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 so people often just say bacteria as the small stuff, not realizing that there's an entire world in bacteria of diversity and different cell sizes and trade-offs and lifestyles and environments. And so even there, we're talking about half the range of body size differences that we see in mammals in terms of orders of magnitude. And so a factor of 10,000 is definitely nothing to sneeze at.
Exactly. So bacteria or prokaryotes, they have some amount of internal structure. And this is something we've learned a lot more about recently. But in general, you can think of them as some sort of membrane. And sometimes that's two layers of membrane. And then inside that membrane is a bunch of more or less free-floating stuff.
That includes a free-floating genome that's not packaged in a nucleus. It includes a bunch of large molecular machines, often called macromolecules, all of the enzymes and proteins of the cell. And mostly this stuff just diffuses around and interacts in a very complicated but messy network to do all of the functions that a cell needs to do to metabolize.
to synthesize new parts and eventually to divide, to import and export material from the cell, and for larger cells to swim around and follow nutrients and make some decisions about their environment. But yes, they lack a lot of the internal structure that we see even for other unicellular organisms. So there are other single cell organisms that have more internal structure,
bacteria tend to be these very simple things. Not quite the simplest life because I'm a viruses are life too sort of person. And we could get into why I think that's the case. So I would count viruses as the smallest life and the simplest example of really an enclosure and some genetic material inside that enclosure. Bacteria then have this active goopy stuff inside.
Exactly. The DNA is just... So it's actually a circular chromosome in many cells. So it's a DNA that forms this entire loop. And that loop is not a rigid loop. It's malleable and moving in time. And so think about sort of a...
a circular chain inside a cell that's jostling around and sort of moving all over the place while interacting with a bunch of molecules that read off the genetic information and copy the genetic information when the cell divides. And so it's in some ways sort of amazing that without much structure and in such a messy environment, we get such reliable organisms, right? Yeah.
is a sort of unbelievable number of prokaryotes on the planet. And they evolve very quickly. They live in every environment you can imagine. They divide reliably. They're just quite amazing creatures.
Yeah, so something we like to say is that if you have a dominant physical constraint, it will also tell you where there's a wall, where something becomes... asymptotically limiting. And what I mean by asymptotically is just something goes off to infinity and you can't keep up with it, or something goes to zero and then it does you no good.
And so often if you're writing down some optimization equation and it has physical constraints in it, you should expect to see a limit at some scale. And so we see these in lots of different groups of organisms, but particularly in bacteria, we have pretty well worked out what the lower limit and the upper limit are.
And so if you think about the smallest bacteria, they have this membrane that is shrinking down. If you imagine a spherical cow, and here we really can imagine a spherical cow because the small bacteria are little spheres. So imagine the small spherical bacterium. And as it shrinks down, the membrane has its own finite thickness. And so imagine you have a really thick membrane.
tire, and you shrink down that tire, eventually the rubber is touching the rubber on all sides. And you don't have a tire anymore with something in the middle, you have just a hockey puck. The same thing is happening for cells. So they have this membrane that is very thin until you get to really small sizes, and then it starts to represent a large fraction of the cell size.
And then you still have to inside of that in the remaining volume fit all of the stuff. So you have to put the DNA in there. You have to put a few functional proteins to actually run the metabolism. And you have to put in this amazing molecular device known as the ribosome, which is sort of this generic tape reading device that takes information from the DNA and turns it into functional proteins.
And so in these small cells, the DNA starts to take up roughly half the cell volume. And so you're running into all of these space constraints where you almost can't fit in just the information of what the cell is. You know, you think about how strange that is. The storage system for information becomes sort of half the cell volume.
And you get to a point where every single gene that you would eliminate kills the cell. So every gene knockout, as we call it, is fatal. And so you can't eliminate any more genes. You're stuck with this minimal genome.
And so the smallest cell is defined by this point where you have a minimal genome, a handful of ribosomes, and the functional proteins that do all the other metabolic and physiological aspects of the cell.
What's amazing is that we write down energetic models of the cell and we think about what's the energy budget of the cell, what it has for running biosynthesis, the production of new stuff, and what it has for maintenance, which is just repairing and the repair and upkeep of existing stuff. then at these tiny cell sizes, the maintenance starts to take over the entire metabolism.
So you start to look more and more like a cell that can only repair itself. It actually can barely grow any new stuff for replication. And this space constraint and this maintenance constraint, where maintenance metabolism becomes the whole metabolism, both happen at basically the same tiny cell size. And so we think it's this dual constraint that sets the smallest possible bacteria.
amazingly, those predictions have agreed with all of the world record holders for smallest bacteria. And it's where you transition to seeing tiny viruses that crop up at these really small cell sizes. So we think this physical constraint tells you the smallest possible bacterium.
And you could. And that's a really great question, Sean, because that's a game, especially in an astrobiological context, we like to think about a lot, which is, well, but the DNA takes up a certain volume. And we're committed to a certain chemistry there. So what if you change that a little bit? And the membrane is made up of this lipid bilayer.
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