It's a little cheesy...but I have to admit as a protein chemist, ATP synthase is probably one of the coolest complexes in biology. I mean, it essentially converts the potential energy of a pH gradient into mechanical energy and then into chemical energy. It's a conventional motor, except made of protein.
Right...it blows my mind. What really freaks me out is how small it is too. It's that complex, yet so fucking small. Makes you wonder how much more complex things can get, that we just can't see yet or make the connections with. The nature of particles intrigues me so much because for me it would only makes sense that they are infinitely small and you can always find smaller and smaller bits of matter... Mind fuck.
On the biological scale, it's not small at all ...and things get orders of magnitude more complex than the ATP synthase. Many orders (oxygenic photosynthesis comes to mind). There are a lot of cool protein (and protein/RNA) complexes so much larger than it that can do even more chemistry. And when you really begin to understand how it works at the molecular level, it's humbling to see the incredibly slick ways in which nature "finds a way". So long as it is thermodynamically favorable, life finds a way to extract energy from just about anything. Another great example of this is anaerobic methane oxidation. Some organisms extract so little energy from their main food source that it takes years to multiply, but as long as there is a steady and consistent stream of it and they've found an ecological niche with little or no competition, what difference does it make if they multiple in 30 minutes (E. coli) or years (anaerobic methane oxidizers)?
Of course, it's all amazing. I was just stating in general, the complex things at that scale blow my mind, no matter what it is.
ATP synthase was actually too much for me, when I first learned about it. I remember it clearly, sitting in a dark basement lecture theatre in 2007, not able to comprehend what the fuck could explain this molecular machine, and not really accepting it. I totally fucked up that class, failed it miserably, failed my course and left university - and it all stemmed from that lecture when I essentially gave up on science (temporarily). So yeah, my hat's off to ATP synthase - it blew my mind in the most defeating way that I have experienced.
This here was a cool experiment. Some folks tethered a long, fluorescent protein called actin to the tip of ATP synthase and took snap shots as the synthase spun around and around. On a side note, I once had a toxicology professor who said that if he could "do it all over again", he'd go back and study this complex.