Primeira Fase da Fotossíntese - Fotoquímica ou Luminosa

Monday, March 21, 2011

     Ocorre quando a planta está iluminada. Simplificando, é quando há a fotólise da água, redução do NADP e do NADPH, desprendimento de oxigênio. Acontece nas membranas dos tilacóides, e apresenta as seguintes etapas: 

1- Absorção da luz

     A energia luminosa é absorvida pelas moléculas da clorofila. Nesse processo a energia luminosa excita a clorofila, que perde elétrons para uma substância aceptora de elétrons. A clorofila recupera seus elétrons a partir da molécula de água, que é então quebrada originando o gás oxigênio (O2). Essa reação é denominada fotólise da água, ou Reação de Hill em homenagem ao seu descobridor.

2H2O + LUZ ----> O2 + 4H+ (íons higrogênio) + 4e- (elétrons)

     O gás oxigênio é liberado para a atmosfera, onde representa 21% do volume do ar atmosférico. Praticamente todo gás oxigênio presente na atmosfera é resultante do processo de fotossíntese.

2- Transporte de elétrons

     Os elétrons da molécula da clorofila que foram excitados e transferidos ao aceptor de elétrons continuam sendo transferídos de aceptor para aceptor atraves de uma CADEIA DE TRANSPORTE DE ELÉTRONS. O último aceptor dos elétrons é o NADP (do inglés Nicotinamide Adenine Dinucleotide Phosphate). que se trensforma em NADPH ao receber os elétrons. Ampliando a reação teremos:

2H2O + 2NADP+ + LUZ ----> O2 + 4H+ + NADPH

3- Produção de ATP

     A energia liberada pelos elétrons ao passar pelos elos da cadeia de transporte força os prótons a passar pela membrana dos tilacóides, no sentido do estroma do para o lúmen. Com o acúmulo de prótons no lúmen, estes tendem da deixar esse compartimento por uma proteína de membrana que sintetiza ATP a partir de ADP e fosfato quando há fluxo de protóns deixando o lúmen dos tilacóides em direção ao estroma.


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