Page 44 - Q & A STPM 2022 Biology

Biology STPM Chapter 7 Gaseous Exchange

Chloroplast Epidermal cell
(subsidiary cell)
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Outer wall of guard cell
Stoma
Nucleus
Inner wall of guard cell

(b) Stomatal opening
– During the day, chloroplast in the guard cells generate ATP by
photophosphorylation during photosynthesis. Semester
– The ATP generated is hydrolysed to provide energy to drive the proton-
pump in the cell membrane of the guard cells.
– Hydrogen ions (H ) are pumped out of the guard cells. 2
+
– This causes the cells to become negatively charged.
– So, the potassium ions (K ) diffuse from the subsidiary cells through
+
the potassium channel down the electrochemical gradient into the guard
cells.
– Chloride ions (Cl ) enter to balance the charges.
–
– The increase in K concentration causes the water potential of the guard
+
cells to become more negative (lower).
– Then, water from the subsidiary cells enters the guard cells.
– The guard cells become turgid, increasing the turgor pressure.
– The thinner and more elastic outer walls of the guard cells stretch more
compared to the thicker and less elastic inner walls.
– This causes the guard cells to curve outwards. Stoma opens.
Stomatal closure
– During the night, potassium ions (K ) move out of the guard cells into
+
the subsidiary cells.
– The water potential in the guard cells increases.
– Water diffuses out of the subsidiary cells by osmosis.
– Turgor pressure in the guard cells decreases.
– The cells become flaccid. Stoma closes.

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Chapter 7.indd 169 11/2/21 9:27 AM