Chris Kempes
speaker
209 appearances
1 recordings
1 series
first heard Jan 2025
last heard Jan 2025
Chris Kempes’s voice in public audio — every appearance, attributed to the second.
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And I want to be clear here that I'm not talking about these recent world record holders for the longest bacteria. So there are these almost centimeter long bacteria. Oh, my God. Right? I'm going to lose sleep now. Yeah, exactly. But the key thing to note about those is they're actually filaments. And so they're sort of like a colony of many –
um little rod-shaped bacteria like e coli stuck end to end and so why they form these very long filaments um there's many copies of the dna and and their dimension in the other radial dimension is is like an e coli it's a it's a small bacterium so they get very long but they don't get volumetrically big and that's really what we're talking about when we say size in this case
Anyway, we look for the world record holder volumetric organism, something that's roughly a sphere. There we find an organism that's about 100 times bigger than our upper bound. So we thought, okay, this is interesting. How is this thing 100 times bigger than our prediction? Especially since we'd been so pleased by our lower bound predictions. And there's two things to note.
One is this organism has these huge internal storage vacuoles. So it has these big internal membranes that it uses to just store inorganic stuff, likely to be able to grow fast later. So it sort of hoards rare stuff and then uses it later. Um, if you remove all of that and you say, let's just talk about the active sort of metabolic portion of the cell, you're still bigger than our limit.
Um, not by as much, but still a little bit bigger. And here then the other crucial thing to note is that this organism lives in a really resource poor environment. So it lives in these, um, ocean, deep ocean sediments, um, running a metabolism that we know is a really poor metabolism that just doesn't give you much energy. And so they grow very, very slowly.
In fact, they're really hard to study and grow in the lab, and you have to do so in sort of environmental settings. And so what that tells us is that this upper bound that we were describing really was a speed limit. It said, no, there's a fastest growth rate. There's a fastest replication rate. If you want to get bigger than this wall, you just have to grow more slowly.
And so that's actually what the next branch of life, the unicellular eukaryotes does is they grow more slowly. And that's also what large bacteria that break this rule do is they just stay away from that fast growth limit, that speed limit.
Exactly, yeah. And it made us realize, to your point, exactly what we were talking about in our theory, what our assumptions were. It was about maximum growth rate. And so we're confident in maximum growth rate, but there's ways to break that law. You can just dial down your metabolism and be a slow-growing organism.
Yeah, so there's lots of interesting ideas there, and I think where it interfaces what we're doing is just to ask questions about what are the fundamental bounds on how efficient you can be, how much energy you need to perform a process, and so forth. And we've done a little bit of statistical mechanics on the energetics of different cellular machines.
So a bunch of us, including David Wolpert a few years ago, worked out sort of what we thought – the optimal efficiency of something like the ribosome would be and then compared it to the efficiency of the ribosome that we have. And so if you abstract the ribosome, what it really is, is
a device for writing strings, for taking a random set of letters that are unorganized and writing those into exactly one string. That string folds up into a functional protein. And so we could compare that thermodynamically to any device that writes strings. And we find that gives us an ultimate efficiency, the best any device in the universe could do at this abstract process.
And we found that the ribosome is at most 20 times worse than that limit. And I say 20 times off the limit, that seems really inefficient. But for reference, our computers, as we talk now, are writing strings at 100 million times worse than the limit.
So the ribosome compared to our computers is like many, many, many orders of magnitude more efficient than our computers at this string writing process. It's bumping up against the limit in some real sense, even though it's 20, a factor of 20 away. Yeah. Exactly. It's feeling the ultimate physical limit.
No, definitely not. I think the statistical mechanics of cells is something people are working on in lots of interesting ways. As I mentioned earlier, I said bacteria mostly don't have structure. There's been this whole revolution really in the last 15 years around phase separation in cells in bacteria and realizing that different types of stuff are actually forming separated phases.
Think liquid and water and how you could have a little bit of
um sorry think about oil and water and how you could have a little oil droplet floating around in water they're both liquids but they're sort of separated in an interesting way and that's happening inside of cells and that has chemical consequences that has structural consequences and so i think there's a lot to uncover there there's a lot of really fundamental statistical mechanics and thermodynamics to do within cells and then i should say that recently we've we've worked out
limits at the large end of bacteria, where we have come to realize how fundamental constraints of diffusion start to set an upper bound for bacteria, where you need to add internal structure just to make things move around more quickly. And so there's a sort of diffusive boundary that you start to run into, and then you have to add transport structures to sort of get around that.
And that occurs also roughly at the same size that you're seeing this ribosome catastrophe. So just like the small end of bacteria, we're seeing a large end where multiple constraints seem to be limiting cells at pretty much exactly the same size.
Yeah, it's a great point. And I think what's interesting is that we would say once you get one of these walls, one of these sort of hard limits coming from physical constraints, it tells you you need to shift something about your architecture to get over that constraint. if you're going to get bigger.
Now there's lots of cases where you can imagine evolution never discovers anything that helps you get bigger. But if you see something that gets bigger, it certainly has found some new architecture. And so one of the great challenges of modern biology is thinking through how and why we got eukaryotes. And so eukaryotes are
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