Question

The Leaf & Gas Exchange
Analyze the diagram of a leaf stoma (pore) below.
Describe the role of diffusion in the leaf. What substances are moving, and in which direction are they likely moving?
COz
H2O
Open Stoma

Answer 1

Diffusion is the passive movement of molecules from regions of higher concentration (or partial pressure) to lower concentration. The stomatal pore provides an open pathway between the leaf’s internal air spaces and the atmosphere, so diffusion drives the gas exchange that supports photosynthesis, respiration and transpiration.

Typical directions (daylight, stoma open):
- CO2: diffuses into the leaf (from the atmosphere into internal air spaces and then into mesophyll cells) to be used in photosynthesis.
- O2: produced in the chloroplasts during photosynthesis, diffuses out of the leaf to the atmosphere.
- H2O (as vapor): diffuses out of the moist internal air spaces to the drier outside air (transpiration).

Notes:
- The rates depend on concentration/partial‑pressure gradients, stomatal aperture (conductance), humidity, temperature and wind. Fick’s law describes the proportionality to the gradient and conductance.
- At night or when photosynthesis is low, the gradients can reverse (CO2 may diffuse out and O2 in) because respiration dominates.
- Stomatal opening itself involves water movement into guard cells (osmosis), but that is separate from the gaseous diffusion through the pore.