The “Electron Transport Chain”, Grossly Oversimplified

Why does breathing work, anyway? And can I possibly explain it in a couple of paragraphs? I don’t know, but I’m going to try…it leads into the subject that got me interested in majoring in Microbiology in the first place. It’s probably kind of foolish to try to cram in this explanation in the half-hour or so before midnight (and hence the informal deadline for getting a post up every day for “Just Science week”), but here goes:

First, a bit of really simplistic background. Since the fundamental principle of the universe is basically that stuff likes to fall apart (dang lazy molecules), in order for a cell (bacterial or otherwise) to make new proteins and strands of DNA and so on, it has to have some kind of energy that it can use to pay for the increasing orderliness that it’s causing. The chemical that’s usually used to provide this energy is ATP. The energy comes from a string of three Phosphate (PO4) groups that are attached to it. The third phosphate in the chain comes off really easily, releasing a bit of energy in the process, like a spring uncoiling. A lot of enzymes work by attaching to ATP, letting ATP fall apart (becoming the slightly more “relaxed” ADP in the process), and using the released energy to power some other process.

In order for this to work, the cell has to be constantly re-charging ADP, cramming that third phosphate back onto the end along with putting back the bit of chemical energy.

The point of this post is a major way that cells provide the energy to reassemble ATP. There are actually a number of ways, but one of the more effective is the “Electron Transport Chain”.

In simple terms, the cell takes an electron from a simple “food” molecule of some sort, and passes it along to a type of protein that reaches through the cell’s membrane. This protein passes the electron along to another protein, but in the process, it goes through a series of changes in shape that allows it to pump a few hydrogen ions (“protons”) from the inside of the cell’s membrane to the outside. Depending on how much energy (as “electrical potential”) was released along with the electron by the “food” molecule’s electron donation, there may be enough energy to shove the electron through up to three different proteins that do this “proton-pumping” trick with each electron.

This process causes there to be a buildup of protons outside of the cell membrane. Since the universe is lazy, it doesn’t want to hold all those crammed-together protons in place – it really wants to shove them back inside the cell so there’ll be an even concentration of them on both sides of the membrane.

The cell has a special sort of gate which lets the protons shove their way through back to the inside of the cell – but in the process, they make part of the ‘gate’ mechanism rotate. The rotating part essentially grabs ADP and loose phosphate and virtually crams them back into place – the energy to do this comes from the force of the protons shoving their way back into the cell.

But what about the electron? Well, at the end, there has to be something that will pull the electron off of the last protein. One of the best “electron acceptors” is oxygen. Oxygen is the second most electron-loving kind of atom there is. Half of an oxygen molecule (O2), a couple of spare protons, and two electrons make a nice, relaxed, stable molecule of H2O.

The reason I find this interesting is because some bacteria can use something besides oxygen, if oxygen isn’t available. They don’t get quite as much energy out of the process since these other “electron acceptors” don’t pull the electron out quite as hard at the end, but it’s better than suffocating. Sulfate-respiring bacteria, for example, can use sulfate (SO4) as the place to dump the electron, converting it to sulfite (SO3) in the process – and eventually converting it to plain “elemental” sulfur (just “S”) or even in some cases using the elemental sulfur in place of oxygen and making H2S – which is that ‘rotten-egg’ smelling gas.

There are some even more exotic “electron acceptors” that some bacteria can use…which will be the topic of another post.

(And, again, please let me know if you spot anything wrong here, and please ask questions if I’m not making any sense – I’m pretty sure I need the practice explaining this kind of thing…)

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The Author is (currently) an autodidactic student of Industrial and Environmental microbiology, who is sick of people assuming all microbiology should be medical in nature, and who would really like to be allowed to go to graduate school one of these days now that he's finished his BS in Microbiology (with a bonus AS in Chemistry). He also enjoys exploring the Big Room (the one with the really high blue ceiling and big light that tracks from one side to the other every day) and looking at its contents from unusual mental angles.

2 thoughts on “The “Electron Transport Chain”, Grossly Oversimplified”

  1. I loved your post on the electron transport chain, and I had a question for you. I am a biology teacher, and a student asked me a question that I did not know the answer to…

    We have been comparing the electron transport chain in the thylakoid of chloroplasts to that in the mitochondria. We discussed the fact that energy from the sun is required to give electrons the energy to move through the electron transport chain for photosynthesis. A student asked me why the electrons do not need this boost of energy to move though the membrane of the mitochondria also. Can you help?


    Nicole DiLuglio
    KIPP Houston High School

  2. I may be able to help here: the reason mitochondria don’t need an extra input of energy is the same fundamental reason why microbial fuel cells work.

    It’s because oxygen sucks.

    Looking back to this post, I really shouldn’t have used the word “shoved” to describe to movement of the electrons in normal aerobic respiration since they’re not really being pushed through, but rather pulled.

    It all really starts at the end of the chain, with an oxygen molecule yanking electrons away from the last protein in the chain (and latching onto a couple of protons to hold onto them with), because oxygen pulls on the electrons harder than the protein does. On the other hand, the protein pulls harder than the next-to-last protein in the chain, so it yanks electrons away from the next-to-last protein…and so it goes all the way back to the beginning of the chain.

    (I’ve noticed that a lot of atoms use protons to hold onto extra electrons when they get reduced – Nitrogen gets reduced to ammonia (NH3) and Sulfur gets reduced to hydrogen sulfide (H2S), just as oxygen gets reduced to water (H2O)…)

    Photosynthesis goes the other way – it has to yank electrons away from water (which you can also think of as “reduced oxygen”). Since the oxygen would rather be in a reduced state instead of being elemental O2, some energy from light is needed to force the electrons loose from the water. (More light is then needed later in the reaction that uses the electron to reduce NADP+ to NADPH, too.)

    I tried to cram a quick explanation of why this works in microbial fuel cells into a 90-second audio which may be found in a more recent post (or during the second half of the November 6th, 2007 “This Week in Science” radio show [Hooray, now I’m an international radio personality!]), as well as a somewhat longer written explanation in a separate post.

    I hope this is helpful!

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