

                               (c)  Nitrate is diffusing against its electrochemical gradient

                               (d)  Sucrose is transported in the same way

     C.   Water Potential

            1.   Definition:  The flow of water from a wet place to a dry place and the physical
                                      force that drives that flow

                  a.   This flow is the result of passive transport

                  b.   Wetness and dryness are relative

                  c.   Pure water is perfectly wet

                  d.   A completely dry substance is perfectly dry

            2.   Pure water

                  a.   Pure water has the highest water potential which is 0 bars at 25oC and one
                        atmosphere of pressure

                  b.   1 bar is approximately equal to one atmosphere of pressure

                  c.   1 megapascal (MPa) = 10 bars

            3.   A completely dry object has the lowest water potential = negative infinite bars

                  a.   Water with anything dissolved in it has a negative water potential

                  b.   If two sides of a semipermeable membrane have different water potentials, water
                        will flow from the side of higher water potential through the membrane to the
                        side of lower water potential

                  c.   WATER FLOWS FROM WET PLACES TO DRY PLACES

                        (1)  Water flows from places of high water potential to places of lower water
                               potential

                        (2)  Water flows from hypoosmotic regions to hyperosmotic regions

            4.   Factors affecting water potentials

                  a.   Temperature - directly proportional

                  b.   Solutes

                        (1)  When solutes are dissolved in water, it causes the water to be drier and thus
                               the water potential to be lower

                        (2)  The contribution of solutes to the over all water potential is called osmotic  
                               potential

                        (3)  The osmotic potential is always negative when solutes are present

                  c.   Pressure

                        (1)  Pressure increases the water potential of the cell as the pressure in the cell                                          increases

                        (2)  The contribution of pressure in the cell to the over all water potential of the
                               cell is called the pressure potential

                        (3)  The pressure potential is either zero or positive

                  d.   Matrix

                        (1)  A matrix is any particle that is too large to go into solution

                        (2)  Examples of a matrix:  solid particles, sand grains, soil particles, cell wall
                              microfibrils (cellulose), and starch

                        (3)  Decreases water potential

                        (4)  The contribution of the matrix to the in the cell to the overall water potential
                               is called the matrix potential

                        (4)  The matrix potential is either zero or negative

            5.   Water Potential = Osmotic Potential + Pressure Potential + Matrix Potential

            6.   How solutions behave under the following conditions:

                  a.   Pressure

                        (1)  The effect of increased pressure within a cell is to decrease the water
                               potential of the cell.

                        (2)  Thus, increased turgor pressure effectively makes a concentrated solution
                               behave like a dilute solution

                  b.   Adhesion - adhesion hold water to the cell walls and cohesion holds the mass of
                                           water together.  Thus, the adhesion forces exerted by wall  
                                           polymers tend to make a dilute solution behave osmotically as
                                           if it were more concentrated in solutes because the water is
                                           less mobile when it is stuck to the wall polymer.

II.  Movement of Substances in Plants

      A.   Four types of movement

             1.   Slow diffusion of molecules and ions across membranes

             2.   Moderate movement of materials carried by cytoplasmic streaming

             3.   More rapid flow of material in sieve tubes

             4.   Very rapid conduction of water and mineral solutes in xylem

      B.   Diffusion and protoplasmic streaming

             1.   Diffusion is very slow and is the slowest movement of all types

             2.   Cytoplasmic streaming

                   a.   Chief method of movement of materials within cells

                   b.   Can be a few mm/hour to several hundred mm/hour

                   c.   The highest rate of cytoplasmic streaming is in slime molds - 486 mm/hr

             3.   Phloem conduction at its maximum is 100 cm/hr

             4.   Xylem conduction is 15 m/hr or faster


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