In the days of proto-life, less-stable membranes built out of simple molecules, like those in soap, may have been an advantage.
Growing Simple Cells with the Help of... Soap?
Published: March 19, 2021
David Pogue: All life on Earth, from the simplest to the most complex, is made of cells with outer membranes.
So, on the road to life, how did that happen?
Scientists like Anna Wang, a former postdoc in Jack Szostak’s lab, now a professor at UNSW, Sydney, have been working with a simple molecule that is one of the prime suspects.
It’s also present here…
Anna Wang: Wow.
Pogue: …shaped into bars, in a wide variety of colors and scents…
Smells good in here.
Wang: Smells amazing.
Pogue: …at Molly’s Apothecary, outside of Boston.
Wang: Oh, that’s wonderful.
Pogue: That’s right: soap.
Soap’s interesting, because a soap molecule is a combination of two different types of molecules, called polar and non-polar. For example, water molecules are polar. Each one has a concentration of electrons in one part, making it negative, which leaves another part more positive. That’s polarity. And it makes water molecules want to stick together, each negative part attracted to another molecule’s positive part.
An oil molecule, made up of carbon and hydrogen, is an example of a non-polar molecule. It has an even distribution of electrons: no polarity and less stickiness between molecules. In fact, polarity is why oil and water don’t mix. The polar water molecules stick together, keeping the oil molecules at bay. The less dense oil floats on top.
That’s also why trying to clean oily grease off your hands with water alone, doesn’t work very well.
It actually won’t come off, it’s super oily.
The two just don’t interact. And that’s where soap molecules come in. They’re hybrids. At one end, are some negatively charged, electron-rich oxygens, ready to interact with polar molecules, like water, but the rest is a long, non-polar hydrocarbon tail, with no positive or negative charge. It’s more comfortable mixing with other non-polar molecules, like grease.
Put some soap on your greasy hands…
…and the soap’s non-polar tails stick into the grease, while its polar heads act like handles, ready to interact with the water, taking the grease along for the ride.
Here’s another interesting soap fact. Drop some soap into water and the molecules form little balls called “micelles,” with their water-loving polar heads sticking out and their water-hating non-polar tails sticking in.
That naturally occurring little container has piqued the interest of scientists like Anna. Back at the lab, she adds some soap molecules to water containing short fragments of R.N.A. They’ve been tagged with a molecule that makes them glow.
Why R.N.A.? The current scientific consensus is that a primitive form of R.N.A. may have been the first molecule with the ability to replicate itself, jump-starting evolution.
Wang: Now, we’re going to go look at it under the microscope.
Pogue: …the microscope room…
Wang: Let’s go.
Pogue: …where Anna loads up a sample she prepared yesterday.
Wang: So, this is what our soap molecules have self-assembled into overnight.
Pogue: What are they? Bubbles?
Wang: Yes, they are almost like bubbles. And so, what we are looking at here is not the soap molecules themselves but what they’ve been able to trap inside these cell-sized structures.
Pogue: Overnight, the soap micelles have self-assembled into larger spheres, trapping the fluorescing R.N.A. inside. And if we could zoom into one of them, we’d see that it actually consists of two layers of soap molecules, arranged with the water-loving heads toward the inside and outside, and the water-hating tails brought together.
Wang: When you have molecules that have a polar head group and a non-polar tail, but you don’t give them any oil to interact with, the oily tails actually want to interact with each other, and so you end up forming these bilayer structures.
Pogue: Wait, so these are soap molecules and these are also soap molecules?
Pogue: And they like to assemble into this position?
Wang: Yeah, that’s right. So, they like to form these really thin envelopes, and you can imagine this structure extending onwards and onwards and curving around and forming a sphere. And that’s what we’re seeing here. We’re seeing this bilayer structure, encapsulating some green-dyed R.N.A. molecules.
Pogue: This lipid bi-layer structure isn’t alive, but it’s familiar to biologists. It’s similar to the bilayer structure of the membranes that surrounds something that is alive, cells.
Of course, those are much more complicated and more stable containers, better at keeping things in or out, though that feature comes at a price.
Wang: If you take the membranes that we have now, but get rid of all the highly evolved protein machinery, what you’re left with is just an inert sack. It can’t grow, it can’t divide. It can’t even get nutrients in and out.
Pogue: That’s why, in the days of proto-life, less-stable membranes built out of simpler molecules, like soap, may have been an advantage.
Anna shows me an example.
Wang: So, what I am about to do is I have some soapy water in here, and I’m just going to add it. What happens is the soap molecules start incorporating onto the existing membrane.
Pogue: Look at this. Look at this.
Pogue: It just split.
Wang: They look pretty spherical now, but they’re starting to wiggle a bit. And all of a sudden, it looks like they might melt.
Pogue: Cell division.
Our cells grow and divide, because we have something giving instructions.
Pogue: But you’re saying that, billions of years ago, none of that existed.
Wang: There’s none of that in here. So, what we’re kind of simulating is a condition where maybe these protocells have floated somewhere down the stream, and they’ve come across a pool of excess soap molecules, and these soap molecules can join the membrane and grow it.
So, I think what it means is that we can still get simple cells to divide by purely physical mechanisms, and that’s what we’re trying to understand in the field, like how do you get to do things that kind of seem like life and mimic life but without any biology?
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