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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Appearances
And what if you used a smaller molecule than lipids? And so one can really absolutely start to play that. And this is a place actually where we're separating the contingencies of evolution from what might be truly universal.
So yeah, if you could find a smaller enclosure that does all the same things that our membrane does in terms of it's flexible enough, it keeps the inside separated from the outside, it has the right sort of hydrophobicity, all those sorts of things. Yeah, there's certainly molecules that would be thinner in a layer. And then yeah, you probably could get a little bit smaller.
So I think this is a case where We think we understand the constraints that matter, why the smallest cells on Earth are the smallest they can be. And then we could play a little bit of games with chemistry to say, how much could you push that lower limit in size? In fact, that game was was played a bit because the Allen Hills meteorite, which was this meteorite from Mars.
Where under a microscope, people thought they were seeing structures that really looked like cells or the remnants of cells or some sort of precipitate from a cell. And this is famous in the astrobiology community because Bill Clinton gave a press conference saying he might have potentially found life on Mars or evidence of life on Mars. I think infamous is the word you're looking for, not famous.
Yeah. You're right. I forgot my prefix. But this, because of how big a news story this was and the implications, the National Academies put together one of these National Research Council reports. where they gathered experts from paleontology and biophysics and biochemistry to try and say, are these things too small?
So one of the initial sort of critiques was, yeah, but these structures are really small. They're smaller than the smallest cells we see. Are we actually talking about fossils here? And so in this report, which is really wonderful, people work through all these different angles to say, yeah, this is probably too small. So people work through, okay, what's the...
what's the most curvature you could get from some polymer or some sort of molecule? And are these things below the curvature radius for that molecule such that we don't think you could actually encapsulate with molecules at this scale? And then there's all sorts of other rich material in that report. And basically every paper in this report says these are too small.
But this is exactly the game that we have to play to try and understand what we might be looking at somewhere else.
That's a great point. Yeah. I mean, you could even play a – I don't know if we have an astrobiology for other universes yet. But you could play that game. You could say, you know, imagine you were in a universe where the atoms were smaller.
Yeah, exactly. Assuming you can still get all the other physics you need, what's the smallest possible cell in such a universe?
Yeah, so the maximum size is really interesting. That actually concerned us for a long time. So when we first sort of started uncovering what we think was limiting the smallest bacteria, we then had nice sort of laws for how things like growth rate would change with increasing cell volume. And so we thought we understood the trends in bacteria as well as the small limit.
But then we had this huge puzzle about why is it that you don't just keep being bigger and bigger bacteria? Right. So what's happening? You know, the biggest bacteria are growing really fast per unit volume. In fact, growth per unit volume is increasing as bacteria get bigger. you have lots of metabolic power. Everything seems great for the biggest bacteria.
Why do you have this major evolutionary transition to unicellular eukaryotes, which we can get into in a bit, where everything shifts and growth rate decreases and all of that? Why is there a motivation for that? Why aren't we all just giant sacks of bacterial stuff walking around and building cities and talking on podcasts? And
What we proposed as a hypothesis in one of those early papers was it's likely that as growth rate gets faster and faster, eventually the basic biochemistry just can't keep up, right? So... if I have some chemical reaction that can only go at some maximum rate or I have some molecular device that can only work at some maximum rate, eventually it can't keep up with my overall growth rates.
So it just can't keep up with how fast I'm trying to move. I could pump more and more energy into it and it just can't turn over quickly enough. And we were very used to thinking about that with macroscopic machines where they all have an upper threshold where you're just trying to push them too quickly.
And so we said, let's look at all of now the detailed biochemistry, all of the macromolecules, all of these different components of the cell, and ask how they change as cells scale up from very small sizes to very large sizes. And as we do that, we see, uh-oh, there's this one component, the ribosome, which again is this device that's sort of the center of biosynthesis.
It's turning genetic information into functional proteins. It's actually building the functional proteins. And we realized for it to keep up with this runaway growth rate, eventually you would need more of that device than you have cell. So you would need to pack more. You need more of this machine that can actually fit inside the cell. We call this the ribosome catastrophe.
And it sets the largest possible cell size. What's really happening there is that the ribosome is making more ribosomes. So it actually makes some of its own components. And so you reach, it has its own replication rate. So it's a machine that replicates itself with one rate. And eventually the cell growth rate starts to push up against that ribosome replication rate. And that is an impossibility.
And so that's where you get one of these, something goes off to infinity. You need infinitely many ribosomes. That can't happen. You overpack the cell. And so we think there's this upper size limit there. Yeah.
Exactly. And so we did that. So we said, okay, we know where this asymptote is, and let's look at the data. And so interestingly, what you see is that you get less and less species, less and less known species of bacteria as you get closer and closer to this wall. And then we said, well, what's the world record holder bacteria?
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