Proton Gradients

The electrochemical proton gradient and ATP synthase

At the inner mitochondrial membrane, a high energy electron is passed along an electron transport chain. The energy released pumps hydrogen out of the matrix space. The gradient created by this drives hydrogen back through the membrane, through ATP synthase. As this happens, the enzymatic activity of ATP synthase synthesiszes ATP from ADP.

The electrochemical proton gradient across the inner mitochondrial membrane is used to drive ATP synthesis in the critical process of oxidative phosphorylation . This is made possible by the membrane-bound enzyme ATP synthase.

The reason proton gradients are required boils back down to chemistry. Life hydrogenates carbon dioxide. In other words, to convert carbon dioxide into organic molecules, life attaches hydrogen atoms to CO2. 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 CO2. 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.

But there's a problem, pointed out by William Martin in collaboration with Russell (Martin & Russell 2007). 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 CO2 is a stable molecule and does not react easily, even with hydrogen — even when thermodynamics says it should react. CO2 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 CO2. Cells need the equivalent of a spark to get CO2 to react, and for cells, that spark is ATP. The problem is that the reaction of CO2 with H2 releases energy, but not a lot — only enough to make 1 ATP. That means cells have to spend 1 ATP to gain 1 ATP, so there's no net gain. If there's no gain, there's no growth; no growth, no life.

Gradients break that cycle. It's not quite true to say that the reaction of CO2 with H2 releases enough energy to make 1 ATP: it's actually enough to make 1.5 ATPs. But of course there's no such thing as 1.5 ATPs, at least not by stoichiometric chemistry; so the spare energy from the reaction, as chemistry, is lost (Figure 4). 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.

The Origin of Complex Life

Two greyscale transmission electron micrographs, which are labeled D and E, show mitochondria in different organisms. The photomicrograph in panel D shows a cross-section through a large dinoflagellate mitochondrion. Because the mitochondrion is folded, different parts of it look like stacked layers in the cross-section. The inner membrane is folded into hundreds of cristae, which look like inward-facing, finger-shaped projections. Panel E shows a cross-section of twelve mitochondria in a Paramecium cell. Each mitochondrion has either a circular or oval shape. The mitochondria are filled with inner membranes, which look like finger-shaped projections or small circles.

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. Why not?

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. 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 . A membrane potential of 150 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. Lacking mitochondria, bacteria cannot grow large and complex, because they can't control respiration over a wide area of 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.

Proton Gradients

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