Gaseous Exchange in Plants

Every living cell needs a steady supply of oxygen to carry out aerobic respiration, and must equally get rid of the carbon dioxide that respiration produces as a waste gas. In animals, this exchange of gases between the body and the environment is usually handled by dedicated respiratory organs and structures -- gills, lungs, tracheae -- built specifically for the purpose, often assisted by a circulatory system that carries the gases to and from every tissue. Plants, in striking contrast, possess no such specialised respiratory organs or system at all. Every part of a plant -- root, stem, leaf, flower -- takes care of its own gaseous exchange needs independently, without relying on any central respiratory organ or on a circulatory fluid to transport respiratory gases from one part of the plant to another.

This absence of specialised respiratory machinery is not a shortcoming; it works precisely because of how plants are built and how they live. A plant's demand for oxygen, and its rate of carbon dioxide output, are both far lower per unit of living tissue than an animal's, since plants are far less active in the mechanical sense -- they do not run, fly or continuously contract muscle. At the same time, most of a plant's living cells lie fairly close to the plant's surface or to some internal air space, so that the distance any respiratory gas has to travel by simple diffusion, down its own concentration gradient, remains short. Diffusion alone, without any pump or active transport step, is therefore entirely sufficient to keep every living cell of a plant adequately supplied with oxygen and to remove the carbon dioxide it produces.

Different plant organs exchange gases through different structures, suited to their own particular surface. In a leaf, the flattened blade is thin and each cell lies close to the surface, and gaseous exchange takes place chiefly through the numerous small pores called stomata (singular: stoma) scattered over the epidermis, especially the lower epidermis in many plants. Each stoma is flanked by a pair of specialised guard cells that can open or close the pore, primarily to regulate the loss of water vapour by transpiration, but the same open stomata simultaneously allow oxygen to diffuse in and carbon dioxide to diffuse out (and, during the day, allow the much larger volumes of gas exchange associated with photosynthesis).

In the woody, bark-covered stems of trees and shrubs, the stomata are largely replaced by a different set of openings called lenticels -- small, slightly raised pores in the bark, filled with loosely arranged cells that leave air spaces between them, through which gases can diffuse in and out despite the otherwise impermeable, corky bark surrounding them.

Roots present a further, and different, situation. Root cells respire actively but have no stomata or lenticels of their own; instead, they rely on the air spaces present between the particles of a well-aerated soil, from which the oxygen dissolved or trapped as soil air diffuses into the root through the thin walls of the epidermal cells and root hairs, which are in close, direct contact with the soil solution. Because roots depend entirely on this diffusion pathway, waterlogged or compacted soils, in which the air spaces between soil particles are filled with water rather than air, severely restrict the oxygen supply available to root cells, forcing them to rely increasingly on anaerobic fermentation and, if prolonged, leading to root damage or death -- a major reason overwatered potted plants and crops in flooded fields often suffer or die.

Underlying all of this is a single simple principle: as long as every living cell has a sufficiently short diffusion path to an external or internal air space, and the plant's own respiratory demand stays comparatively modest, no dedicated respiratory organ or gas-transport system is needed. This is why plant respiration, unlike animal respiration, can be studied almost entirely at the level of the individual cell and its immediate surroundings, rather than at the level of an integrated organ system.