Discuss the various mechanisms of opening and closing of stomata.

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   Photosynthetic  theory;  during  the  day,  guard  cells  carry  out  photosynthesis  manufacturing glucose;  This  increases  the  osmotic  pressure  of  the  sap  vacuole;  which  becomes  higher  than that  of  the  neighbouring  epidermal  cells;  guard  cells  therefore  take  in  water  by  osmosis;  and become  turgid;  the  outer  thin  wall  stretches  easily;  pulling  the  thicker  inner  wall  outwards;  thus the  stomata  opens;  At  night,  there  is  no  light  hence  no  photosynthesis  takes  place;  plant  cells respire  using  up  more  glucose;  the  osmotic  pressure  of  the  sap  vacuole  of  the  guard  cells reduces;  becoming  lower  than  the  neighbouring  epidermal  cells;  the  guard  cells  lose  water  by osmosis;  to  adjacent  epidermal  cells;  they  then  become  flaccid;  pulling  together  the  thick  inner walls;  and  stomata  closes;  Enzymatic  inter-conversion  between  starch  and  glucose/sugar;  At day  time,  plants  continuously  use  carbon  (IV)  oxide  for  photosynthesis;  leading  to  an  increase in  the  pH  of  the  guard  cells;  this  causes  starch  to  be  converted  to  sugar/glucose;  the  glucose increases  the  osmotic  pressure  of  the  guard  cells;  hence  water  is  taken  in  by  osmosis;  the  cells become  turgid  and  bulge  outwards;  causing  the  stomata  to  open;  At  night,  no  photosynthesis occurs  but  respiration  takes  place;  carbon  (IV)  oxide  accumulates  in  guard  cells;  lowering  the pH;  the  low  pH  favours  conversion  of  glucose  into  starch;  starch  is  osmotically  inactive;  this lowers  the  osmotic  pressure  of  guard  cells;  guard  cells  therefore  lose  water  by  osmosis  to  the adjacent  epidermal  cells;  become  flaccid;  pulling  together  the  thick  inner  walls;  and  the stomata  closes;  Active  ion  exudation;  during  the  day,  there’s  an  accumulation  of  potassium  and sodium  ions;  as  a  result  of  active  pumping  of  the  ions  by  the  ATP  formed  through photosynthesis;  carbon  (IV)  oxide  fixation  occurs  in  the  guard  cells;  the  guard  cells  become turgid;  and  stomata  open;  At  night,  before  the  stomata  close,  the  ions  diffuse  out  of  the  guard cells  into  epidermal  cells;  the  osmotic  pressure  of  guard  cells  is  lowered;  they  lose  water  to epidermal  cells  by  osmosis;  and  become  flaccid;  thereby  closing  the  stomata;
 Max.  20  mks     

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