quinta-feira, 16 de julho de 2009


Metabolismo: Respiração Celular

A respiração celular pode ser aeróbica (com presença de O2) ou anaeróbica (sem presença de O2).
A respiração celular aeróbica é composta por 3 fases:
Glicólise: Ocorre no citoplasma, tanto em eucariotos como em procariotos. Não necessita de O2 para ocorrer.
Ciclo de Krebs (ou ciclo do ácido cítrico): Em eucariotos ocorre no interior das mitocôndrias e nos procariotos ocorre no citoplasma.
Cadeia respiratória: Nos procariotos ocorre no citoplasma e nos eucariotos ocorre nas cristas mitocondriais.
Ao longo da cadeia respiratória ocorre libertação gradual de energia, à medida que os elétrons passam de um transportador para outro. Esta energia libertada vai ser utilizada na síntese de moléculas de ATP (fosforilação oxidativa), a partir de ADP+Pi, dissipando-se alguma sobre a forma de calor. Cada molécula de NADH permite a síntese de três moléculas de ATP, enquanto que a molécula de FADH2 apenas permite a síntese de duas moléculas de ATP. No final da cadeia transportadora, os elétrons são transferidos para um aceitador final - oxigênio, que capta dois prótons H+, formando-se uma molécula de água.

Saldo energético:Glicólise: 2 moléculas de ATP + 2 NADH.
Ciclo de Krebs: 2 moléculas de ATP + 8 NADH + 2 FADH2.
Cadeia respiratória:
2 NADH da glicólise → 6 ATP
8 NADH do ciclo de Krebs → 24 ATP
2 FADH2 do ciclo de Krebs → 4 ATP
Total: 34 ATP.




A respiração anaeróbia envolve um receptor de elétrons diferente do oxigênio e existem vários tipos de bactérias capazes de usar uma grande variedade de compostos como receptores de elétrons na respiração: compostos nitrogenados, tais como nitratos e nitritos, compostos de enxofre, tais como sulfatos, sulfitos, dióxido de enxofre e mesmo enxofre elementar, dióxido de carbono, compostos de ferro, de manganês, de cobalto e até de urânio.
Outras moléculas, como NO2, SO2 são os aceptores finais na cadeia de transporte de elétrons.

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