During the process of photosynthesis green plants need carbon dioxide and release oxygen.

For cellular respiration plant cells need oxygen and release carbon dioxide.

Some of the unique features regarding plant respiration are:

• Plants do not have specialized organs for gas exchange.
• Each part of the plant takes care of its own gas exchange.
• The rate of respiration is relatively much slower in plants compared to animals.
• The liquid transport system of plants is not involved in gas transport
• The parenchymal cells of leaves, stems and roots are loosely packed with air spaces . The gases diffuse through thee intercellular air spaces.
• Oxygen and carbon dioxide pass through the cell wall and plasma membrane by diffusion. Membrane channels called aquaporin also help in the diffusion of carbon dioxide.

Gaseous exchange in the leaves

The exchange of oxygen and carbon dioxide in the leaf  occurs through stomata .

Refer to chapter 2.2 to understand more about the Stomata and its role in gas exchange and transpiration.

Factors like temperature, humidity , light intensity and atmospheric concentration of CO_{2 i}nfluence the density of stomata produced on leaves .

Interestingly studies have reported that  as the concentration of CO₂ goes up, the number of stomata produced goes down, and vice versa. 

Stomatal index : it is  the ratio of the number of stomata in a given area divided by the total number of stomata and other epidermal cells in that same area.

A 45-amino acid peptide called stomagen released by mesophyll cells  induces the formation of stomata in the epidermis .

 Could fossil leaves tell us about past levels of CO₂ in the atmosphere?

It is intresting to know that fossil leaves could tell us about the levels of atmospheric CO_{2 .}

In a study reported in  Nature, 411:287, 17 May 2001) by Gregory Retallack  it was said that the fossil leaves of the ginkgo and its relatives showed high stomatal indices  late in the Permian period (275–290 million years ago) and again in the Pleistocene epoch (1–8 million years ago). Geological evidence show that both these periods are known for low levels of atmospheric CO₂ and ice ages. On the contrary  during the Cretaceous period, a time of high CO₂ levels and warm climate , stomatal indices were  reported to be low.

These studies support the role of  carbon dioxide as a greenhouse gas and its contribution in global warming.

Gaseous Exchange in Roots and Stems

Mature woody stems and roots are covered by dead cells called Cork cells that are filled with a waxy, air and water proof substance called Suberin which makes the roots and stems impervious to water and air . But  the cork of both mature roots and woody stems  contain non suberized  pores called lenticels. These enable the gaseous exchange in roots and stem.

The stems of many annual plants are green and they take part in photosynthesis . In such stems the stomata are used for gas exchange rather than the lenticels.

In Cacti and succulents have a different adaptation . Their stomata remain closed during in order to prevent moisture loss in the hot, dry environments .However their stomata open at night instead, and the CO₂ is stored as an acid in large sacs (called vacuoles) within their cells until it’s needed for photosynthesis . These fluid-filled vacuoles create the thick, fleshy leaves and stems that are characteristic of these plants. 

Test your Understanding by attempting the Drag and Drop exercise below