Stomata and Gas Exchange

Gas Exchange in Plants 

Leaf Adaptation

Stomata: free unhindered diffusion of CO2 into leaf and O2 out of leaf.

High Stomata Density: the more stomata the greater the amount of gas exchanged.

Thin: the shorter the distance the faster the rate of diffusion of CO2 and O2.

Great Surface Area: the greater the surface area the greater the gas exchanged.

Flat: maintains the highest possible concentration difference for fastest diffusion.

Internal Air Spaces: diffusion of CO2 and O2 is much faster in air than in water.

Moist Internal Surface: required for the absorption and release of gas from the leaf cells.

Textbook Diagram: external view of a leaf.

Textbook Diagram: transverse section of a leaf to show internal anatomy.

A high rate of photosynthesis can only be maintained if atmospheric carbon dioxide can pass swiftly to the photosynthetic cells and oxygen gas escape from the leaf.

Control of Gas Exchange

Textbook Diagram: closed and open stomata.

The stomata are by far the most influential structures in gas exchange.

Gas exchange is essential in light when photosynthesis is taking place.

The guard cells respond to carbon dioxide concentration in the leaf.

At low CO2, during photosynthesis, the guard cells are turgid and the stomatal pore is open.

If photosynthesis stops the CO2 level rises in the leaf and the guard cells become flaccid.

Flaccid guard cells cause the stomatal pore to close and so gas exchange stops.

Generally: stomata open in light and closed in the dark.

Closed in the dark to reduce transpiration and so conserve water.

Lenticels

Textbook Diagram: structure of lenticel.

Lenticels are raised loose cork tissue in woody stems, roots and some fruits.

Function in gas exchange for aerobic respiration.

Live non-photosynthetic cells tissue cells below the dead cork layer need oxygen gas.

They also need to get rid of carbon dioxide waste gas.

Lenticels allow oxygen in and carbon dioxide out.