So a cell takes the energy that comes from burning simple sugars and stores it in small units that are held in a molecule called ATP. The ATP is then shipped to where the cell needs it, and when that part of the cell requires energy, ATP is broken down so that the energy is released. The two molecules into which it is broken down (ADP and P) eventually make their way back to the mitochondrion, so that they can be put back together to store another unit of energy. The process by which all this is done is mind-bogglingly complex. Ask any biochemistry student who is required to memorize all the chemical reactions that take place in order for this to happen in a cell!
Now we know that this process is not only mind-bogglingly complex, but part of it is nearly 100% efficient!
Researchers from Japan studied the step in which ATP is actually formed. Essentially, the process by which the simple sugars are burned produces a high concentration of hydrogen ions (H+) on one side of a membrane. The hydrogen ions want to move from that area of high concentration to an area of lower concentration, but the only way to do that is to move through a molecule called ATP synthase, which is illustrated above. The molecule has two basic components: a rotor system that turns as the ions pass through it (called the Fo portion), and a synthesis unit that forces ADP and P to combine to form ATP (called the F1 portion). As the hydrogen ions flow through the rotor portion (shown in purple in the drawing above), their motion causes the rotor to turn. When three ions pass through, the rotor has made one-third of a complete turn, which is enough energy for the synthesis unit (shown in green in the drawing above) to force ADP and P to join together, making one ATP.1 You can watch this short Youtube video to see an animation of how the system works.
One question that you can ask is, “How efficient is this method for making ATP?” After all, the protons moving through the ATP synthase have a certain amount of energy. When the ATP synthase makes ATP, the result is a certain amount of energy stored in ATP. How much energy is wasted in that process? For every 1 Joule of energy in the hydrogen ion motion, for example, how much energy ends up in the ATP? If 0.9 Joules ends up in the ATP, the system is 90% efficient. If only 0.5 Joules ends up in the ATP, the system is only 50% efficient.
The researchers decided to answer this question, at least for the F1 part of the system. What they found was that the system is about as efficient as the laws of physics allow. Here’s how they put it:2
We found that the maximum work performed by F1-ATPase per 120° step is nearly equal to the thermodynamical maximum work that can be extracted from a single ATP hydrolysis under a broad range of conditions. Our results suggested a 100% free-energy transduction efficiency and a tight mechanochemical coupling of F1-ATPase.
Remember, an ATP is formed for every one-third turn, which is a 120° step. So the researchers found that under lots of different conditions, this part of the process is essentially 100% efficient:3 it converts into ATP all of the energy that the laws of physics allow it to convert! How surprising is that? Well, let’s consider what Gizmag calls the most efficient engine in the world today. It is just over 50% efficient.
I don’t know of any marvel of human technology that comes close to the efficiency of the F1 portion of ATP synthase. Of course ATP synthase was designed by the Almighty Engineer, so it really isn’t reasonable to expect that people could come up with anything so efficient!
1. Neil A. Campbell and Jane B. Reece, Biology, Sixth Edition, Benjamin Cummings, 2002, p. 167.
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2. Shoichi Toyabea, et. al., “Thermodynamic efficiency and mechanochemical coupling of F1-ATPase,” Proceedings of the National Academy of Sciences of the United States of America, 10.1073/pnas.1106787108, 2011.
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3. Please note that this does not mean the process is actually 100% efficient. That is physically impossible. Any time energy is converted, some portion of it must be used to disorder the universe. As a result, the laws of physics tell us that no machine can be truly 100% efficient. What the researchers mean is that the process essentially converts 100% of the energy that the laws of physics allow it to convert.
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