In my new book What Do You Think You Are? I look at a whole range of influences that make you the person you are, from the evolutionary process that shaped us as humans to your consciousness, personality and genetics. But one fundamental aspect is the atoms making up your body.
At the basic level, you are made of just four types of particles, which have been around for the majority of the lifetime of the Universe. In one sense this is very reductionist. You might rightly argue that you are far more than a collection of particles. However, it would be silly to deny that they exist and that they are the objects that make you up.
Building blocks of everything
The particles inside you aren’t quite the ones that you may have come across if you only studied science to high school level (or some time ago).
Most familiar will be the electron. Like the other three, this is a ‘fundamental’ particle, meaning that they aren’t made of anything simpler. Flows of electrons create electrical currents, while it’s the quantity and distribution of electrons around the outside of atoms that determine how they will behave chemically. Electrons are small. Around a million, trillion, trillion of them weigh a kilogram.
We have known about electrons since the late 1890s. But the names of the other component particles that make you up – quarks and gluons – did not enter the language until the 1960s.
You might be wondering what happened to protons and neutrons. These are the familiar particles making up the central nucleus of an atom, but each has sub-components. Every proton is made up of two up quarks and one down quark, held together by a flow of gluons, while each neutron contains one up quark and two down quarks, again linked by gluons.
Read more about the human body:
In one sense, it’s worth noting, though, that there is a seriously important component missing from that analysis. That is nothing. By far the biggest constituent of you, is emptiness. The void. This isn’t a nihilistic plunge into the darkness of the soul, but a realistic assessment of your composition.
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Let’s take a zoom in on the simplest type of atom in your body, a hydrogen atom. If we could somehow visualise what goes on at the sub-microscopic scale, somewhere in the middle would be a single proton, made up of quarks and gluons.
Around the outside, in a fuzzy region of probability, would be an electron. And in between would be loads of nothing. The hydrogen atom is around 99.9999999999996 per cent empty space.
An old simile for the relative size of the nucleus and the hydrogen atom is a fly in London’s Albert Hall. Another way to look at it is that if the atom were the size of the Earth, the nucleus would be about 200 metres across – the rest is empty space.
Chemical components of you
What we’ve seen so far is a physicist’s view. For many, a more familiar way to look at those building blocks of you is as chemical elements.
This pushes the number of basic components up from four basic particles, but bearing in mind there are about 7 billion billion billion atoms in a typical 70 kg (150 lb) human, it’s still quite a simplification to realise that we can account for 99.95 per cent of your body weight with just eleven elements.
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You’ve probably heard that the majority of your body is water. It might seem unlikely: the body feels quite solid. But most of you is made up of cells filled with water. There’s enough structure to make it unlikely that you will run down the drain, but there is plenty of water there. The most common figure is that around 60 per cent of your body is water – even your bones are about 30 per cent aqueous.
Knowing as we do that water is made up of hydrogen and oxygen – H2O – it might seem that the elements topping the charts for body weight should be hydrogen and oxygen, but there’s also a huge amount of that most versatile of atoms, carbon, present. All life as we know it incorporates water and is based on carbon structures.
Because carbon atoms are 12 times the weight of hydrogen atoms, carbon comes second to oxygen by weight, with about 65 per cent oxygen, 18 per cent carbon and 10.2 per cent hydrogen in your body.
Throw in a small amount of nitrogen (3.1 per cent), a pinch of calcium for those bones (1.6 per cent), 1.2 per cent phosphorous, around 0.25 per cent each of potassium and sulphur, with smaller percentages of sodium, chlorine and magnesium, and you’ve hit that 99.95 per cent mark.
How much is your body worth?
One way to assess what goes into making you up at is to look at the value of the elements in your body on the open market. This isn’t an easy calculation, but one estimate is around £125 ($160).
Such estimates vary hugely. To see why, consider the oxygen and hydrogen. The estimate above used a cost per kilogram of £0.17 ($0.20) for both. But water costs less than this – my latest water bill gives a charge of £0.13 per kilogram, and that’s not a cheap way to obtain water.
In total, by this estimate, the hydrogen and oxygen in your body are worth around £9.40 ($11.40), but this is far outpriced by the 160g of potassium in your makeup, which was given a value of £86 ($104), dominating your body’s chemical worth.
Again, if we try to buy an equivalent amount of potassium, we get wildly varying prices. For lab-quality potassium metal, for example, I’d have to pay around £414 ($500). On the other hand, a banana contains around 0.4g of potassium – so 400 bananas give us the required 160g. I can get these from a supermarket at a cost of £56 ($68), and even less wholesale.
It’s clear that we’re never going to reach an exact value. Others estimate the chemical makeup of the body costing anything from £0.83 ($1) to £1,650 ($2,000).
In the high figure, for example, hydrogen dominates, because it has been priced at £83 ($100) per kilo, based on the price of hydrogen fuel for cars. The low value used old data and almost certainly involves a calculation error. Even so, we get a feel for the cost of what is inside you.
The life story of an atom
Each atom in you came from somewhere. The atoms in your body are constantly being replaced at different rates – some remain only hours, others for a few years, but over a ten-year period the majority will have been replaced. And there are only two obvious ways to join your body – the air that you breathe and the food and drink that you consume. The atoms that become incorporated in your body were previously in the air, plants, animals and minerals.
If we could follow an individual atom back through its history, it will have been incorporated many times into other animals and plants. There are so many atoms involved that your body incorporates atoms that were previously in the body of the historical celebrity of your choice.
Bear in mind that you contain around 100,000 times more atoms than the number of humans that have ever existed. In fact, your atoms have been in pretty well every type of living thing, from trees to grass, insects to dogs.
Read more about atoms:
- If atoms are mostly empty space, why is matter not transparent?
- Could we ever study particle physics in a more elegant fashion than just smashing atoms?
The very same atoms will have been in dinosaurs, while throughout the presence of life on Earth many will also have been in bacteria. With the exception of a few atoms produced by radioactive decay, every atom in your body already existed when the Earth formed around 4.5 billion years ago.
So, if your atoms were already in existence when the Earth came into existence, where are you actually from? The Solar System formed from space-borne gas and dust which itself could only have had two sources.
The earliest of these is, effectively, the Big Bang around 13.8 billion years ago, responsible for the production of your hydrogen. The rest of the atoms were produced in stars, which then exploded in vast cosmic convulsions known as supernovae.
Over time, the clouds of dust and gas spewed out started to pull together under the force of gravity. The largest quantities of material congregated in the centre, eventually producing a star – in our case, the Sun.
Further out, accumulating dust and gas resulted in the formation of planets. Finally, we had the opportunity for all those elements that would end up in your body to do something. The very active young Earth mixed things up nicely. The dice were loaded for your eventual atomic assembly.
Brian is a writer of popular science books, with a background in experimental physics. The topics he writes on range from infinity to how to build a time machine. He has also written regular columns, features and reviews for numerous magazines and newspapers, and given lectures at the Royal Institution in London, Oxford and Cambridge Universities, and Cheltenham Festival of Science.