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
During Day: The mechanism of stomatal opening
-
K+ are
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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