Saturday, March 24, 2012

STPM Biology - chapter 7: Revision essay question 1

1. With the aid of labelled diagram, explain,
    a. i. The structure if a stoma                 [2]
       ii. The mechanism of stomata opening and closing based on the potassium ions accumulation                                                          
            hypothesis [8] 
b. Outline the environmental factors which influence the opening and closing of a stoma   [5]


Answer:


a. i.  



Label -1
Diagram - 1


a. ii.  
 

During Day: The mechanism of stomatal opening
-          Kare pumped from neighboring cells into the guard cells, H+ are pumped out of the neighboring cells to maintain the electro neutrality
-          The increase of ion K+ and sugar (form photosynthesis) concentration makes the water potential of the guard cells more negative (lower), therefore
-          The water from neighboring cells moves into the guard cells
-          The resultant increase in hydrostatic pressure causes the guard cells to become turgid
-          The uptake of water causes increased bowing og the guard cells (owing to the greater expansion of the outer walls than the inner wall) and the stoma open
During Night: The mechanism of stomatal closing
-          K+ ions are actively transported out from the guard cells into the neighboring cells, H+ ions are transported into the neighboring into the guard cells.
-          Photosynthesis does not occur and carbon dioxide concentration increases and the pH of the guard cell fall.
-          Sugar is converted into insoluble starch, therefore the water potential of the guard cell increases.



b.

-  light/blue light stimulate guard cells to accumulate potassium and become turgid, stoma open; or by driving photosynthesis in guard cells chloroplast making ATP available for active transport of H+.
- When temperature increases, stoma opens
- Air movement
- Dehydration (water stress):  in case of water deficiency, guard cells lose turgid and stoma closes
- Mesophyll cells produce hormone abscisic acid which signals the guard cells to close.
- Concentration of carbon dioxide: depletion of of CO2 within the air spaces of the leaf causes the stoma to open
- Moisture/humidity


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