Thermogenesis

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The Indian lotus flower heats the stigma to 30 to 35 ° C.

Thermogenesis or heat generation is the production of heat through the metabolic activity of living things. Heat is an inevitable by-product of metabolic processes such as energy metabolism , digestion and muscle activity in animals. In addition, there are special metabolic pathways (decoupling of the chemiosmotic coupling , substrate cycles ) in which chemical energy is used exclusively to produce heat.

Thermogenesis in plants

In plants , normal breathing , in which even under optimal conditions 60% of the energy is released in the form of heat, does not lead to a warming of the breathing organs, since the heat is immediately released to the environment due to the environmentally-friendly construction of the plants. However, some plants are able to use thermogenesis to heat certain organs.

The arum ( Arum maculatum ) has an appendage in the flower bulb in which starch is stored before the anthesis . Simultaneously with the opening of the flower bulb, there is an increase in breathing. In Sauromatum guttatum , the hormonal signal commonly known as calorigen has been identified as salicylic acid . In the case of the arum, around 75% of the dry matter of the appendage is breathed in in 12 hours, the breathing intensity is 200 to 400 µL CO 2 · h −1 · mg dry matter −1 , 20 to 40 times that of a mammalian brain. This is achieved by a greatly increased number and size of the mitochondria , the enzymes of the citric acid cycle and the respiratory chain increase sharply. As with all thermogenic plants, electrons are transported exclusively via the cyanide-resistant bypass, the released energy is given off directly as heat and is catalyzed by the alternative oxidase (AOX).

In the Indian lotus flower ( Nelumbo nucifera ), thermogenesis takes place in the scar while it is ready for conception. The temperature is kept constant in the range between 30 and 35 ° C, regardless of the ambient temperature. This is a very rare case of thermoregulation in plants .

In general, thermogenesis occurs in plants mainly in the basal groups of the bed covers , but also in all cycads . The attraction of pollinators is usually assumed as a function , whereby the increase in temperature either spreads more fragrances or the insects warm themselves up directly.

Thermogenesis in animals

Numerous animal species, especially mammals and birds as endothermic animals , have special mechanisms of thermoregulation . The heat generation can be divided into muscular and biochemical thermogenesis. In particular, for the extra heat generation in brown adipose tissue , the term in the literature often jitter-free thermogenesis (English non-shivering ) is used.

Muscular thermogenesis

Heat is generated in the skeletal muscles in

Since the efficiency of the skeletal muscles rarely exceeds 20%, most of the energy used in physical work is converted into heat. Without dissipation from the body, this leads to a corresponding warming. If the muscles are involuntarily tensed in a cold environment and the muscle tone is increased, heat is generated without doing any mechanically useful work.

With shivering from cold, there is a further increase in muscle tone up to visible tremors in the muscles. The activated motor units contract independently of one another; Agonistic and antagonistic muscles, which alternately contract during normal movements, are now activated simultaneously. In shivering, the primary function of muscle contractions is to use energy to generate heat; the heat output that can be achieved in this way can reach 320 to 400  watts, four to five times the basal metabolic rate . Real shivering as energetic hard work can be carried out by humans for a maximum of 2 hours.

Biochemical thermogenesis

Even in the resting state, heat is generated ( basal thermogenesis, basal metabolic rate ), every further increase in the metabolic rate leads to further, obligatory thermogenesis. If necessary, additional required heat can be generated by burning fatty acids ; in vertebrates this happens in the liver and - if available - in the brown adipose tissue . The heat production in brown adipose tissue is more effective due to its decoupling from the ATP synthesis . The production and activity of the decoupling protein thermogenin in brown adipose tissue is induced by cold stimuli.

A substrate cycle based on the activity of the enzymes phosphofructokinase and fructose-1,6-bisphosphatase was found in bumblebees of the genus Bombus . Substrate cycles do not form new metabolic products, but rather hydrolyse the cell's own energy carrier, ATP, while generating heat. The cycle described here enables the animals to reach the body temperature required for flying while they are still resting.

Postprandial thermogenesis

Course of postprandial thermogenesis, here as a percentage of basal metabolism shown

When food is digested, heat is also released, as energy has to be expended for the absorption, splitting, transport, conversion and storage of the nutrients. In humans, around 10% of the energy absorbed is immediately "consumed" or converted again in the form of postprandial (= occurring after the meal) thermogenesis and increases the basal metabolic rate for several hours . There are big differences depending on the food component : lipids only convert 2 percent of their energy content into heat, glucose 8 percent, proteins 20 to 30 percent and ethanol 22 percent. In obese men, on the other hand, thermogenesis rates of 15 percent up to 7 hours after ingestion of a protein meal were found in phases of strong weight gain.

Adaptation to changed ambient temperatures

If the ambient temperature suddenly drops, shivering can set in immediately as a regulatory mechanism, while adjusting the biochemical thermogenesis requires a certain amount of time. It is known from laboratory rats that if the temperature is lowered for a longer period of time, the initially violent shivering will visibly decrease after a few days, while the metabolic rate remains increased. If rats adapted to the cold are injected with curare in order to prevent shivering from the cold, the metabolic rate of these animals remains increased due to the further acting thermoregulation. The term tremor-free thermogenesis for heat production, especially of brown adipose tissue, is derived from such observations.

Thermogenesis in prokaryotes

Thermogenic bacteria were described at the beginning of the 20th century.

literature

  • Peter Schopfer, Axel Brennicke: Plant Physiology. Elsevier, Munich 2006, ISBN 3-8274-1561-6 , p. 248f.
  • Christopher D. Moyes, Patricia M. Schulte: Animal Physiology. Pearson Studies, Munich 2008, ISBN 978-3-8273-7270-3 , p. 678ff.

Individual evidence

  1. ^ Matthias Schaefer: Dictionary of Ecology. 4th edition. Spektrum Akademischer Verlag, Heidelberg / Berlin 2003, ISBN 3-8274-0167-4 , p. 347.
  2. ↑ Robert Roemer, Irene Terry, Christina Chockley, Jennifer Jacobsen: Experimental evaluation and thermo-physical analysis of thermogenesis in male and female cycad cones. In: Oecologia. Volume 144, 2005, pp. 88-97. doi: 10.1007 / s00442-005-0045-0 .
  3. ↑ Werner Müller, Stephan Frings: Animal and Human Physiology. Springer, Berlin 2007, ISBN 978-3-540-32728-8 , p. 258.
  4. ↑ Huiyun Liang, Walter Ward: PGC-1alpha: a key regulator of energy metabolism. In: Advan. Physiol. Edu. 30, 2006, pp. 145-151. doi: 10.1152 / advan.00052.2006 Full text (English) ( Memento of the original from November 23, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@ 2Template: Webachiv / IABot / advan.physiology.org
  5. ↑ EA Newsholme et al.: The activities of fructose diphosphatase in flight muscles from the bumble-bee and the role of this enzyme in heat generation. In: Biochem J . 128 (1) 1972, pp. 89-97. PMID 4343671 PMC 1173573 (free full text).
  6. ↑ Y. Protection: The energy metabolism of obese patients. In: J. Wechsler (Ed.): Obesity - Causes and Therapy. Blackwell, Berlin / Vienna 1998, p. 108 of the Google Books preview.
  7. ↑ J. Steiniger, H. Karst, R, Noack, H.-D. Steglich: Reduced food-induced thermogenesis in obesity. (Short scientific report). In: The food. 26, 4, 1982, pp. K23-K26 (PDF, 124 KB).
  8. ↑ Richard W. Hill, Gordon A. Wyse, Margaret Anderson: Animal Physiology. Sinauer Associates, Sunderland, Mass. USA, 2004, pp. 220-221.
  9. ↑ Berens: Thermogenic Bacteria. In: Lafar's Handbook of Technical Mycology. Volume I, Berlin 1908.