In cells, it is very similar. Proton pumps are a special kind of transporter that push hydrogen ions from areas of low concentration to areas with high concentration.
Ions moving down a gradient release energy, but when they move up a gradient, it takes energy. Diffusion can then use this gradient to capture energy again, as the ions move downhill.
In a cell, using energy to move ions uphill when there are already lots of ions there seems a bit odd. Yet this work gives many benefits to the organisms, including plants.
If plants did not do this, there would eventually be no more concentration gradient to release energy, and the plant would die. Plants can also strap a sugar molecule to a hydrogen ion to be carried across a membrane. So, when concentrations of hydrogen ions are high, the plant can move more sugar into plant tissues.
So far, we have hydrogen ions moving downhill through transporters and releasing energy. We also have these ions moving uphill, into areas of higher concentration, using a proton pump. Pumping against a gradient can be difficult, so the job of proton pumps is hard work. There is one pump that isn't as strong, and so acts a little out of the ordinary.
It is called the Proton Pyrophosphatase Pump. Joshua Haussler, Karla Moeller. Proton Pump Particularities. This illustration shows neural cells sending signals. By volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program.
If you are interested in helping with the website we have a Volunteers page to get the process started. Digging Deeper. Digging Deeper: Depression and the Past. Digging Deeper: Germs and Disease. Digging Deeper: Milk and Immunity.
How Do We See? How Do We Sense Smell? How Do We Sense Taste? How Do We Sense Touch? More to the point, how don't they? Back in the s, Efraim Racker had just figured out the mechanism by which cells glean a little energy from the breakdown of glucose in the absence of oxygen, a pathway known as glycolysis.
The whole pathway is pure chemistry, involving the reaction of one molecule with another, and therefore obeys the laws of stoichiometry ; that is, you can balance the equations. Not surprisingly, Racker and others immediately tried to transfer their insights to the quantitatively far more important process of aerobic respiration, which supplies more than 80 percent of our ATP. But one glaring problem with aerobic respiration is that it doesn't balance.
Exactly how much ATP is produced per oxygen molecule consumed? The amount varies, but it's somewhere around 2. That works out to 28—38 ATPs per glucose — again, a variable number, and never an integer Silverstein Aerobic respiration is not stoichiometric, so it's really not chemistry.
And that's why the long search for a high-energy chemical intermediate a molecule able to transfer the energy from the oxidation of glucose to form ATP was doomed to failure. In place of such an intermediate, Mitchell proposed a proton gradient across a membrane: the proton motive force Mitchell It works much like a hydroelectric dam.
The energy released by the oxidation of food via a series of steps is used to pump protons across a membrane — the dam — creating, in effect, a proton reservoir on one side of the membrane. The flow of protons through amazing protein turbines embedded in this membrane powers the synthesis of ATP in much the same way that the flow of water through mechanized turbines generates electricity. This explains why respiration is not stoichiometric: a gradient, by its very nature, is composed of gradations.
Figure 2: The structure of complex I, the largest protein complex involved in respiration in bacteria and mitochondria, as revealed by X-ray crystallography The structure a suggests the piston mechanism shown b , whereby shunting the piston drives protons across the membrane through three separate channels. The architecture of respiratory complex I. Nature , — Figure Detail Mitchell was completely wrong about some of the details, but his overall conception was right, even revolutionary — literally revolutionary, as in, the ATP synthase enzyme revolves.
The flow of protons through the membrane turbines rotates the stalk of the ATP synthase, and the conformational changes induced by this rotation catalyze ATP synthesis.
This mechanism was first proposed by Paul Boyer, who long disagreed with Mitchell over the mechanics and was later proved right in this particular by John Walker, using X-ray crystallography. Boyer and Walker shared the Nobel Prize in The molecular biological achievements of the last two decades culminated in with the deciphering of the crystal structure of another respiratory complex, the enormous for a protein complex I, by Walker's Cambridge colleague Leonid Sazanov Efremov et al.
Again, the structure betrays the mechanism — in this case not a rotary motor but, even more surprisingly, a lever mechanism not unlike the piston of a steam engine Figure 2. Yet without in any way decrying these virtuosic accomplishments, the questions that drove Mitchell in the first place remain surprisingly unanswered.
We know in nearly atomic detail how respiration works. We know far less about why it works that way. Mitchell worked on mitochondria because he could; they are a tractable experimental model. But he came at the question from the standpoint of bacterial physiology — how do bacteria keep their insides different from the outside? Throughout his life, Mitchell saw the detailed mechanism of respiration in this far broader sense: Membrane proteins can create gradients across a membrane, and these gradients can in turn power work.
Proton gradients powering ATP synthesis were just a special case to Mitchell. What he can hardly have envisaged so clearly was the pervasive role of protons.
Although cells can generate sodium, potassium, or calcium gradients, proton gradients rule supreme. Protons power respiration not only in mitochondria, but also in bacteria and archaea members of another domain of prokaryotes, which look much like bacteria but have very different biochemistry. Proton gradients are equally central to all forms of photosynthesis , as well as to bacterial motility via the famous flagellar motor , a rotary motor similar to the ATP synthase and homeostasis the import and export of many molecules in and out of the cell is coupled directly to the proton gradient.
Even fermenters, which don't need proton gradients to generate ATP, maintain the proton motive force, using ATP derived from fermentation to power proton pumping. In short, Mitchell knew protons were important, but he could hardly have guessed at just how important. But why protons? Figure 3 Figure Detail For the last two decades, Russell has been the dynamic force behind the emerging paradigm shift in our understanding of the origin of life. Drawing on a background in ore geochemistry many ores are precipitated by hydrothermal vent systems , Russell postulates that alkaline vents, akin to the modern Lost City vent system in the mid-Atlantic Figure 3 , were the ideal incubators for life, providing a steady supply of hydrogen gas, carbon dioxide, mineral catalysts, and a labyrinth of interconnected micropores natural compartments similar to cells, with filmlike membranes; Lane et al.
Alkaline vents are, in essence, electrochemical reactors that operate in a state far from equilibrium. But the centerpiece of Russell's conception lies in natural proton gradients. Four billion years ago, alkaline fluids bubbled into what would then have been mildly acidic oceans CO 2 levels were about a thousand times higher than they are today, and CO 2 forms carbonic acid in solution, rendering the oceans mildly acidic. Acidity is just a measure of proton concentration, which was about four orders of magnitude four pH units higher in the oceans than in vent fluids.
That difference gave rise to a natural proton gradient across the vent membranes that had the same polarity outside positive and a similar electrochemical potential about millivolts [mV] across the membrane as modern cells have.
Russell has long maintained that natural proton gradients played a central role in powering the origin of life. There are, of course, big open questions — not least, how the gradients might have been tapped by the earliest cells, which certainly lacked such sophisticated protein machinery as the ATP synthase. There are a few possible abiotic mechanisms, presently under scrutiny in Russell's lab and elsewhere. But thermodynamic arguments, remarkably, suggest that the only way life could have started at all is if it found a way to tap the proton gradients Lane et al.
Net growth is not possible. In the graph, energy is shown on the y-axis. A horizontal, dashed line shows the starting level of ATP. The graph is a bell-shaped curve starting at the dashed line, rising above it, and ending below it. The rising portion of the curve shows that one ATP molecule is necessary for the activation energy to get the chemical reaction started. The production of only a single ATP molecule is counteracted by the energy usage of one ATP molecule, so there is no net gain in energy.
Life hydrogenates carbon dioxide. In other words, to convert carbon dioxide into organic molecules, life attaches hydrogen atoms to CO 2. There are only so many ways of doing this, and all life uses just five primary pathways. All but one of these costs energy for example, the energy of the sun in photosynthesis. The exception is an ancient pathway called the acetyl-CoA pathway, in which hydrogen gas is reacted, via a few steps, with CO 2. This pathway is exothermic releasing energy that can be captured as ATP right through to pyruvate, one of the central molecules in cell metabolism.
It's "a free lunch that you're paid to eat," in the words of Everett Shock. All cells that use the acetyl-CoA pathway today depend on proton gradients. None of them can grow by fermentation — that is, by the chemistry of glycolysis. Why not? Because CO 2 is a stable molecule and does not react easily, even with hydrogen — even when thermodynamics says it should react. CO 2 is a bit like oxygen in this respect: Once it starts to react, it's not easily stopped.
But a fire needs a spark to get it going, and so, too, does CO 2. If there's no gain, there's no growth; no growth, no life. Figure 5: Why chemiosmosis solves the problem If a reaction doesn't release enough energy to generate 1 ATP, it can be repeated endlessly until it has pumped enough protons to generate 1 ATP.
Chemiosmosis allows cells to save loose change, so to speak. Seventeen protons are shown on one side of the membrane as a result of this proton pumping. The accumulation of protons drives ATP synthesis by ATP synthase, which is depicted on the right side of the diagram as a red circle and cylinder in the membrane. Grey boxes show that energy release by the electron transport chain on the right can be separated from ATP synthesis by ATP synthase on the left.
It's not quite true to say that the reaction of CO 2 with H 2 releases enough energy to make 1 ATP: it's actually enough to make 1. But of course there's no such thing as 1. But that doesn't happen with a gradient Figure 5. In principle, a reaction can be repeated over and over again, just to pump a proton over a membrane.
When enough protons have accumulated, the proton motive force powers the formation of ATP. So a gradient allows cells to save up protons as "loose change", and that makes all the difference in the world — the difference between growth and no growth, life and no life.
Figure 6 Despite their power, protons have their share of problems, and these problems might explain why life got stuck in a rut for 2 billion years. All complex life on Earth today is composed of a certain type of complex cell, known as a eukaryotic cell.
Generally much larger than bacteria or archaea, the eukaryotic cell contains a nucleus , and a much larger genome , and all kinds of specialized organelles little organs , such as mitochondria. The strange thing is that eukaryotes have repeatedly given rise to large, complex, multicellular organisms like plants, animals, fungi, and algae — but prokaryotes show little or no tendency to evolve greater morphological complexity, despite their biochemical virtuosity.
One possible answer relates to the control of proton gradients. All eukaryotic cells turn out to have mitochondria, or once had them and later lost them by reductive evolution back toward a prokaryotic state. No mitochondria, no eukaryotes Figure 6. All mitochondria capable of oxidative phosphorylation have retained a tiny genome of their own, which appears to be necessary to maintain control over membrane potential Allen A membrane potential of mV across the 5-nanometer membrane gives a field strength of 30 million volts per meter — equivalent to a bolt of lightning.
This huge electrochemical potential makes the mitochondrial membranes totally different from any other membrane system in the cell such as the endoplasmic reticulum which, according to Allen, is why mitochondrial genes are needed locally in cellular subregions. In effect, by responding to local changes in electrochemical potential, they prevent the cell from electrocuting itself. No mitochondrial genome, no oxidative phosphorylation.
It could be, then, that bacteria can't expand in cell and genome size because they can't physically associate the right set of genes with their energetic membranes. If that's the case, the acquisition of mitochondria and the origin of complexity could be one and the same event. The question is, what kind of a cell acquired mitochondria in the first place? Most large-scale genomic studies suggest that the answer is an archaeon — that is, a prokaryotic cell that is in most respects like a bacterium.
That begs the question, how did mitochondria get inside an archaeon? The answer is a mystery but might go some way toward explaining why complex life derives from a single common ancestor, which arose just once in the 4 billion years of life on Earth. Peter Mitchell's demonstration that ATP synthesis is powered by proton gradients was one of the most counterintuitive discoveries in biology, and it took a long time to be accepted. The precise mechanisms by which a proton gradient is formed and coupled to ATP synthesis chemiosmotic coupling is now known in atomic detail, but the broader question that drove Mitchell — why are proton gradients so central to life?
Recent research suggests that proton gradients are strictly necessary to the origin of life and highlights the geological setting in which natural gradients form across membranes, in much the same way as they do in cells. But the dependence of life on proton gradients might also have prevented the evolution of life beyond the prokaryotic level of complexity, until the unique chimeric origin of the eukaryotic cell overcame this obstacle.
Allen, J. The function of genomes in bioenergetic organelles. Efremov, R. Nature , — doi Lane, N.
0コメント