Respirazione anaerobica

Il termine respirazione anaerobica indica una via del metabolismo energetico in cui una catena di trasporto degli elettroni usa come accettore finale un composto ossidato diverso dalla molecola d'ossigeno (ossigeno molecolare) O₂^[1]^[2] che è invece alla base della respirazione aerobica o cellulare^[3].

Questo tipo di metabolismo è presente tra i batteri e i funghi come Neurospora crassa, Aspergillus nidulans e Hansenula anomala che sono in grado di attuare una respirazione anaerobica con accettore finale di elettroni lo ione nitrato NO₃^-^[4] (organismi anaerobi). Si conoscono casi di batteri che usano come accettori finali di elettroni nitrati (NO₃^- derivato dall'acido nitrico), solfati (SO₄^2- derivato dall'acido solforico) o carbonati CO₃^2-^[5].

Batteri denitrificanti

I batteri denitrificanti utilizzano il nitrato (NO₃^-) come accettore di elettroni riducendolo ad azoto molecolare (N₂). La reazione complessiva può essere riassunta in questo modo:

{\ce {2NO3- + 10e- + 12H+ -> N2 + 6H2O}}

Questo metabolismo energetico è stato studiato in organismi anaerobi facoltativi appartenenti ai generi Pseudomonas, Bacillus, Thiobacillus e altri, che utilizzano molecole di nitrato solo quando vivono in carenza di O₂.

La denitrificazione è uno stadio del ciclo dell'azoto, che avviene sia nel suolo che nelle acque, ed è un processo importante per l'eliminazione dei nitrati, soprattutto negli ambienti acquatici, dove questi composti possono provocare fenomeni di eutrofizzazione

Batteri solfato-riduttori

A questo gruppo appartengono micoorganismi anaerobi obbligati appartenenti ai generi Sulfobacteria e Clostridium.

In questo caso l'accettore degli elettroni è lo ione solfato (SO₄⁼) che viene ridotto a ione solfuro (S⁼). La reazione può essere schematizzata così:

{\ce {SO^{2-}4 + 8e- + 8H+ -> S^2- + 4H2O}}

La riduzione degli ioni solfato è una tappa del ciclo dello zolfo

Questi (ed altri) batteri sono alla base di una catena trofica presente nelle profondità oceaniche, in prossimità di sorgenti idrotermali associate alle dorsali: i black smoker.

Batteri metanogeni

L'uso di anidride carbonica (CO₂) come accettore di elettroni da parte di microorganismi porta alla produzione di metano (CH₄) secondo una reazione (metanogenesi) semplificabile in questo modo:

{\ce {4H2 + CO2 -> CH4 + 2H2O}}

Accanto a questa via, però questi organismi presentano la possibilità di produrre metano partendo da composti organici parzialmente ridotti come il carbonio metilico dell'acido acetico. Un'altra via ancora possibile è quella a partire dal Metanolo CH₃OH:

{\ce {4CH3OH -> 3CH4 + CO2 + 2H2}}

I microorganismi in grado di svolgere questo tipo di metabolismo appartengono ad alcune particolari classi di Archea e in particolare agli Euryarchaeota.

Questi batteri sono coinvolti nella produzione del biogas.

Note

^ Davis, Dulbecco, Eisen, Ginsberg, Wood - Microbiology - Harper International Editions, 1973
^ Cimmino - Lezioni di microbiologia - Ed. Porziuncola, 1974
^ In questo caso l'ossigeno si lega all'idrogeno proveniente dalla demolizione della sostanza organica con formazione di acqua secondo la seguente reazione: sostanza organica + O₂→CO₂ +H₂O
^ W J Payne, Bacteriological reviews, Reduction of Nitrogenous Oxides by Microorganisms, dec 1973
^ Stanier, Doudoroff, Adelberg Il mondo dei microrganismi - Zanichelli, 1977

Voci correlate

Altri progetti

Wikimedia Commons contiene immagini o altri file su respirazione anaerobica

Collegamenti esterni

(EN) anaerobic respiration, su Enciclopedia Britannica, Encyclopædia Britannica, Inc.

V · D · M Metabolismo
Metabolismo	Via metabolica · Catena metabolica · Ciclo metabolico · Via anfibolica
Catabolismo	Glicolisi · Beta ossidazione · Decarbossilazione ossidativa del piruvato · Ciclo di Krebs · Fosforilazione ossidativa · Glicogenolisi · Chetogenesi
Anabolismo	Gluconeogenesi · Sintesi proteica · Sintesi degli acidi grassi · Glicogenosintesi · Via dei pentoso fosfati · Gliceroneogenesi
Altre voci	Ciclo di Cori · Degradazione degli amminoacidi · Fotosintesi clorofilliana · Fotorespirazione · Ciclo di Calvin · Ciclo dell'urea · Fermentazione · Ciclo del gliossilato

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