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14.2.314-15 · Homeostasis and Control

Abscisic Acid and Stomatal Closure — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 13 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

14.2.3 Abscisic Acid and Stomatal Closure — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how abscisic acid causes stomata to close during water stress.Hormone — abscisic acid (ABA) is produced by plants during water stress.Causes of water stress — very high…

  • Effect on pumps
  • Anion channels
  • K+ exit channels
  • K+ entry channels
  • Chloroplast
  • Second messenger

Part 1 of 5When ABA is produced

When ABA is produced
Hormone
  • Hormone abscisic acid (ABA) is produced by plants during water stress.
Causes of water stress
  • Causes of water stress very high temperatures or reduced water supplies.
Function
  • Function stimulates the closing of the stomata.

Part 2 of 5Binding of ABA

Receptors
  • Receptors guard cells have ABA receptors on their cell surface membranes.
Binding
  • ABA binds to the receptors.
Effect on pumps
  • Effect on pumps the proton pumps are inhibited, stopping active transport of H+ out.

Part 3 of 5Calcium ions as second messengers

Entry
  • ABA causes Ca2+ to move into the guard cell cytoplasm through the surface membrane.
Role
  • Ca2+ acts as a second messenger.
Anion channels
  • Anion channels open and negatively charged ions leave.
K+ exit channels
  • K+ exit channels open, so K+ leaves.
K+ entry channels
  • K+ entry channels are closed.

Part 4 of 5Closure of the stoma

1
Ions leave, so the water potential of the guard cells rises.
2
Water leaves by osmosis.
3
The guard cells become flaccid.
4
The stoma closes.
Benefit
  • Benefit water loss by transpiration is reduced during water stress.
Cost
  • CO2 uptake and photosynthesis decrease.

Part 5 of 5More from the specification

1Cellulose microfibril
  • Strand of cellulose that restricts cell expansion.
2Carbon
  • Element with four valence electrons that forms the backbone of organic molecules.
3Gradient
  • A difference in concentration.
4Diffusion
  • The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
    • as a result of random movement.
5Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
6Constants
  • Factors deliberately kept the same so the result has one cause.
7Chloroplast
  • The organelle of photosynthesis.
8Mechanism
  • The sequence of steps by which something happens

Key terms

TermMeaning
Abscisic acidHormone causing stomatal closure in water stress.
Second messengerMolecule passing a signal within a cell.
Water stressShortage of water relative to loss.
FlaccidLacking turgor pressure.
ReceptorProtein on the surface membrane that binds a signal molecule.

Avoid these mistakes

1
Saying ABA makes guard cells turgid — it makes them flaccid.
2
Saying Ca2+ is the hormone — it is the second messenger inside the cell.
3
Saying ABA activates the proton pumps — it inhibits them.
4
Forgetting that K+ leaves — both K+ and anions leave.

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