Editing Gas exchange (section)

{{Short description|Process by which gases diffuse through a biological membrane}}
{{Continuum mechanics}}
'''Gas exchange''' is the physical process by which gases move passively by [[Diffusion#Diffusion vs. bulk flow|diffusion]] across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable [[membrane]], or a [[biological membrane]] that forms the boundary between an organism and its extracellular environment.

[[File:Maiah Flores - Wikipedia Digital Diagram.pdf|thumb|Gas exchange]]

Gases are constantly consumed and produced by [[Metabolism|cellular and metabolic reactions]] in most living things, so an efficient system for gas exchange between, ultimately, the interior of the cell(s) and the external environment is required. Small, particularly unicellular organisms, such as [[bacterium|bacteria]] and [[protozoa]], have a high [[Surface-area-to-volume ratio|surface-area to volume ratio]]. In these creatures the gas exchange membrane is typically the [[cell membrane]]. Some small multicellular organisms, such as [[flatworm]]s, are also able to perform sufficient gas exchange across the [[skin]] or [[cuticle]] that surrounds their bodies. However, in most larger organisms, which have small surface-area to volume ratios, specialised structures with convoluted surfaces such as [[gill]]s, [[Pulmonary alveolus|pulmonary alveoli]] and [[Leaf#Mesophyll|spongy mesophyll]]s provide the large area needed for effective gas exchange. These convoluted surfaces may sometimes be internalised into the body of the organism. This is the case with the alveoli, which form the inner surface of the [[mammal]]ian [[lung]], the spongy mesophyll, which is found inside the leaves of [[Euphyllophyte|some kinds of plant]], or the gills of those [[mollusc]]s that have them, which are found in the [[Mantle (mollusc)|mantle]] cavity.

In [[aerobic organism]]s, gas exchange is particularly important for [[Respiration (physiology)|respiration]], which involves the uptake of [[oxygen]] ({{Chem|O|2}}) and release of [[carbon dioxide]] ({{Chem|CO|2}}). Conversely, in [[photosynthesis|oxygenic photosynthetic organisms]] such as most [[Embryophyte|land plants]], uptake of carbon dioxide and release of both oxygen and water vapour are the main gas-exchange processes occurring during the day. Other gas-exchange processes are important in less familiar organisms: ''e.g.'' carbon dioxide, [[methane]] and [[hydrogen]] are exchanged across the cell membrane of [[methanogen|methanogenic archaea]]. In [[nitrogen fixation]] by [[diazotroph]]ic bacteria, and [[denitrification]] by [[heterotrophic]] [[bacteria]] (such as ''[[Paracoccus denitrificans]]'' and various [[pseudomonadaceae|pseudomonads]]),<ref>{{cite journal | last1 = Carlson | first1 = C. A. | last2 = Ingraham | first2 = J. L. | year = 1983 | title = Comparison of denitrification by ''Pseudomonas stutzeri'', ''Pseudomonas aeruginosa'', and ''Paracoccus denitrificans'' | journal = Appl. Environ. Microbiol. | volume = 45 | issue = 4| pages = 1247–1253 | doi = 10.1128/AEM.45.4.1247-1253.1983 | pmid = 6407395 | pmc = 242446 | bibcode = 1983ApEnM..45.1247C | doi-access = free }}</ref> nitrogen gas is exchanged with the environment, being taken up by the former and released into it by the latter, while [[giant tube worms]] rely on bacteria to oxidize [[hydrogen sulfide]] extracted from their deep sea environment,<ref>C.Michael Hogan. 2011. [http://www.eoearth.org/article/Sulfur?topic=49557 ''Sulfur''. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC] {{webarchive |url=https://web.archive.org/web/20121028080550/http://www.eoearth.org/article/Sulfur?topic=49557 |date=October 28, 2012 }}</ref> using dissolved oxygen in the water as an electron acceptor.

[[Diffusion]] only takes place with a [[concentration gradient]]. [[Gases]] will flow from a high [[concentration]] to a low concentration.
A high [[oxygen]] concentration in the [[Pulmonary alveolus|alveoli]] and low oxygen concentration in the [[capillaries]] causes oxygen to move into the capillaries.
A high [[carbon dioxide]] concentration in the capillaries and low [[carbon]] [[dioxide]] concentration in the alveoli causes carbon dioxide to move into the alveoli.