14.2 Homeostasis in plants

Question 1

Conditions where Stomata Close

Answer 1

• darkness
• high carbon dioxide concentrations in the air spaces in
  the leaf
• low humidity
• high temperature
• water stress, when the supply of water from the roots is
  limited and/or there are high rates of transpiration.

Question 2

Conditions where Stomata Open

Answer 2

• increasing light intensity
• low carbon dioxide concentrations in the air spaces
  within the leaf.
• high humidity
• low temperature

Question 3

Opening of the Stomata

Answer 3

1) ATP-powered proton pumps in the cell surface
membrane (guard cells) actively transport H+ out of the guard cell.
2) The low H+ concentration and negative charge
inside the cell causes K+ channels to open
3) K+ diffuses into the cell down an electrochemical
gradient.
4) The high concentration of K+ inside the guard cell
lowers the water potential (ψ).
5) Water moves in by osmosis, down a water potential gradient.
6) The entry of water increases the volume of
the guard cells, so they expand. The thin outer
wall expands most, so the cells curve apart

Question 4

Closing of the Stomata

Answer 4

1) Hydrogen ions are not pumped out anymore from the guard cells
2) Potassium ions move out of stomata by simple diffusion down concentration gradient
3) Water potential in stomata increases higher than outside the stomata
4) Water moves out by stomata by osmosis

Question 5

ABA stands for

Answer 5

Abscisic Acid

Question 6

Role of ABA

Answer 6

• coordinate the responses to stress - stress hormone
• If a plant is subjected to difcult environmental conditions, such as very high temperatures or much reduced water supplies, then it responds by secreting ABA.
• ABA causes stomata to close by inhibiting proton pumps

Question 7

What ABA does

Answer 7

• Theoretical *
  1) Guard cells have ABA receptors on their cell surface membranes
  2) When ABA binds with these it inhibits the proton pumps to stop hydrogen ions being pumped out
  3) ABA also causes calcium ion protein channels to open - stimulates the movement of calcium ions into the cytoplasm through the cell surface membrane and the tonoplast (membrane around the vacuole).
  4) Calcium acts as a second messenger to activate channel proteins to open that allow negatively charged ions to leave the guard cells.
  5) This, in turn, stimulates the opening of channel
  proteins which allows the movement of potassium ions out of the cells.
  6) At the same time, calcium ions also stimulate
  the closure of the channel proteins that allow potassium
  ions to enter.
  7) The loss of ions raises the water potential of the cells, water passes out by osmosis, the guard cells become flaccid and the stomata close.

Question 8

Daily rhythms of Stomata

Answer 8

• Stomata show daily rhythms of opening and closing.
• They open during morning to midday and close from midday throughout the night
• Even when kept in constant light or constant dark, these rhythms persist
• Opening during the day maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen. - - However, it also allows the outward diffusion of water vapour in transpiration
• The closure of stomata at night when photosynthesis cannot occur reduces rates of transpiration and conserves water.