The electron transport chain

The electron transport chain





Electron Transport Chain

The electron transport chain is a series of protein complexes embedded in the mitochondrial membrane. Electrons captured from donor molecules are transferred through these complexes.

Coupled with this transfer is the pumping of hydrogen ions. This pumping generates the gradient used by the ATP synthase complex to synthesize ATP.

The following complexes are found in the electron transport chain: NADH dehydrogenase, cytochrome b-c1, cytochrome oxidase, and the complex that makes ATP, ATP synthase.

In addition to these complexes, two mobile carriers are also involved: ubiquinone, and cytochrome c.

Other key components in this process are NADH and the electrons from it, hydrogen ions, molecular oxygen, water, and ADP and Pi, which combine to form ATP.

At the start of the electron transport chain, two electrons are passed from NADH into the NADH dehydrogenase complex. Coupled with this transfer is the pumping of one hydrogen ion for each electron

Next, the two electrons are transfered to ubiquinone. Ubiquinone is called a mobile transfer molecule because it moves the electrons to the cytochrome b-c1 complex.

Each electron is then passed from the cytchrome b-c1 complex to cytochrome c. Cytochrome c accepts each electron one at a time. One hydrogen ion is pumped through the complex as each electron is transfered.

The next major step occurs in the cytochrome oxidase complex. This step requires four electrons. These four electrons interact with a molecular oxygen molecule and eight hydrogen ions. The four electrons, four of the hydrogen ions, and the molecular oxygen, are used to form two water molecules. The other four hydrogen ions are pumped across the membrane.

This series of hydrogen pumping steps creates a gradient. The potential energy in this gradient is used by ATP synthase to ATP from ADP and inorganic phosphate. The ATP synthesis steps you see here are discussed in greater detail in the ATP sythase gradients animation.

This animation illustrates two full cycles of electron donation. In biological systems, however, many electron transport cycles occur simultaneously--helping to ensure that the proton gradient is always maintained.



The electron transport chain is embedded in the inner membrane of the mitochondria. It consists of four large protein complexes, and two smaller mobile carrier proteins.



NADH is the electron donor in this system. It initiates the electron transport chain by donating electrons to NADH dehydrogenase (blue).



NADH donates two electrons to NADH dehydrogenase. At the same time, the complex also pumps two protons from the matrix space of the mitochondria into the intermembrane space.



The two electrons are now transferred to the mobile carrier protein known as ubiquinone. Ubiquinone transports the electrons, two at a time, to the next complex in the chain.






Ubiquinone (pink) delivers two electrons at a time to cytochrome b-c1 (red).



As each electron makes its way through the complex, a hydrogen ion, or proton, is pumped from the matrix space of the mitochondria into the intermembrane space, helping to maintain the proton gradient.



After affecting the pumping of a proton across the membrane, the electron leaves cytochrome b-c1 and enters the mobile carrier protein, cytochrome c (purple).



The mobile carrier protein cytochrome c (purple) transfers electrons, one at a time, to cytochrome oxidase (orange). Four electrons must be transferred to the oxidase comples in order for the next major reaction to occur.



The next major event is the reaction of the four electrons, a molecule of O2 (oxygen), and eight protons.



The reaction results in the pumping of four hydrogen ions across the inner membrane into the intermembrane space, and the release of two H20 (water) molecules into the matrix space.



ATP synthase accepts one proton from the intermembrane space and releases a different proton into the matrix space to create the energy it needs to synthesize ATP. It must do this three times to synthesize one ATP from the substrates ADP and Pi (inorganic phosphate).



With the supply of NADH exhausted, the electron transport chain can no longer maintain the proton gradient that powers ATP synthase, and ATP synthesis comes to a stop.

Electron Transport Chain 1

The byproducts of most catabolic processes are NADH and [FADH2] which are the reduced forms. Metabolic processes use NADH and [FADH2] to transport electrons in the form of hydride ions (H-). These electrons are passed from NADH or [FADH2] to membrane bound electron carriers which are then passed on to other electron carriers until they are finally given to oxygen resulting in the production of water. As electrons are passed from one electron carrier to another hydrogen ions are transported into the intermembrane space at three specific points in the chain. The transportation of hydrogen ions creates a greater concentration of hydrogen ions in the intermembrane space than in the matrix which can then be used to drive ATP Synthase and produce ATP (a high energy molecule).

Points to Ponder 2
It was the extremely high improbability of any one of the thousands of biologically significant molecules, like the twenty or more enzymes and carrier molecules involved in cellular respiration, being formed by just chance and the laws of nature (never mind the need for the untold millions of each one of them to allow for life) that alerted scientists to there having to be an intelligent agent within the cells telling them how to make them. This is what first motivated scientists to search for and find the DNA molecule and everything else connected to it that has been, and continues to be, discovered. But, paradoxically, modern evolutionary biologists see all of the information packed into the DNA molecule and still conclude that it all came about by just chance and the laws of nature alone rather than a mind at work(i.e. an intelligence). In other words, scientists, using their ability to detect intelligence, recognized that there had to be an intelligent agent inside the cell instructing it on how and when to produce these complex and vital molecules. But after finding it they concluded that this intelligent agent itself had come about by chance and the laws of nature alone. Alternatively, many people now believe and teach that it was nature itself, as the intelligent agent, that through evolution, brought about DNA and all of the innovations needed for life because that was what was needed. They seem to forget that, by definition, evolution, as described by evolutionary biologists, is a blind process which has no goals.

The body’s ability to get the energy it needs to live from the breakdown of glucose in the presence of molecular oxygen is dependent on over twenty different enzymes and carrier proteins arranged in a specific order. Each one of these proteins, which contribute to cellular respiration, is made up of three hundred or more amino acids in a specific order that work together in a specific pathway to get the job done. Clinical experience teaches that the absence or malfunction of any one of these components results in death as shown above by the examples of cyanide and arsenic. Dr. Michael Behe has called a system where the absence of any one part renders it useless as being irreducibly complex. It certainly looks like the numerous enzymes and carrier proteins needed for cellular respiration demonstrates irreducible complexity.

However, just having an irreducibly complex system does not automatically allow for survival. In the case of cellular respiration, to get enough energy to stay alive, not only does there have to be enough of each of the enzymes and carrier proteins present but they must also work in the right order and be effective enough as well. A chain is only as strong as its weakest link and a machine is only as efficient as its slowest part. So too, the body’s ability to get the energy it needs to work properly, whether at complete rest or at high levels of activity, is dependent on, not only the mere presence of the various enzymes and transport proteins of cellular respiration, but also the amount and efficiency of each of them as well. Besides being irreducibly complex, systems that allow for life must also have a natural survival capacity. By this I mean that each system must give the organism the capacity to survive by taking into account the laws of nature. This usually involves having a knowledge of what is needed to keep the organism alive within the laws of nature and then being able to do what needs to be done. Cellular respiration, which uses specific molecules in a specific order to allow the body to get the energy it needs to survive, seems to know what is needed to get the job done and does it naturally.

To hear evolutionary biologists tell it, all you have to do is show how each of the enzymes and carrier proteins involved in cellular respiration may have come about from some natural process, such as gene duplication, and that in and of itself should be sufficient to confirm that the cellular respiration came about solely by chance and the laws of nature alone. But this is a preposterous notion. It’s like trying to explain how the engine that is able to power a go kart eventually developed into one that can power a dump truck without taking into account everything else that’s different between them. For these innovations, like cellular respiration being able to obtain enough energy from the glucose molecule, did not develop within a vacuum and therefore should not be considered in isolation from what must have been going on within these intermediate organisms for them to survive in the first place. To do so seems to be a tad simplistic and frankly, unscientific. Moreover, nowhere is it discussed by evolutionary biologists how incredibly lucky it was that each of the enzymes and carrier proteins needed in the pathway just happened to come together in the right order to do the job. Nor how fortunate it was that the final result of cellular respiration was to provide the human body with enough of the energy it needs to survive in the world.

Given what we know about how life actually works and how easily it dies when it doesn't have enough energy, it is evident that for cellular respiration to have developed naturally within living organisms that could reproduce, would have required several simultaneous innovations. What those innovations were and exactly how those intermediate organisms were able to get enough energy to live in these intermediate phases may never be known. This is because further changes which may have come about have since gone by the wayside of historical science and evolution and we can only see what is present now. This is one way to explain how cellular respiration may have evolved without having to seriously consider the physiology of the now extinct intermediate organisms. But this is not Science, where every aspect of the reverse engineering needed to come up with a plausible explanation should be explored before a theory is proclaimed to the public. No, this is faux science and just wishful thinking. It’s also how evolutionary biologists have been able convince themselves, and others, of the supposed irrelevance or even impossibility of irreducible complexity. Some scientists have argued that the positions of intelligent design and irreducible complexity are arguments from ignorance which lack enough imagination. I would submit that the concerns put forth above are based, not on ignorance, but on what we actually do know about how life actually works and how easily it dies. But I wholeheartedly agree that, based on the current evolutionary theory of how cellular respiration came into being, with no consideration of any of the factors mentioned above, that evolutionary scientists do indeed have very good imaginations. Alas, we who believe that the design seen in nature is real, and not an illusion, are forced to limit our imaginings to what is already known about what it takes for life to survive within the laws of nature.

The laws of nature have put up many obstacles to prevent life from existing. Just as a car can die from not having enough gas for energy, or oil for seizing parts, or anti-freeze for engine overheating, so too, all physicians know that there are many different pathways to death. If you really want to begin to understand how life came into existence, you first have to understand how easily it can become non-existent. Did life really come about solely by random chemicals coming together to form cells, then simple organisms, and then complex ones like us? In other words, without a mind at work to make it happen? Do you think that the over twenty different enzymes and carrier proteins, each consisting of over 300 amino acids, just happened to come together in a specific pathway, called cellular respiration, to provide our cells with the energy they need to live? No, when it comes to the origin of life it seems to me that Science still has a lot of explaining to do. Meanwhile, as we wait for evolutionary biologists to admit the deficiencies within their theory, our children and the whole world continue to be misled!



1) http://chemwiki.ucdavis.edu/Biological_Chemistry/Metabolism/Electron_Transport_Chain
2) http://www.arn.org/docs/glicksman/bic1502.html

Tan, Change; Stadler, Rob. The Stairway To Life:
The Electron Transport Chain 
Assuming for the moment that a natural means exists for producing the membrane, we next need a means of extracting energy from food to generate a proton gradient. In life, this involves an electron transport chain (except for methanogens and acetogens ). The most common electron transport chain includes the coordinated effort of three complexes of molecules that are attached to the membrane. The word “complex” here has a double meaning and cannot be underestimated. The following description (although somewhat technical) is essential to provide some perspective of the complexity of the common electron transport chain. The three complexes strip electrons (typically from a carbon source of “food”) and pass the electrons very carefully down a chain of reactions in a process that simultaneously pumps protons across the membrane. The common waste products are CO2 and H2O—similar to the combustion of fuel in your car, except that the cell does not use violent explosions; it uses a careful and extraordinarily precise sequence of about fifteen reactions. Stripping electrons from carbon is not so difficult; harvesting the resulting energy is the real challenge. Electrons are highly reactive and difficult to control. The three complexes handle the electrons like a game of hot potato, quickly passing them from one stop to the next without dropping them. If electrons escape the chain of reactions, free radicals result, causing damage to important biomolecules. If a cell, even a simple bacterium, detects too many free radicals from the electron transport chain, the cell is smart enough to realize that it could become a danger to its neighbors. The altruistic cell responds by entering “programmed cell death mode,” otherwise known as apoptosis, wherein an organized and safe disassembly of the cell occurs. The complexity of sensing free radical formation and the resulting apoptosis are another chapter in the complexity of life. In the electron transport chain, the extraction of energy by passing the electrons from one step to the next requires proteins of exquisite accuracy. The process involves electron tunneling, a quantum phenomenon that occurs over distances of only a dozen or so angstroms (an angstrom is approximately the diameter of a single hydrogen atom). An increase of one angstrom in distance between steps of the electron transport chain decreases the speed of the reaction by about tenfold. Regressing to our childhood for a moment, the process has some similarity to a Slinky “walking” down a staircase. The Slinky at the top of the stairs has potential energy that dissipates as it descends the steps. But unlike what the Slinky commercials show, we all know how rarely a Slinky succeeds in reaching the bottom of the stairs. Success requires precision and no obstacles; failure means that the Slinky will be stuck on a step and will block the next Slinky from traveling down the steps. The electron transport chain is similar, except with substantially higher precision (each step must be within a few angstroms of the expected distance or else the reaction will stop) and a substantially higher penalty for failure (free radical formation could destroy the step or the entire cell). This implies that very precisely formed and folded proteins must be responsible for the process, calling into question the possibility of arriving at such complex proteins via thousands of successive small innovations, fueled by random mutations and honed by natural selection. The three complexes are creatively named Complex I (also known as NADH-Q Oxidoreductase), Complex III (also known as coenzyme Q-cytochrome c oxidoreductase), and Complex IV (also known as cytochrome c oxidase). Complex II exists but is an alternative entry point to the electron transport chain. A simplified version of Complex I, as found in the bacterium Thermus thermophilus, consists of an assembly of sixteen separate proteins, where each protein is composed of hundreds of amino acids. In contrast, Complex I in eukaryotes requires forty-four separate proteins. For every two electrons shuttled through Complex I, four protons are pumped across the membrane. The composition of Complex III varies from three proteins in simple organisms to eleven distinct proteins in vertebrates. The passage of two electrons through Complex III also pumps four protons across the membrane. Complex IV consists of six proteins in simple organisms and fourteen proteins in mammals. Complex IV pumps two protons across the membrane for each pair of electrons that is transferred to oxygen. For those who are keeping count, the complete electron transport chain pumps ten protons across the membrane for each transported pair of electrons. Complex I, III, and IV together require a minimum of twenty-five proteins (sixty-nine in mammals) just for the process of pumping protons, and these proteins must fold and combine such that successive “steps” in the electron transport pathway align to within about ten angstroms of each other. With these three complexes, a proper membrane, and a source of fuel, the cell is now capable of producing electricity. In our houses, electricity typically maintains 110 or 220 volts between two insulated conductors. In mitochondria, the insulating membrane is about fifty angstroms thick, and the voltage difference maintained across the thin membrane is between 0.15 and 0.2 volts. That may not seem like a lot of voltage, but it equates to an electric field of roughly thirty million volts per meter, which is similar in strength to a bolt of lightning.