Energy metabolism
| Parent |
| Subordinate |
|
Energy generation from inorganic substances: Energy generation from organic materials: |
| Gene Ontology |
|---|
| QuickGO |
With energy metabolism (including basal metabolism called) is that portion of the metabolism of living beings , of the extraction of energy used. It differs from the building metabolism ( anabolism ), which is used to build up body components and consume energy.
The energy metabolism consists of chemical metabolism, which in sum are exergonic , i.e. energy-releasing. So material systems are used that are in a thermodynamic imbalance and are converted into a lower-energy, more stable state of equilibrium during their implementation, with energy being released. This type of energy generation is called chemotrophy , the living beings operating it as "chemotroph".
Another way of generating energy is phototrophy , in which light is used as an energy source. Living beings that use light as an energy source are called "phototrophic". Most phototrophic organisms can also gain chemotrophic energy, i.e. through an energy metabolism, for example when there is a lack of light.
Storage of energy and transport of energy carriers
Chemotrophic organisms use the energy released during the exergonic metabolism. They store them for a short time by using them to synthesize energy-rich substances, from which the energy can easily be released again when the synthesis is reversed. Nucleoside phosphates are suitable for this, since energy is released when their phosphate residues are split off and phosphate residues can be bound to them again through the use of energy (for more information, see adenosine triphosphate ). With nucleoside phosphates, energy can be stored and also transported in this way. The most important energy-storing nucleoside phosphates in living organisms are the tri- and diphosphates of adenosine and guanosine (abbreviations ATP , ADP , GTP and GDP ).
Fatty acid synthesis can be viewed as another method of storing energy . This works via the generation of the malonyl coenzyme A through oxidative decarboxylation of pyruvate in glycolysis , through the breakdown of amino acids or through β-oxidation of fatty acids.
Amount of energy
The energy released during the conversion of substances is the change in free energy caused by the conversion , i.e. the difference between the energy content of the converted substances (the educts) and those of the substances formed from them (the products). It depends on the amount converted, the energy content and the concentration of the substances involved in the conversion (the reactants), the temperature and the pressure. The energy content of the substances is defined as the energy required to form these substances from chemical elements . These energy contents are listed in tables.
Often the concentration of the reactants is not known and it changes in the course of the reaction. In these cases the change in free energy cannot be calculated or can only be calculated with difficulty. A clue for the energy released during the conversion of a substance can be obtained by calculating the change in free energy under standard conditions ( denoted by ΔG 0 ). The following standard conditions were agreed: temperature 25 ° C, pressure 1.013 bar, concentration of the substances (reactants) involved in the conversion 1 mol / l with the exception of that of water, for which 55.6 mol / l (pure water) is agreed, and that of gases for which a concentration in solution equilibrium with a partial pressure of 1 bar in the gas phase has been agreed. In biological systems, however, it is not the concentration of 1 mol / l, which is usually not tolerated by living beings, that is agreed upon for the H + ion concentration, but 10 −7 mol / l, corresponding to pH 7, and the value of the change in the Free energy under these conditions as ΔG 0 '.
If the actual conditions deviate from these standard conditions, the amount of change in free energy is also different; it can deviate considerably from the standard value. In living systems, standard conditions are usually not given and often change during the conversion of substances. The amount of change in free energy under standard conditions therefore only provides an indication of the energy released during chemical conversion in living beings.
According to the second law of thermodynamics , part of the energy is converted into heat in all energy conversions . Accordingly, only part of the energy released during energy metabolism can be used by living beings for purposes other than heat generation.
Types of energy metabolism
A distinction is made between fermentative and oxidative energy metabolism.
Anaerobic energy metabolism , also called fermentation (in English fermentation ) referred, leaves the gross reaction, no redox reactions seen. Examples of fermenting energy metabolism:
- Lactic acid fermentation is the conversion of milk sugar (lactose) to lactic acid in lactic acid bacteria :
- C 12 H 22 O 11 + H 2 O → 4 C 3 H 5 O 3 - + 4 H +
- ΔG 0 '= - 478 kJ per mole of lactose
At oxidative energy metabolism ( oxidative phosphorylation , cellular respiration ), redox reactions can also be recognized in the gross sales, based on the consumption of an oxidizing agent and a reducing agent . Examples of oxidative energy metabolism:
- Conversion of grape sugar (glucose) (= reducing agent) with molecular oxygen (O 2 ) (= oxidizing agent) to carbon dioxide (CO 2 ) and water (H 2 O) in animals , humans , many bacteria , also in plants:
- C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O
- ΔG 0 '= - 2822 kJ per mole of glucose
- Conversion of carbon dioxide (CO 2 ) (= oxidizing agent) and molecular hydrogen (H 2 ) (= reducing agent) to methane (CH 4 ) and water (H 2 O) in methanogenic archaea:
- CO 2 + 4 H 2 → CH 4 + 2 H 2 O
- ΔG 0 '= - 139 kJ per mole of carbon dioxide
- Conversion of sulfate (SO 4 2− ) (= oxidizing agent) and molecular hydrogen (H 2 ) (= reducing agent) to hydrogen sulfide (H 2 S) and water (H 2 O) in sulfate-reducing bacteria ( desulfurication ):
- SO 4 2− + 4 H 2 → HS - + 3 H 2 O + OH -
- ΔG 0 '= - 112 kJ per mole of sulfate
- Conversion of nitrate (NO 3 - ) (= oxidizing agent) to nitrogen (N 2 ) and water (H 2 O) in denitrifying bacteria ( denitrification ):
- 2NO 3 - + 12 H + + 10 e - → N 2 + 6 H 2 O
Measurement of energy metabolism
There are different ways of measuring the energy turnover:
- At rest, the organism does not do any external work, if one disregards the practically immeasurable amount for the acceleration of the breath in animals actively breathing air. All energy conversions occurring within the organism, for example in higher animals also the work of the heart and the respiratory muscles, are converted into heat. In equilibrium, i.e. H. in this case at constant temperature, all of the converted energy is given off as heat. The energy conversion can therefore be measured as the amount of heat given off / time unit (“direct” calorimetry ).
- The energy converted in heterotrophic organisms oxidizing with O 2 comes under certain circumstances only from the oxidation of high-energy substances. For a certain substance, there is a stoichiometric relationship between the amount of substance consumed, the amount of O 2 taken in, the amount of CO 2 given off and the energy released. The amount of O 2 taken up and the amount of CO 2 given off can easily be measured. If the oxidized substances and their contribution to total sales are known, the energy released at the same time can be calculated from the gas quantities absorbed and released ("indirect" calorimetry ).
- The energy converted in the organism comes from the nutrients that are supplied with food. In the body's equilibrium, the amount of energy converted must be equal to the difference between the energy content of the food ingested and the energy content of the excretions and can therefore be physically determined in the calorimeter .
literature
- Albert L. Lehninger, David L. Nelson, Michael M. Cox: Principles of Biochemistry. 2nd Edition. Spectrum Academic Publishing House, Heidelberg / Berlin / Oxford 1998, ISBN 3-8274-0325-1 .
- Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: Biochemistry. 6th edition. Elsevier Spektrum Akademischer Verlag GmbH, Heidelberg 2007, ISBN 978-3-8274-1800-5 .
- Rudolf K. Thauer, Kurt Jungermann, Karl Decker: Energy conservation in chemotrophic anaerobic bacteria. In: Bacteriological Reviews. Volume 41, No. 1, 1977, pp. 100-180.