Chemotrophy (literally "chemical nutrition", cf. trophy ) describes the metabolic type of all living beings that gain energy for their metabolism from chemical reactions of substances that they absorb from the environment. Living beings with such an energy metabolism are called chemotrophic . An alternative is phototrophy , in which light is used as an energy source.
The first life forms and the primal ancestor from which all living beings today descended were probably chemotrophic. Phototrophic organisms developed later.
Adenosine triphosphate (ATP)
ATP can be formed through energy-supplying redox reactions . The captured reducing agent or catabolic electron donors formed therefrom react with external electron acceptors. These reactions are mostly catalyzed by enzymes located on biomembranes . There, according to the chemiosmotic principle, ATP is formed by an ATP synthase (Fig. 1, right).
ATP can also be formed through fermentations , in which ATP is formed inside the cell through the chemical conversion of organic substances ( substrate chain phosphorylation ( SKP ), Fig. 1, bottom center).
Use of organic matter
Membrane transport and fermentation using the example of a lactic acid bacterium
Chemotrophic organisms that use organic substances absorbed from the outside often have to use energy in the form of ATP in order to transport the energy-supplying substances into their cell interior ( active transport ). In the metabolism of a lactic acid bacterium , shown in simplified form in Fig. 1, a sugar is transported from the outside into the cell interior while consuming energy. The sugar is esterified with phosphate. In the case of this bacterium, the absorbed energy supplier is then broken down via substrate chain phosphorylation ( SCP ). This allows ATP to be regenerated from ADP .
The end product is lactic acid, which is transported out of the cell by a transport protein. This creates an electrochemical pH gradient that the bacterium can use. Using the enzyme ATP synthase , further ATP can be obtained when the H + ions pumped outwards flow back into the cell interior.
Lactic acid bacteria are one of the few organisms that only need to absorb one substance to generate energy. As a rule, at least two substances are required, namely a reducing agent and an oxidizing agent.
Oxidation of organic substances
Animals are chemotrophic and can gain energy from ingested organic substances such as sugar. But because they oxidize the organic substances with O 2 , their energy gain is significantly higher than with their fermentation. The oxidation of the NADH formed by the oxidation of organic substances takes place in animals on the mitochondrial membrane in the respiratory chain . Here an ATP synthase supplies the required ATP according to the principle of chemiosmotic coupling .
The oxidizing agent oxygen does not have to be transported through the mitochondrial membrane while consuming energy, since O 2 can diffuse freely through the biomembrane as an uncharged, small molecule. Some anaerobic chemotrophic organisms use oxidizing agents under anoxic conditions, i.e. in the absence of O 2 , which must be transported into the cell interior using energy. These oxidizing agents are weaker oxidizing agents than O 2 and as a result less ATP is formed than in the case of oxidation with O 2 .
The number of oxidants used by chemotrophic organisms is limited. In contrast, the number of organic substances that are oxidized by these organisms (aerobically or anaerobically) is enormous. It covers almost the entire spectrum of all organically produced substances ( dogma of biological infallibility ) and not a few xenobiotics .
Use of inorganic substances
Even in organisms that use inorganic substances with small molecules to generate energy, the energy-supplying redox reactions take place on a biomembrane with the formation of an electrochemical proton gradient that drives the ATP synthase. The proton gradient is often generated by oxidizing the inorganic reducing agents on the outside of the membrane and reducing the oxidizing agents on the inside of the membrane (see figure). There are also many of these organisms which instead of oxygen use other oxidizing agents with a lower redox potential , so that the molar ATP yield is lower.
Chemoautotrophy and chemoheterotrophy
Chemotrophic organisms that only use inorganic substances to generate energy can usually cover their carbon needs from CO 2 ( autotrophy ). Then one speaks of chemoautotrophy . Such organisms can only be found among bacteria and archaea .
As a rule, chemoautotrophic organisms use inorganic electron donors to reduce CO 2 . This use is known as lithotrophy and in these organisms as chemolithoautotrophy.
- Albert L. Lehninger: Principles of Biochemistry : Walter de Gruyter, Berlin, New York 1987, ISBN 3-11-008988-2
- Rudolf K. Thauer, Kurt Jungermann, Karl Decker: Energy conservation in chemotrophic anaerobic bacteria . In: Bacteriological Reviews . Vol. 41, No. 1, 1977, pp. 100-180.
- Entry on chemotrophy. In: Römpp Online . Georg Thieme Verlag, accessed on July 8, 2015.
- Otto R et al .: Lactate Efflux-Induced Electrical Potential in Membrane Vesicles of Streptococcus cremoris 1982 J. Bact. 149.2 pp. 733-738.
- G. Gottschalk: Bacterial Metabolism. 2nd Edition. Springer, New York 1986, pp. 223f.