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In the highlighted paragraph from NCERT textbook, they mention that cellular organisation is the defining property of life forms while metabolism without exception is also the defining property of life forms. My doubt is whether the term 'metabolism' also includes the metabolism reaction in vitro condition too or metabolism only means all the chemical reactions occuring in the cell excluding in vitro condition. If what I mean is true, does cellular organisation and metabolism both mean the same thing? But then again if both are the same thing, metabolism also should be the defining property of life forms, but it is not the case as mentioned by my textbook. So that means both metabolism and cellular organisation are different things. Is this right?

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This paragraph appears to draw a distinction between "the sum total of all reactions in cells" and "some isolated reactions that occur in cells". The former is "metabolism" and one defining trait of life; the latter are "metabolic reactions" and can occur in a test tube, and if so they do not fit tidily in a living/nonliving distinction (the paragraph talks about "not living things" but "surely living reactions", which is one way of illustrating the confusion I guess).

As to whether "metabolism" and "cellular structure" are the same thing, I think it's not so much that they're literally the same thing but they're not really separable, are they. The cellular structure is made by "all the chemical reactions in the cell", and "all the chemical reactions in the cell" are mediated by the cellular structure. It's a bit like trying to separate out "a race" from "the people running in the race". You can have one or two metabolic reactions happening in vitro, but if you had all of them in your test tube, happening the same way they do in the cell, you'd have a cell.

The reality of course is that there isn't a single all-encompassing definition of "life", so don't try and nitpick things too much. Metabolism is important because it's what allows living things to do what living things do, thermodynamically speaking. Cellular structure is important because it happens to be the way all living things implement their metabolism, and arguably it is a necessary requirement for metabolism to occur (there are arguments for why you need a small space enclosed by a membrane for example). Other things are also important like reproduction (and other near-universal feature of life that is a basic thing living things do, and allows Darwinian evolution), being subject to Darwinian evolution (because that is the process that leads life to being optimized in the way it is, and able to do the things it does to begin with), sensing and reacting to its environment (all this stuff about "what life does", well, those are universal things life does)... That means you can have arguable examples like viruses, which I would expect your textbook to call "nonliving" because they do not have metabolism or a cellular structure, but some others might call "living" because they have genetic information, replicate, are subject to Darwinian evolution and share a common ancestor with all other known life. The labels are less important than the features one is discussing at any one time. I expect your textbook probably says something like this at some point.

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  • $\begingroup$ thank you so much! $\endgroup$
    – R. Anusha
    Dec 17 '20 at 11:18

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