Autor: PROFESSORA REGINA MADEIRA | Publicado: 2012-03-25T10:46:00-07:00

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C.E.S.F.T. - BIOLOGIA - TURMAS 1001/1002 - 09/02/2012 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;


&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ORIGEM DA VIDA


Até o presente momento, a Teoria do Big Bang é utilizada para explicar o
surgimento da Terra. Acredita-se que nosso planeta se formou há 4,5 bilhões de
anos e, durante cerca de um bilhão de anos, sofreu processos importantes, como
seu resfriamento, viabilizando o surgimento da vida.



Estudiosos mais antigos acreditavam que os seres
vivos surgiam espontaneamente da matéria bruta – a hipótese da geração
espontânea, também chamada de abiogênese. Entretanto, por meio de diversos
experimentos, executados por cientistas, como Redi, Needham, Spallanzani e Pasteur,
foi possível descartar essa hipótese, adotando a biogênese, que afirma que os
micro-organismos surgem a partir de outros preexistentes.

Embora tenha respondido uma grande questão, a
biogênese não explica como se dá o processo de surgimento de uma espécie a
partir de outra. Assim, existem algumas explicações para tal, sendo a origem
por evolução química a mais aceita pela categoria científica. Essa teoria
propõe que a vida surgiu a partir do arranjo entre moléculas mais simples,
aliadas a condições ambientais peculiares, formando moléculas cada vez mais
complexas, até o surgimento de estruturas dotadas de metabolismo e capazes de
se autoduplicar, dando origem aos primeiros seres vivos. Oparin, Haldane e
Miller são os precursores dessa hipótese.

Atualmente, acredita-se que o primeiro ser vivo
era autotrófico. Dois motivos justificam sua ampla aceitação: o fato do planeta
provavelmente não dispor de moléculas orgânicas suficientes para sustentar as
multiplicações dos primeiros seres vivos até que a fotossíntese surgisse, e o
fato de que, em razão da instabilidade do planeta, estes organismos só
conseguiriam sobreviver se estivessem em locais mais protegidos, como fontes
termais submarinas dos mares primitivos. Assim, a hipótese autotrófica sugere
que os primeiros seres vivos surgiram primeiramente em ambientes mais extremos,
nutrindo-se a partir da reação entre substâncias inorgânicas, tal como algumas
archaeas atuais: processo este denominado quimiossíntese. Essa hipótese sugere
ainda que, a partir desses primeiros seres vivos, surgiram aqueles capazes de
realizar fermentação, depois os fotossintéticos e, por último, os seres
heterotróficos.

Acredita-se que esses primeiros indivíduos eram
procarióticos, compartilhando diversas semelhanças com as arqueas; e, há cerca
de dois bilhões de anos, surgiu a célula eucariótica.
Mariana Araguaia

Graduada em Biologia

Equipe Brasil Escola



&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ATIVIDADES NO CADERNO DO ALUNO