9.2 Gaseous Exchange in Plants | Diversity In Plant Functions | Biology 11

Gaseous exchange in plants refers to the movement of $O_{2}$ and $C O_{2}$ between the plant and its environment. Unlike animals, plants lack a dedicated respiratory system; instead, gases diffuse passively through specialized structures.

Structures for Gaseous Exchange

1. Stomata (Leaves)

Stomata are microscopic pores found mainly in the lower epidermis of leaves. Each stoma is flanked by two guard cells that regulate its opening and closing.

Spongy mesophyll air spaces: Gases diffuse from stomata into the large intercellular air spaces of the spongy mesophyll, reaching all photosynthetic cells.
Direction of exchange:
- Day: Net uptake of $C O_{2}$ , net release of $O_{2}$ (photosynthesis > respiration)
- Night: Net uptake of $O_{2}$ , net release of $C O_{2}$ (only respiration)

2. Lenticels (Woody Stems and Roots)

Mature woody stems and roots are covered by cork (suberized cells), which is impermeable to gases. Gaseous exchange occurs through lenticels — permanent openings in the bark where loosely arranged complementary cells allow passive diffusion of $O_{2}$ inward and $C O_{2}$ outward.

3. Root Hair Cells

In young roots, gaseous exchange occurs directly through the thin, moist walls of root hair cells by simple diffusion.

Mechanism of Stomatal Opening and Closing

$K^{+}$ Ion Influx Theory

The opening and closing of stomata is controlled by changes in the turgor pressure of guard cells, regulated by $K^{+}$ ion movements.

Stomatal Opening (Day):

Light activates ATPase (proton pump) in guard cell membranes.
$H^{+}$ ions are pumped out of guard cells.
Starch in guard cells is converted to malic acid, which dissociates into malate $^{-}$ and $H^{+}$ .
$K^{+}$ ions move into guard cells (influx) via $K^{+}$ channels, driven by the electrochemical gradient.
Water potential inside guard cells decreases.
Water enters guard cells by osmosis → cells become turgid → stoma opens.

Stomatal Closure (Night / Drought):

In darkness or under water stress, Abscisic Acid (ABA) is released.
ABA triggers $K^{+}$ efflux (movement out) from guard cells.
Water potential inside guard cells increases.
Water leaves guard cells by osmosis → cells become flaccid → stoma closes.

Condition	$K^{+}$ Movement	Guard Cell State	Stoma
Light / Day	Influx (in)	Turgid	Open
Dark / Drought / ABA	Efflux (out)	Flaccid	Closed

Photorespiration

Definition

Photorespiration is a metabolic process in which the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) fixes $O_{2}$ instead of $C O_{2}$ , occurring in the presence of light.

Conditions Favouring Photorespiration

High temperature (above 25°C)
High $O_{2}$ concentration
Low $C O_{2}$ concentration (e.g., when stomata are closed)

Events of Photorespiration

RuBisCO binds $O_{2}$ to RuBP (ribulose-1,5-bisphosphate) instead of $C O_{2}$ .
This produces phosphoglycolate (a 2-carbon compound) instead of two molecules of 3-PGA.
Phosphoglycolate is metabolized in peroxisomes and mitochondria, releasing $C O_{2}$ and consuming $O_{2}$ .
No ATP is produced — it is an energy-wasting process.
Net result: loss of fixed carbon and reduction in photosynthetic efficiency.

Why Did Photorespiration Evolve?

When RuBisCO first evolved (~3 billion years ago), the atmosphere had very low $O_{2}$ and high $C O_{2}$ — so RuBisCO's oxygenase activity was negligible. As photosynthesis by early organisms raised atmospheric $O_{2}$ levels, photorespiration became a significant (and disadvantageous) side reaction. RuBisCO could not easily evolve to exclude $O_{2}$ because the active sites for $C O_{2}$ and $O_{2}$ fixation overlap.

Compensation Point

The compensation point is the light intensity (or $C O_{2}$ concentration) at which:

$Rate of Photosynthesis = Rate of Respiration$

At this point, the $C O_{2}$ released by respiration is exactly equal to the $C O_{2}$ fixed by photosynthesis, and the $O_{2}$ produced equals the $O_{2}$ consumed. There is no net gaseous exchange.

Below compensation point: Respiration > Photosynthesis → net $C O_{2}$ release
Above compensation point: Photosynthesis > Respiration → net $C O_{2}$ uptake

Photorespiration raises the compensation point because it wastes fixed carbon, meaning the plant needs higher light intensity to achieve net carbon gain.

Summary of Gaseous Exchange Sites

Plant Part	Structure	Mechanism
Leaves	Stomata + air spaces	Diffusion through guard-cell-regulated pores
Woody stems/roots	Lenticels	Passive diffusion through complementary cells
Young roots	Root hair cells	Direct diffusion through moist cell walls

Structures for Gaseous Exchange

1. Stomata (Leaves)

Stomata are microscopic pores found mainly in the lower epidermis of leaves. Each stoma is flanked by two guard cells that regulate its opening and closing.

Spongy mesophyll air spaces: Gases diffuse from stomata into the large intercellular air spaces of the spongy mesophyll, reaching all photosynthetic cells.
Direction of exchange:
- Day: Net uptake of $C O_{2}$ , net release of $O_{2}$ (photosynthesis > respiration)
- Night: Net uptake of $O_{2}$ , net release of $C O_{2}$ (only respiration)

Light activates ATPase (proton pump) in guard cell membranes.
$H^{+}$ ions are pumped out of guard cells.
Starch in guard cells is converted to malic acid, which dissociates into malate $^{-}$ and $H^{+}$ .
$K^{+}$ ions move into guard cells (influx) via $K^{+}$ channels, driven by the electrochemical gradient.
Water potential inside guard cells decreases.
Water enters guard cells by osmosis → cells become turgid → stoma opens.

Stomatal Closure (Night / Drought):

In darkness or under water stress, Abscisic Acid (ABA) is released.
ABA triggers $K^{+}$ efflux (movement out) from guard cells.
Water potential inside guard cells increases.
Water leaves guard cells by osmosis → cells become flaccid → stoma closes.

Condition	$K^{+}$ Movement	Guard Cell State	Stoma
Light / Day	Influx (in)	Turgid	Open
Dark / Drought / ABA	Efflux (out)	Flaccid	Closed

High temperature (above 25°C)
High $O_{2}$ concentration
Low $C O_{2}$ concentration (e.g., when stomata are closed)

Events of Photorespiration

RuBisCO binds $O_{2}$ to RuBP (ribulose-1,5-bisphosphate) instead of $C O_{2}$ .
This produces phosphoglycolate (a 2-carbon compound) instead of two molecules of 3-PGA.
Phosphoglycolate is metabolized in peroxisomes and mitochondria, releasing $C O_{2}$ and consuming $O_{2}$ .
No ATP is produced — it is an energy-wasting process.
Net result: loss of fixed carbon and reduction in photosynthetic efficiency.

Why Did Photorespiration Evolve?

Compensation Point

The compensation point is the light intensity (or $C O_{2}$ concentration) at which:

$Rate of Photosynthesis = Rate of Respiration$

Below compensation point: Respiration > Photosynthesis → net $C O_{2}$ release
Above compensation point: Photosynthesis > Respiration → net $C O_{2}$ uptake

Photorespiration raises the compensation point because it wastes fixed carbon, meaning the plant needs higher light intensity to achieve net carbon gain.

Summary of Gaseous Exchange Sites

Plant Part	Structure	Mechanism
Leaves	Stomata + air spaces	Diffusion through guard-cell-regulated pores
Woody stems/roots	Lenticels	Passive diffusion through complementary cells
Young roots	Root hair cells	Direct diffusion through moist cell walls