How Old Are You?: How The Atomic Age Solved One of Biology’s Greatest Mysteries

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By Bradley Stockwell

My two favorite arenas of academia are science and history, and the more I study the two, the more I see how interwoven they really are. There’s no greater example of this than something called the “bomb pulse”. Whether you know it or not, lurking inside of you is a piece of Cold War history—even if you weren’t alive at the time—and it is this little memento that finally solved one of biology’s most elusive secrets: How old are you? And I don’t mean how many times has the cellular clump of mass known as you swung around the sun, but how old are the individual cells that make up that mass? Your skin cells, heart cells, neurons—your body is constantly renewing itself with new cells and it is only as of 2002 that we began to have a definitive answer for how old each one was. With this post, my intentions are twofold. One: I want to tell you about one of the greatest scientific discoveries of the 21st century, and two: I’m hoping that by wrapping them in this titillating story, I can also slip in a few basic principles of nuclear chemistry. With that said, let’s begin!

Between 1945 and 1963, over four hundred nuclear bombs were detonated, unleashing an untold number of extra neutrons into the atmosphere. Some of these neutrons found their way into nitrogen atoms, causing them to eject a proton. If you’re familiar with some basic chemistry, when a seven-proton nitrogen atom loses a proton, it becomes a six-proton carbon atom. However, because these carbon atoms still have two extra neutrons from when they were nitrogen, they become something called an isotope, a variant of an element which differs in neutrons, but has the same amount of protons. In this case, these slightly more massive and radioactive isotopes become an isotope of carbon called carbon-14.

When I say radioactive, all I mean is that the atom’s nucleus is unstable; that it is emitting energy in the form of ejected subatomic particles or energetic light waves to stabilize itself until it becomes a stable isotope, or a completely new element altogether. This radioactive decay comes in three main forms: alpha, beta and gamma. Alpha decay—which only happens with heavy elements like uranium—is the ejection of something physicists call an alpha particle, but chemists just call it a helium atom, a bundle of two protons and two neutrons. In fact, almost all the helium here on Earth came from this type of decay. Think about that the next time your sucking down a helium balloon; you’re inhaling the atomic leftovers of uranium, thorium and other heavy, radioactive elements. Beta minus decay is the ejection of an electron and beta plus decay is the ejection of the electron’s antiparticle, the positron. Gamma decay is the emission of an extremely energetic light wave called a gamma ray and it is often emitted in conjunction with alpha and beta decay.

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The time it can take for a radioactive element to reach a stable form can be anywhere from instantaneous to far longer than the age of the universe. Because individual atoms decay unpredictably, the way in which we measure this loss is through probability, or something called a half-life. This is the time it takes for half a quantity of radioactive material to decay into a more stable form. This is not to say if you have four radioactive atoms, in x amount of time you’ll have two necessarily, but more that each individual radioactive atom has a fifty percent chance that it will decay to a more stable form in x amount of time. For example, carbon-14, the star of our story, has a half-life of 5,730 years. This means if you had a pound of it, after 5,730 years you’d have a half pound of carbon-14 and half a pound of nitrogen-14, carbon-14’s more stable form. Then after another 5,730 years you’d have a quarter pound of carbon-14 and three-quarters pound of nitrogen-14, and so forth. This is how carbon dating works; by measuring the relative portions of carbon-12 and carbon-14 in a sample of organic matter, archeologists are able to determine its age.

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The period between 1945 and 1963 in which all this atomic testing was happening is now called the “bomb pulse” by the scientific community. It was called this because the amount of carbon-14 in the atmosphere was doubled during this period from all those free neutrons crashing into nitrogen. In 1963, when the Soviet Union, the U.K. and the U.S. agreed to the Limited Test Ban Treaty which prohibited all above-ground detonations, the amount of carbon-14 began to decrease by half every eleven years and will eventually be depleted somewhere around 2030 to 2050. This isn’t because the carbon-14 is decaying into nitrogen-14 (remember the half-life of carbon-14 is 5,730 years), but because it is being absorbed by the life inhabiting our planet, which includes us. Although carbon-14 is an altered carbon atom—a carbon isotope, it still behaves like a carbon atom because it is the number of protons in an atom that determines its chemical behavior, while the number of neutrons determines its mass; and like a regular carbon atom, these carbon-14 atoms have been binding to oxygen, forming CO2, which is sucked up by plants during photosynthesis and then fed to the rest of us through the food chain. Like the plants, our bodies too can’t tell the difference between carbon and carbon-14, so for the last seventy-plus years all this extra carbon-14 has been used by every living creature to build new cells, proteins and DNA.

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While our bodies can’t tell the difference between carbon and carbon-14 (because they have the same amount of protons), scientists can because of their slight difference in mass (remember carbon-14 has two extra neutrons). The difference in mass is measureable through a technique called mass spectrometry, which sorts atoms by weight. Without getting too technical, an instrument called a mass spectrometer strips atoms of some of their electrons and launches them into a magnetic field, which alters the atoms’ course, and because of inertia, heavier atoms take a wider path than lighter ones. By measuring how many atoms travel along certain paths, scientists can determine how much of a specific atom—in this case a carbon-14 atom—is in a sample.

So what does this have to do with determining a cell’s age? Well for a long time nothing. But somewhere around 2002, Krista Spalding, a postdoc at the Karolinska Institute in Stockholm, Sweden, wanted to challenge the longtime doctrine that said the human brain couldn’t create any new neurons after the age of four. There had been growing evidence that the adult hippocampus—a seahorse-shaped region deep in the brain that is important for memory and learning—could regenerate neurons, but no one knew for sure. Spalding and her postdoc advisor, Jonas Frisén, had a hunch that the “bomb pulse” period could somehow offer a solution and it did, culminating in a paper by Spalding, Frisén and their team published in June 2013, which conclusively found that the hippocampus did produce approximately 700 to 1,400 new neurons per day, and these neurons last twenty to thirty years. How you ask? Well there’s an episode of Radiolab (a wonderful science podcast I recommend you all listen to) that has a much more colorful version of Spalding and Frisén’s journey here, but because I know I’m probably already pushing your attention spans, I’ll just give a brief overview. You see, atmospheric scientists have been measuring the amount of carbon-14 and other elements in the atmosphere every two weeks since the late 1950s, giving us an extremely accurate timetable of how much carbon-14 is and was in the atmosphere at any given time after. By correlating this data to the amount of carbon-14 found in a cell’s DNA (while other molecules are regularly refreshed throughout a cell’s life, DNA remains constant), researchers can determine not just the age of a hippocampal neuron, but any cell. So by accident, the nuclear age finally shed light on when tissues form, how long they last and how quickly they’re replaced.

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You—and every other living organism—are continually creating new cells. Cells that make up your skin, hair and the lining of your gut are constantly being replaced, while others, like cells that make up the lens of your eye, the muscles of your heart and the neurons of the cerebral cortex, have been with you since birth and will stay with you until you die.

So why is this so important? Well firstly, it gives us a key insight into the mechanisms behind many neurodegenerative diseases such as amyotrophic lateral sclerosis (Lou Gehrig’s disease), Parkinson’s, Alzheimer’s, Huntington’s and many more. Really we’ve only just begun to dig into this Pandora’s box so to speak, and unfortunately time is limited (well unless we start blowing up a bunch of atomic weapons again, but let’s hope humanity has moved past this) because, as I said, this measureable spike of carbon-14 in our atmosphere from the “bomb pulse” will eventually be depleted somewhere between 2030 and 2050.

Despite what the “bomb pulse” is and will offer to scientific research, isn’t it cool just knowing which cells have been at the party of you the longest? Or that like the rings of a tree, or the sedimentary layers of rock, our bodies too tell the story of our times? With that, until next time, stay curious my friends.

 

 

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How Pencil Lead and Sticky Tape Won a Nobel Prize

 

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By Bradley Stockwell

First off, I want to apologize to all of my six followers to this blog. I know I left you in anxious anticipation over my follow-up post to Climate Change Part I on future green technologies. However, after three months of procrastination, I confess I still haven’t written it. I’m sorry, but I’m easily distracted and while attempting to assemble it I came across a story too good not to tell about a fascinating material called graphene. Graphene is the thinnest, strongest and stiffest material on Earth; it conducts electricity and heat better than any other known material; it is transparent and two-dimensional and is the basis for all future technologies and A.I. At the moment, its potential of applications looks limitless. Oh did I mention it was discovered with nothing more than pencil lead and tape? They even gave the guys who discovered it the 2010 Nobel Prize in Physics. Shit, if I knew it was that easy I could’ve scratched Nobel prize in physics off the bucket list a long time ago.

So what is graphene exactly then? In short, it’s a sheet of pencil lead (graphite) an atom thick. But to understand how we arrived at the discovery of graphene, we need to tell another story, the story of carbon. Graphene is an allotrope of carbon which simply means it’s one possible way to structure carbon atoms. The carbon atom has six protons and typically six neutrons in its nucleus. Sometimes the nucleus has eight neutrons, in which case the carbon atom is known as carbon-14. Carbon-14 is unstable, meaning it radioactively decays, but the decay is consistent over long periods of time. Because this form of carbon is found in many materials, measuring its presence gives us a way to age materials—or what is known as carbon dating. Carbon-14 however is not an allotrope of carbon, it is what is known as an isotope, something covered in detail in a previous post, Flight of the Timeless Photon.

Allotrope formation is dependent on the electrons of a carbon atom and the way in which they bond to other carbon electrons. Carbon has six electrons, two of which are buried in its innermost shell near the nucleus, and four in its outermost shell which are called valence electrons. It is these four outermost electrons—and a ton of heat and pressure—that make the difference between a lump of coal and a diamond, another allotrope of carbon. In diamond, a carbon atom’s four valence electrons are bonded with four other carbon valence electrons. This produces an extremely stiff crystalline structure. In fact, a typical diamond is made up of about a million billion billion atoms (1 with 24 zeros after it) all perfectly arranged into a single pyramidal structure, which is key to its extraordinary strength. But diamond is not the strongest and most stable allotrope of carbon. Although DeBeers may want you to think otherwise, a diamond is not forever; every diamond in existence is actually slowly turning into graphite. The process however takes billions of years so no need to worry about your wedding ring just yet.

Graphite is not a crystalline structure like diamond, but planes of carbon atoms connected in a hexagonal pattern, with each plane having an extremely strong and stable structure—stronger and more stable than diamond. Some of you may be asking, is this not the same graphite we write with and grind up into fine powder lubricants? Yes indeed it is, and the conundrum of descriptives can be blamed on electrons. In diamond, a carbon atom shares its four valence electrons with four other carbon atoms, whereas in graphite it shares its electrons with only three (see graphic below). This results in graphite having no electrons left over to form strong bonds between layers, leaving it up to something called van der Waals forces, a weak set of forces generated by fluctuations in a molecular electric field. Basically it’s the universal glue of matter and is something all molecules naturally possess. Because these forces are so weak is why you’re able to write with graphite—a.k.a. pencil lead. As you press your pencil to paper, you’re breaking the van der Waals forces, allowing layers of graphite to slide across one another and deposit themselves on a page. If it weren’t for the weak van der Waals bonds, pencil lead would be stronger than diamond and this is behind the advent of carbon fiber. Carbon fiber is spun graphite, lathered in an epoxy glue to overcome the weak van der Waals forces. Restriction of van der Waals forces is also behind the phenomenality of graphene.

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Since graphene is a single layer of graphite one atom thick, there is no need to worry about weak van der Waals forces. By default this makes graphene the strongest and thinnest material known to man. Also, because its carbon atoms are not structured in a crystalline lattice like diamond, which leaves no free electrons, it also conducts electricity and heat better than any known material. This means because of its transparency and thinness, we could literally add touch sensitivity to any inanimate object and possibly entire buildings. It also allows for something called Klein tunneling, which is an exotic quantum effect in which electrons can tunnel through something as if it’s not there. Basically it means it has the potential to be an electronic dynamo and may someday replace silicon chips and pave the way for quantum computing. Graphene was purely hypothetical until 2004 when Andre Geim and Konstantin Novoselov discovered it. As stated in the title of this post, they discovered it with nothing more than a lump of graphite and sticky tape. They placed the tape on the graphite and peeled off a layer. They then took another piece of tape and stuck it to the piece of tape with the graphite layer and halved the layer. They continued to do this until they were left with a layer of graphite one atom thick. I’m not exaggerating the simplicity of the procedure in any way. Watch the video below and you can replicate the experiment yourself, the only catch is you need an electron microscope to confirm you indeed created graphene. Until next time my friends, stay curious.