Plant gas exchange — By what physical process do CO2 and O2 normally enter and leave plant tissues through stomata?

Introduction / Context:
Gas exchange in plants underpins photosynthesis and respiration. Carbon dioxide must diffuse in for photosynthesis, and oxygen diffuses out (or in for respiration). Understanding the dominant transport mechanism across stomata and intercellular spaces is foundational in plant physiology.

Given Data / Assumptions:

• Stomata regulate pore aperture but do not actively pump gases.
• CO2 and O2 are small, nonpolar molecules that readily diffuse down concentration gradients.
• Leaf internal spaces provide short diffusion paths to mesophyll cells.

Concept / Approach:
Diffusion is the random molecular motion leading to net movement from higher to lower partial pressure. Stomatal opening sets the boundary conditions; once open, CO2 and O2 move by diffusion driven by concentration and partial pressure differences between ambient air and the moist internal leaf air spaces.

Step-by-Step Solution:

Verification / Alternative check:
Fick’s law describes flux proportional to gradient and conductance; empirical measurements of stomatal conductance and intercellular CO2 concentration align with diffusion models.

Why Other Options Are Wrong:

• Osmosis: applies to water movement, not gases.
• Active transport: no ATP-dependent gas pumps operate for CO2/O2 in leaves.
• Evaporation: describes water vapor loss, not CO2/O2 influx.
• Facilitated diffusion via aquaporins: aquaporins move water, not CO2/O2 as a primary mechanism.

Common Pitfalls:
Equating stomatal opening with active import. The guard cells use energy to change aperture, but gas molecules move passively by diffusion.

Final Answer:
Diffusion