- as internal respiration or cell respiration : enzymatically catalyzed metabolic processes of energy production in the cell , during which oxygen is usually consumed and carbon dioxide is produced;
- as external respiration or gas exchange : passive and active processes of the exchange of oxygen and carbon dioxide with the environment and their transport in the organism.
Cellular respiration can be aerobic or anaerobic . Aerobic breathing requires oxygen and has therefore only been possible in geological terms since elemental oxygen has been available in the atmosphere and in water. Its formation goes back to the first photosynthetically active prokaryotes , probably forerunners of today's cyanobacteria . Without oxygen, only anaerobic respiration can take place, in which other substances take over the role of oxygen. In addition, all organisms can gain energy through fermentation .
Anaerobic respiration occurs only in prokaryotes. All eukaryotes can only use oxygen as an oxidant; In an oxygen-free environment they can only gain energy through fermentation. The vast majority of eukaryotes are therefore dependent on aerobic respiration (obligatory aerobic). In contrast, yeasts , which are also eukaryotes, can live without oxygen solely on the basis of fermentation (facultative anaerobic). An example of the rare case in which eukaryotes have lost the ability to use oxygen and are therefore obligately anaerobic are the Neocallimastigaceae , which live in the rumen of ruminants and specialize in the utilization of cellulose . Among prokaryotes, Escherichia coli is an example of how it is possible to switch between aerobic and anaerobic respiration.
Oxygen is required for aerobic breathing. Normally, organic compounds such as carbohydrates or fatty acids are oxidized and energy is obtained in the form of ATP . Three sub-processes follow one another: glycolysis , the citric acid cycle and electron transfer in the respiratory chain , with O 2 being the terminal electron acceptor. Essential sub-steps of glycolysis and the citric acid cycle are three different oxidative decarboxylations in which carbon dioxide is released and reduction equivalents in the form of NADH are obtained, which are fed into the respiratory chain.
If - as is usually the case - glucose is used as a substrate, the sum equation is:
- One molecule of glucose and six molecules of oxygen become six molecules of carbon dioxide and six molecules of water
In anaerobic respiration, which occurs only in prokaryotes , the electrons obtained from the oxidation of an energy carrier are transferred to other external, reducible substrates instead of oxygen. The various anaerobic respiratory systems are classified based on the substrate or metabolic end products inhaled.
|Breathing type||Organisms||"Substantial" response|
|aerobic breathing||obligatory and facultative aerobes (e.g. eukaryotes )||O 2 → H 2 O|
|Iron breathing||facultative aerobes, obligate anaerobes (e.g. Desulfuromonadales )||Fe 3+ → Fe 2+|
|Nitrate breathing||facultative aerobes (e.g. Paracoccus denitrificans , E. coli )||NO 3 - → NO 2 -|
|Fumarate breathing||facultative aerobes (e.g. Escherichia coli )||Fumarate → succinate|
|Sulphate breathing||obligate anaerobes (e.g. Desulfobacter latus )||SO 4 2− → HS -|
|Thiosulfate breathing||z. B. Ferroglobus||H 2 S 2 O 3 → 2 H 2 S|
|Methanogenesis (carbonate respiration)||methanogenic and obligate anaerobes (e.g. Methanothrix thermophila )||CO 2 → CH 4|
|Sulfur breathing||facultative aerobes and obligate anaerobes (e.g. Desulfuromonadales)||S → HS -|
|Respiration of arsenate||Pyrobaculum||AsO 4 2− → AsO 3 -|
|Acetogenesis (carbonate breathing)||homoacetogenic and obligate anaerobes (e.g. Acetobacterium woodii )||CO 2 → CH 3 COOH|
The following variants can be distinguished, which can also occur in combination:
- the skin respiration , in which the gas exchange with water or air through the diffusion over the entire body surface is effected,
- the gill breathing , in which the gas exchange with water takes place via thin, blood-supplied skin protuberances, the gills . It occurs in many invertebrates , including terrestrial animals, in fish and in amphibians .
- The Tracheenatmung over tubular invaginations of the body skin. It is found in insects , millipedes and some spiders .
- The lungs breathing at snails , lungfish , amphibians, reptiles , birds and mammals (including humans ).
- Various other variations of air breathing in bony fish ,
- Gas exchange of the plants via the stomata ,
- the plastron respiration or "physical gill" in aquatic insects,
- the distribution of the gases to the target cells in the respiratory fluid (blood or lymph ), mostly with oxygen transport vectors ( hemoglobin or hemocyanin ), partly cellular ( erythrocytes ).
Physical basics: diffusion
Diffusion is a physical process of equalizing the concentration of substances of different concentrations through thermally induced molecular movement . This takes place from areas with a higher concentration to areas with a lower concentration along a concentration gradient .
The transport rate or transport speed is described by the diffusion laws according to Adolf Fick :
According to Fick's First Law , the transport rate, i.e. the change in the amount of substance (dQ s ) after time (dt), is proportional to the exchange area A and proportional to the concentration gradient, which for gases, as in the case of respiration, is also a partial pressure gradient (dp / dx ) can be described. Another decisive factor is the Krogh diffusion coefficient K , which is the product of the solubility coefficient a and the diffusion coefficient D :
The following applies to the distance covered:
The diffusion time thus increases to the second power with an increase in the diffusion distance.
An effective gas transport by diffusion therefore requires:
- a large surface,
- a high pressure gradient or a high pressure difference between internal pressure and external pressure (p i -p a ),
- a small thickness of the "respiratory membrane" or a short diffusion path (x).
In multicellular differentiated organisms, special organs as part of external respiration are often responsible for gas exchange. The lung is optimized anatomically for gas exchange, by passing through the alveoli has (alveoli) over a large surface with low diffusion distance. CO 2 diffuses 20 times better than oxygen: Although the diffusion coefficient for CO 2 in the alveolar membrane is somewhat poorer due to the larger molecule size, the solubility is 24 times greater, which means a concentration difference that is just as many times greater.
Transport of respiratory gas by convection in animals
In the case of small aquatic animals such as roundworms , flatworms and rotifers , the process of diffusion is sufficient to meet the oxygen requirement (skin breathing). Also Hohltiere exclusively Hautatmer; They have a large surface area and a very low metabolic intensity due to their tentacles . Larger animals have to renew their breathing water or air through ventilation. Especially with vertebrates, there is also the transport of breathing gases within a circulatory system with circulating fluid.
Since the solubility of oxygen in aqueous solutions is very low (see: Henry's Law ), the solubility in the blood is increased by respiratory pigments. In addition to the well-known hemoglobin , this group also includes chlorocruorin , hemerythrin and hemocyanin . Hemoglobin increases the transport capacity of oxygen by a factor of 50 due to its high O 2 binding capacity .
Hemoglobin is a chromoprotein and is the most common respiratory pigment in animals. It consists of a protein ( globin ) and a light-absorbing prosthetic group ( heme ). The special structure of the heme consisting of a protoporphyrin ring with iron as the central ion determines the red color of the blood by absorbing light in the short-wave spectrum (primarily blue tones). The heme group of all hemoglobins and myoglobins is identical. However, hemoglobins differ in the structure of the protein component (globin). This is shown primarily in the different oxygen binding behavior . The O 2 affinity of the hemoglobin of small and more active representatives of mammals is lower than that of larger representatives. This enables better delivery of oxygen to the surrounding tissue. Hemoglobins of cold-blooded vertebrates, on the other hand, have a higher O 2 binding affinity than birds or mammals. Invertebrates also show a significantly higher O 2 binding affinity of their hemoglobin.
Gas exchange in humans
Composition of the inhaled and exhaled air
|Inspirational Group||gas||Expiratory fraction|
|1 %||Noble gases||1 %|
Inhaled air of atmospheric air with averaged composition. Even in indoor spaces used by people with limited ventilation - in favor of heating or cooling, and protection against wind and dust - higher CO 2 concentrations are present. MIK value = 0.30% CO 2 , occupational exposure limit value AGW (replaces the previously used MAK value ) = 0.50% CO 2 .
Gas exchange disruptions
The pulmonary emphysema creates a diffusion disorder by reducing the exchange area. The pulmonary edema creates a diffusion disorder by increasing the diffusion path. Disorders of the oxygenation of the blood can also result from insufficient or incorrectly distributed blood flow to the lungs. Isolated respiratory disorders manifest themselves in hypoxia without hypercapnia , since the CO 2 diffusion still works well for the reasons mentioned when the oxygen diffusion has long been significantly restricted. With an intact breathing pump, respiratory disturbances can be compensated for by ventilators : Deep breathing increases the oxygen partial pressure in the alveoli, which increases the difference in concentration and thus the rate of diffusion. However, a reduced CO 2 partial pressure in the alveoli must be accepted, which is transferred to the blood and disturbs the acid-base balance ( respiratory alkalosis ). The symptomatic therapy of gas exchange disorders is carried out by administering oxygen .
Breathing in plants
Also photoautotrophic organisms (plants in the broadest sense), their total energy through photosynthesis win their energy needs through aerobic respiration, when photosynthesis is not possible, at night and in parts or stages of development, which no active chloroplasts contain (about roots or germinating seeds). The substances inhaled in the process ultimately come from photosynthesis and are supplied from other parts of the plant or were previously stored as reserve materials.
While the air contains more than 20% oxygen, water absorbs only a little of it, especially at higher temperatures (cf. oxygen saturation ). Algae and other aquatic plants can absorb it from the surrounding water by diffusion because they have a large surface and no impermeable cuticle . Swamp plants , some of which grow submerged, and aquatic plants with floating leaves form special ventilation tissues ( aerenchyma ) to supply their submerged parts with oxygen.
With cyanide-resistant breathing , which only occurs in plants , energy is only released in the form of heat, i.e. without the formation of ATP . Cyanide- resistant breathing is so named because it is not affected by cyanides . It is important for many arum plants , the inflorescences of which are heated up as a result, and increasingly emit fragrances to attract pollinators. In the case of the arum , the cob is temporarily around 20 ° C warmer than the surrounding area. When many fruits ripen, cyanide-resistant respiration occurs and accelerates it ( respiratory climacteric ).
Another process, formally referred to as breathing in the sense of a reversal of photosynthesis, is photorespiration , which always takes place alongside photosynthesis in the chloroplasts and reduces its effectiveness. It is interpreted as a relic from the geological time, when the oxygen content of the air was still quite low.
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