Assimilation (biology)

Assimilation (Latin assimilatio approximation, integration ) is the mass and energy exchange in which recorded, foreign substances - are converted into endogenous compounds - mostly by supplying energy. Ie substances from the environment are converted into components of the organism. A distinction is made depending on your perspective between carbon, nitrogen, sulfur, phosphate and mineral assimilation.

Carbon assimilation

The carbon assimilation (or C- assimilation ) is the most important process of assimilation.

Heterotrophic organisms ( animals, fungi, protists, most bacteria) build on endogenous substances from organic material, which they found in the environment and thus serve among other things as a carbon source.

Autotrophic organisms exhibit carbon dioxide ( CO2) in the carbon dioxide assimilation by the supply of energy and with the aid of a reducing agent -energy, simple organic substances forth, which are converted in the further metabolism to more complex molecules.

Photoautotrophic organisms use light as an energy source. This form of assimilation is therefore called photosynthesis. An example of this is the formation of D- glucose ( C6H12O6 ) from carbon dioxide ( CO2) and water using light energy:

The light energy that is needed for this is, under standard thermodynamic conditions 2872 kJ / mol.

• All green plants and some bacteria ( cyanobacteria ) use in photosynthesis water as a reducing agent or electron source. It also oxygen is generated so that a oxygenic photosynthesis is present.
• Different bacteria use hydrogen (H2), hydrogen sulfide (H2S ), sulfur or iron ( II) ions as reducing agent. Since in this case no oxygen is released, this is called a anoxygenic photosynthesis.

Chemoautotrophic organisms (some bacteria) use chemical energy they gain from exergonic chemical conversion processes. This form of assimilation is therefore called chemosynthesis.

• As a reducing agent using inorganic substances, for example, hydrogen (H2), hydrogen sulfide (H2S), sulfur, iron ( II) ions, ammonia (NH3) or nitrite. These reducing agents are oxidized to produce energy at the same time.

Phosphate assimilation in plants

The roots of the plants to import phosphate ions ( HPO42 ) from the soil via H / PO43 - symporter in the cell membrane of rhizodermis cells. Phosphate is used, inter alia, as a substrate for the phosphorylation of adenosine diphosphate ( ADP) to adenosine triphosphate ( ATP) in the cytosol ( glycolysis ), in mitochondria ( citric acid cycle ), and the chloroplasts (photosynthesis ). The added phosphate group can be used in further reactions for the synthesis of sugar phosphates, phospholipids, or nucleotides.

Sulfur assimilation in plants

Sulfur is mainly in the form of sulfate ( SO42 - ) from verwitterndem rock on H / SO42 - recorded ( cf. phosphate assimilation ) and especially in leaves symporter reduced in several steps to the amino acid cysteine. Glutathione, ferredoxin, NADH, NADPH and O- acetylserine act here as electron donors. Cysteine ​​is in the plastids of the substrate for the synthesis of methionine is, the second sulfur-containing amino acid. The sulfur of the two amino acids can subsequently into proteins, acetyl -CoA, or S -adenosylmethionine are incorporated and transported in the form of glutathione via the phloem to shoot, root tips and fruits, where there is no sulfur assimilation occurs.

Nitrogen assimilation in plants

Plants and many bacteria exhibit nitrate ( NO3- ) or ammonium ago (NH4 ) nitrogen-containing organic compounds.

Nitrate assimilation

The nitrate assimilation of plants is artabhängig mainly in the root or in the shoot. Via reduction to nitrite and ammonium it flows into the synthesis of asparagine and glutamine.

Nitrate ( NO3- ), is taken as sulfate and phosphate via a H - symport in the roots. In the cytosol, nitrate is reduced to nitrite by nitrate reductase ( NO2 ). The reducing agent used mainly NADH, NADPH addition also in non-green tissues. The conditional dephosphorylation by light of a specific serine residue of nitrate leads to the activation of this enzyme, while darkness leads to phosphorylation and thus enzyme inactivation. Therefore, nitrate is assimilated mainly in the day ( during photosynthesis ). Nitrite is transported into the plastids and there reduced by the nitrite to ammonium. The electrons required for the reduction yields ferredoxin, which receives from the NADPH formed in the pentose phosphate pathway in oxidative roots electrons. In green tissues, the electrons from the photosynthetic electron transport chain come from. Expression of nitrite reductase genes is increased by light and increased nitrate concentration while asparagine and glutamine inhibit as end products of nitrate assimilation enzyme production.

In summary, in formulas:

Step 1 ( nitrate reductase ): NO3- NADH H → NO2 NAD H2O

Step 2 ( nitrite reductase ): NO2 6 8 Fdred H → NH4 6 Fdox 2 H2O

Ammonium assimilation

In the plastids of the glutamate - ammonium ligase and glutamate synthase catalyze the incorporation of nitrogen into the amino acids glutamine and glutamic acid ( glutamate ):

As an electron donor for the synthesis of glutamic acid is used in root plastids and NADH in the chloroplasts of leaves ferredoxin. Ammonium can be assimilated simultaneously glutamate dehydrogenase:

Electron donor for this reaction is NADH or NADPH in the mitochondria in the chloroplasts.

The built- in glutamine and glutamate nitrogen is inserted via transamination with other amino acids. These reactions catalyzed by aminotransferases in accordance with the binding of the amino group of an amino acid to the carbonyl group of an intermediate of glycolysis (3- phosphoglycerate, phosphoenolpyruvate and pyruvate ), or from the tricarboxylic acid cycle ( α - ketoglutarate and oxaloacetate ).

Provides an example of transamination of aspartate aminotransferase:

The aspartate formed here (an amino acid ) is a substrate for asparagine:

Asparagine as amino acid is not only a substrate for protein synthesis, but is also based on the high N: C ratio of the storage and transport of nitrogen.

The expression of asparagine synthetase genes is reduced by light and carbohydrates. Therefore, the regulation of this enzyme occurs complementary to the regulation of enzymes involved in glutamine and glutamate synthesis (glutamine or glutamate synthase). Consequently, (a lot of light, high carbohydrate concentrations) favors the synthesis of carbon-rich materials of glutamine and glutamate with sufficient power availability; in energy scarcity ( low light, low carbohydrate concentrations) outweighs the synthesis of asparagine -carbon for storage and transport of nitrogen.

Nitrate and ammonium assimilation allow plants to produce all the amino acids necessary for their metabolism. People and animals can not synthesize certain amino acids and have these as essential amino acids from their food that comes directly or indirectly from plants. The essential amino acids include histidine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Many legume species relate in the context of nitrogen assimilation of ammonia ( NH3) from the symbiosis with bacteria of the genus Rhizobium ( " rhizobia " ), the elemental nitrogen (N2 ) reduce to ammonia, while algae ferns ammonia from the symbiosis with N2 -reducing cyanobacteria receive.