Biological Foundations 112, Lecture 12

Stomatal Opening and Closing

Chapter 32, pages 701-705

I.   Stomatal Opening/Closing

     A.   Structure of the guard cells

            1.   Crescent-shaped or semicircular in form

            2.   Walls adjacent to pore are thicker and less elastic

     B.   Physics of opening and closing

            1.   Depends upon the relative turgor pressure of the guard cell

            2.   Turgor controlled by adding and removing solutes

            3.   Water moves in from adjacent cells to causes a buildup of pressure in the guard cell

            4.   Guard cell changes shape (outer wall of guard cell expands)

            5.   Results in the bowing of the thickened cell wall adjacent to the pore

            6.   When this occurs to both guard cells, it produces an elliptical aperture in the stoma

     C.   Physiology of opening and closing (adding and removing solutes)

            1.   Potassium ions, protons and Cl-

                  a.   Sun shining on the guard cell causes these ions to be taken up by the guard cell
                        from the surrounding cells

                  b.   These ions are released to the adjacent cells when the sun goes down

                  c.   Active transport is what causes these ions to be taken up by the guard cells

            2.   Starch

                  a.   Starch is a polymer of glucose molecules

                  b.   An enzyme cuts up the starch molecule into separate glucose molecules

                  c.   Glucose molecules are hooked back up into a polymer by another enzyme when
                        the stoma closes

            3.   Theoretical physiological mechanism

                  a.   CO2 dissolved in water makes a weak acid called carbonic acid

                  b.   In sunlight, the cell sap becomes less acid (pH increase) because the cell is using
                        CO2 to make sugar

                  c.   In the presence of potassium ions and increased pH, an enzyme is activated to
                        break down starch to glucose

                  d.   The water potential decreases as more solute (glucose) is added to the cell sap 
                        of the guard cell

                  e.   As the water potential decreases in the guard cell, the guard cell becomes more  
                        turgid because water from the surrounding cells moves in by osmosis

                  f.   When the cells become fully turgid the stomata are fully open

                  g.   The reverse of this occurs when the sun goes down at night

     D.   Factors affecting stomatal opening

            1.   Time of day

                   a.   Majority of plants have stomates open during the day time

                   b.   A few plants open their stomates at night

                         (1)  Carrier molecule picks up the CO2 and holds it until daylight

                         (2)  Carrier molecule releases its CO2 for photosynthesis during the day

            2.   Dehydration and water stress

                  a.   Extreme water loss causes the stomata to close because there is not enough  
                        water in the plant to maintain turgor pressure in the guard cells

                  b.   Moderate water loss causes abscisic acid to be released which causes stomatal

            3.   Carbon dioxide inside of the leaf

                  a.   High concentrations of CO2 cause the stomata to close

                  b.   Low concentrations of CO2 cause the stomata to open

     E.   Rate of transpiration is affected by the following factors:

            1.   Humidity

                  a.   The higher the humidity the slower transpiration goes

                  b.   The lower the humidity the faster transpiration goes

            2.   Air temperature

                  a.   As air temperature increases the relative humidity decreases

                  b.   As air temperature decreases the relative humidity increases

                  c.   Air temperature heats up the leaf and causes evaporation on the surface of the  
                        cell walls of the spongy parenchyma to increase

            3.   Midday

                  a.   Midday is the time of day when transpiration is at it highest rate

                  b.   If moderate temperatures and humidity occurs during this time of day,  the  
                        transpiration rates do not go beyond what the plant can handle

                  c.   High temperatures and low humidity causes transpiration to occur faster than the
                        plant can handle which begins to deplete the protoplast of water causing the
                        plant to temporarily wilt

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