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Thread: Electron transport chain: free electrons flying around, or molecules exchanging electrons? Or both?

  1. #1 Electron transport chain: free electrons flying around, or molecules exchanging electrons? Or both? 
    Forum Freshman GreatBigBore's Avatar
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    So my beginning biology book talks about the electron transport chain in cellular respiration. It mentions many times that electrons are being "pushed up the energy hill" and then "falling down the energy hill". I get the metaphor, but I'm trying to understand the actual mechanics just a bit (not a lot! please don't blow me away with a PhD answer!). The book talks about the electrons changing energy levels, but doesn't that really mean that an electron on a given atom is pushed up to the next shell on that atom? Or maybe that the electron on a given atom is pushed onto another atom, but again, into a further-out shell? And isn't this "pushing up" driven by photons? And doesn't the "falling down" release photons?

    Or are there really electrons just flying around free, as in an electrical current? And their energy levels actually changing, independently of any atoms or molecules?


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    If you look at your biology book there's probably a picture of several parts of respiration including the citric acid cycle (often called Krebs cycle), as well as the transport chain within the mitochondria. Look for any molecules in those pictures with the + sign, the most common ones are NAD+, because of their polarity they can capture e- and are electron receptors. Another one is simply H+ which is easy to combine with an electron. So this isn't so much a change in electric potential shells, but a chain of transfers of electrons as they get passed from receptor to receptor.

    PS, I hope that helps but it's been a while since I looked at respiration (way out of my field)...so I hope some kind biologist comes in and comments.


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    Quote Originally Posted by GreatBigBore View Post
    So my beginning biology book talks about the electron transport chain in cellular respiration. It mentions many times that electrons are being "pushed up the energy hill" and then "falling down the energy hill". I get the metaphor, but I'm trying to understand the actual mechanics just a bit (not a lot! please don't blow me away with a PhD answer!). The book talks about the electrons changing energy levels, but doesn't that really mean that an electron on a given atom is pushed up to the next shell on that atom? Or maybe that the electron on a given atom is pushed onto another atom, but again, into a further-out shell? And isn't this "pushing up" driven by photons? And doesn't the "falling down" release photons?

    Or are there really electrons just flying around free, as in an electrical current? And their energy levels actually changing, independently of any atoms or molecules?
    Yeah, this is the reason I was bored by biochemistry at university: too many woolly-seeming flow schemes and not enough precision about what really happens in terms of chemical reactions. There's another typically frustrating (for me) explanation here: Electron transport chain - Wikipedia, the free encyclopedia

    For example, I can see that NADH -> NAD⁺ + H⁺ involves loss of 2 electrons (i.e. it is an oxidation process), and these electrons must go somewhere, and I can see that if H+ is pumped out of the cell against a concentration gradient it will tend to return, which can be a source of energy. But as usual I cannot fathom, from this article, exactly what the reactions are or what the thermodynamic balance is.

    So as Lynx Fox says, if some kind biochemist is able to explain what is really happening in processes such as this, I too would be very grateful.
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by GreatBigBore View Post
    So my beginning biology book talks about the electron transport chain in cellular respiration. It mentions many times that electrons are being "pushed up the energy hill" and then "falling down the energy hill". I get the metaphor, but I'm trying to understand the actual mechanics just a bit (not a lot! please don't blow me away with a PhD answer!). The book talks about the electrons changing energy levels, but doesn't that really mean that an electron on a given atom is pushed up to the next shell on that atom? Or maybe that the electron on a given atom is pushed onto another atom, but again, into a further-out shell? And isn't this "pushing up" driven by photons? And doesn't the "falling down" release photons?

    Or are there really electrons just flying around free, as in an electrical current? And their energy levels actually changing, independently of any atoms or molecules?
    Yeah, this is the reason I was bored by biochemistry at university: too many woolly-seeming flow schemes and not enough precision about what really happens in terms of chemical reactions. There's another typically frustrating (for me) explanation here: Electron transport chain - Wikipedia, the free encyclopedia

    For example, I can see that NADH -> NAD⁺ + H⁺ involves loss of 2 electrons (i.e. it is an oxidation process), and these electrons must go somewhere, and I can see that if H+ is pumped out of the cell against a concentration gradient it will tend to return, which can be a source of energy. But as usual I cannot fathom, from this article, exactly what the reactions are or what the thermodynamic balance is.

    So as Lynx Fox says, if some kind biochemist is able to explain what is really happening in processes such as this, I too would be very grateful.

    I will consult my books and see what I can do.
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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  6. #5  
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    Quote Originally Posted by Cogito Ergo Sum View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by GreatBigBore View Post
    So my beginning biology book talks about the electron transport chain in cellular respiration. It mentions many times that electrons are being "pushed up the energy hill" and then "falling down the energy hill". I get the metaphor, but I'm trying to understand the actual mechanics just a bit (not a lot! please don't blow me away with a PhD answer!). The book talks about the electrons changing energy levels, but doesn't that really mean that an electron on a given atom is pushed up to the next shell on that atom? Or maybe that the electron on a given atom is pushed onto another atom, but again, into a further-out shell? And isn't this "pushing up" driven by photons? And doesn't the "falling down" release photons?

    Or are there really electrons just flying around free, as in an electrical current? And their energy levels actually changing, independently of any atoms or molecules?
    Yeah, this is the reason I was bored by biochemistry at university: too many woolly-seeming flow schemes and not enough precision about what really happens in terms of chemical reactions. There's another typically frustrating (for me) explanation here: Electron transport chain - Wikipedia, the free encyclopedia

    For example, I can see that NADH -> NAD⁺ + H⁺ involves loss of 2 electrons (i.e. it is an oxidation process), and these electrons must go somewhere, and I can see that if H+ is pumped out of the cell against a concentration gradient it will tend to return, which can be a source of energy. But as usual I cannot fathom, from this article, exactly what the reactions are or what the thermodynamic balance is.

    So as Lynx Fox says, if some kind biochemist is able to explain what is really happening in processes such as this, I too would be very grateful.

    I will consult my books and see what I can do.
    Thank you René!

    I'll look forward to it.
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    GreatBigBore,

    The electron transport chain is "just" a series of coupled redox reactions, conceptually there is little more to it than that. Getting to grips with redox chemistry first will make understanding the ETC a whole lot simpler. In some reactions electrons are indeed being transferred from one molecule to another, in other cases the electrons are actually hydrogen atoms; in "totting" up, it's easier to follow the electrons.

    Also, don't confuse electron energy levels within atoms with "falling down the energy hill". These are two entirely different things. Instead, think of the standard free-energy change that occurs in each reaction (again, a good grasp of redox chemistry is required). Terms like "high energy" or "High energy electron" are very unhelpful terms that pop up all over biochemistry texts; they seem to confuse more often than not.



    exchemist,

    The scale, dynamics and complexity of biology often escapes non-biologists. There is no way to teach the electron transport chain to students in a couple of days or so as part of a larger introductory biochemistry/molecular biology course. Interested students can, if they so desire, dedicate an entire research career to studying just one small aspect of the ETC. Reaction mechanisms galore can be found either in the primary literature or in more advanced texts.
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    Quote Originally Posted by Zwirko View Post
    GreatBigBore,

    The electron transport chain is "just" a series of coupled redox reactions, conceptually there is little more to it than that. Getting to grips with redox chemistry first will make understanding the ETC a whole lot simpler. In some reactions electrons are indeed being transferred from one molecule to another, in other cases the electrons are actually hydrogen atoms; in "totting" up, it's easier to follow the electrons.

    Also, don't confuse electron energy levels within atoms with "falling down the energy hill". These are two entirely different things. Instead, think of the standard free-energy change that occurs in each reaction (again, a good grasp of redox chemistry is required). Terms like "high energy" or "High energy electron" are very unhelpful terms that pop up all over biochemistry texts; they seem to confuse more often than not.



    exchemist,

    The scale, dynamics and complexity of biology often escapes non-biologists. There is no way to teach the electron transport chain to students in a couple of days or so as part of a larger introductory biochemistry/molecular biology course. Interested students can, if they so desire, dedicate an entire research career to studying just one small aspect of the ETC. Reaction mechanisms galore can be found either in the primary literature or in more advanced texts.
    Ha, I had a horrible feeling that might be the answer. Perhaps I should just pick one step and research that in depth as far as I can on the internet. And then see how much energy I have left for any more....

    (I hate things I can't bottom out and have a faint feeling of guilt about all this from 40 years ago when I said "Sod it" and closed the book on it.)
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    Forum Professor Zwirko's Avatar
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    You're definitely not wrong about the "woolly" aspect though. For some reason, that I can't fully understand, biology suffers from a lot of dumbing down. It's always dismayed me to see young people turned off biology completely from their experiences of it at school. My niece has recently dropped biology from her curriculum because it's "so boring and simplistic". She's now focussing on physics, chemistry and maths. At undergraduate level too, students are very much shielded from the difficult stuff. I don't know whether educators think their students are too stupid to cope (maybe they've learned this through experience) or if their courses are far too general and concept based.
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    Quote Originally Posted by Zwirko View Post
    You're definitely not wrong about the "woolly" aspect though. For some reason, that I can't fully understand, biology suffers from a lot of dumbing down. It's always dismayed me to see young people turned off biology completely from their experiences of it at school. My niece has recently dropped biology from her curriculum because it's "so boring and simplistic". She's now focussing on physics, chemistry and maths. At undergraduate level too, students are very much shielded from the difficult stuff. I don't know whether educators think their students are too stupid to cope (maybe they've learned this through experience) or if their courses are far too general and concept based.
    Well, this was Oxford in the 1970s, so I'm not sure I'm brave enough to accuse them of dumbing down! I suspect it is more what you said before, about the layers of complexity being such that it was not practical to go into details about each step if you were trying to convey the whole process. But I felt uncomfortable with things where I could not say how they REALLY worked. To me, that was the beauty of university level study - really getting to the bottom of it all....which is why I went for the quantum chemistry option rather than biochem. Guess these things are personal.

    What you say about biology is apparently also - shockingly- said about school maths in the UK, nowadays. The brighter students are not stretched, their imagination is not fired and they switch off. Tragic.
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    Quote Originally Posted by PhDemon View Post
    Quote Originally Posted by exchemist
    Well, this was Oxford in the 1970s, so I'm not sure I'm brave enough to accuse them of dumbing down! I suspect it is more what you said before, about the layers of complexity being such that it was not practical to go into details about each step if you were trying to convey the whole process.
    It was the same when I did the first year biochem. course there in the 90's -- lots of pretty pictures, very few details of the chemistry. I blamed the tutor but maybe it's a common theme...

    I can assure you that, after 20 years, biochemistry textbooks and teachers alike still use lots of pretty pictures.
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