2.2 - Main Organ for Gaseous Exchange

The Necessity of Gaseous Exchange

• Plants require carbon dioxide ($C{O}_2$) for photosynthesis and oxygen ($O_2$) for cellular respiration.

• Gaseous exchange occurs primarily through stomatal pores on the leaves.

Mechanism of Stomatal Opening and Closing

The opening and closing of stomata are regulated by the changes in turgor pressure of the guard cells, which is driven by two main factors:

1. Potassium Ion ($K^{+}$) Uptake

• Opening of Stoma (In the presence of light):

  • Active transport pumps potassium ions ($K^{+}$) from surrounding epidermal cells into the guard cells.
  • The accumulation of $K^{+}$ increases the solute concentration of the guard cells, which lowers their water potential.
  • Water from surrounding cells diffuses into the guard cells by osmosis.
  • The guard cells become turgid. Because the inner cell wall of the guard cell is thicker and less elastic than the outer wall, the guard cells curve outwards, causing the stoma to open.

• Closing of Stoma (In the absence of light/darkness):

  • Potassium ions ($K^{+}$) diffuse out of the guard cells into the surrounding epidermal cells.
  • Solute concentration in the guard cells decreases, raising the water potential.
  • Water diffuses out of the guard cells by osmosis.
  • The guard cells lose turgidity (become flaccid), and the stoma closes.

2. Concentration of Sucrose

• Opening of Stoma:

  • During the day, photosynthesis takes place in the chloroplasts of the guard cells, producing glucose which is then converted into sucrose.
  • The accumulation of sucrose increases the solute concentration in the guard cells, lowering the water potential.
  • Water enters the guard cells by osmosis, making them turgid and causing the stoma to open.

• Closing of Stoma:

  • At night, photosynthesis stops, and sucrose is either converted to starch or transported away.
  • The solute concentration in the guard cells decreases, raising the water potential.
  • Water leaves the guard cells by osmosis, making them flaccid, and the stoma closes.

Effect of Water Shortage on Stomatal Movement

• When a plant experiences water stress or wilting, the plant secretes a plant hormone called abscisic acid (ABA).

• Abscisic acid triggers the rapid efflux of potassium ions ($K^{+}$) out of the guard cells.

• This causes water to leave the guard cells by osmosis, reducing turgidity.

• The stomata close to prevent excessive water loss through transpiration, protecting the plant from dehydration.