Where Do Trees Get Their Mass?

As someone with a great curiosity for the world around me, it’s more than a little embarrassing for me to admit that I never wondered where trees get all their mass. A seed gets placed in a pot of dirt or straight in the ground, and years later it’s this massive fixture, roots deep in the earth, branches full of countless leaves. Where does it get all the matter to build itself?

Once the question was asked, it kind of ate at me, and perhaps it’s eating at you now. Fear not, because I will provide not only the answer, but also an understanding of what was necessary to prove it was correct. By the time we’re done you’ll learn something very cool, and won’t feel bad for not knowing the answer immediately. Let’s start with intuition, because that’s often the best place to start with science.

As kids we learn everything a plant needs to grow. It needs sunlight, water, and soil. Plants need sunlight as an energy source for photosynthesis, which is how the plant produces the fuel that it needs to live. (Animals consume food and – through a separate but similar chemical process – accomplish the same end.) Water is a key component of all known forms of life, so it’s not surprising that plants need it as part of their metabolic process – as part of photosynthesis. That leaves the soil, which apart from being something to hold the plant in place doesn’t have an obvious job here. The roots are used to take up water, but maybe they are taking up something else and that’s what the plant is made of.

That was the predominant theory for a very long time. The first truly scientific test of this idea was conducted by 17th century chemist Jan Baptist van Helmont, and his experiment was simple yet ingenious. Helmont placed a willow tree in a pot of soil and weighed it. Over the course of five years he tended to the plant, but was careful to neither add nor remove soil from the pot. After five years he found that the plant had gained a mass of 74 kg but the soil had only lost tens of grams, which he estimated to be within the error of his measurement. The only logical conclusion was that the 74 kg the tree gained could not have possibly come from the soil. By Helmont’s accounting the only other thing he added to the tree was water, so trees must be composed almost entirely of water!

Many living organisms (humans included) are largely made of water, and if you’ve ever ripped a living branch off a tree and tried to burn it then you won’t be surprised to learn that plants have a lot of water in them. Does it make sense for plants to be almost entirely water? Knock on a sturdy tree trunk; it sure doesn’t feel like water.

Ah, but from our learned position we know that each water molecule is made up of one oxygen and two hydrogen atoms (H2O). Perhaps the tree breaks down the water into hydrogen and oxygen. Trees produce oxygen, so it must be given off and the hydrogen retained. So trees are made of hydrogen!

Wait, no. That won’t work. If trees were made of pure hydrogen then they would go up like the Hindenburg when they caught fire. That can’t be the case, so we must still be missing something.

What we’re missing went unrealized for another hundred years after Helmont’s work. It’s easy to miss because it’s invisible.

Plants need air.

Joseph Priestly was a renowned late-18th century natural philosopher (an excellent old term for a scientist) who began experimenting with different gasses, and among his numerous important contributions to science was the discovery of oxygen. What concerns us here is his discovery that plants “revitalize” enclosed volumes of air; a finding that would later contribute to the discovery of photosynthesis after biologist and chemist Jan Ingenhousz discovered that sunlight was a key component to plants “revitalizing” – meaning oxygenating – the air.

I could go on exploring the history of the discoveries that led to photosynthesis, but I’ve yet to answer the key question: Where do plants get their mass?

Let’s jump ahead through history and arrive at the chemical equation for photosynthesis, which involves the other major constituent of air, carbon dioxide (CO2).

6 CO2 + 6 H2O + (sunlight) → C6H12O6 + 6 O2

Meaning that six carbon dioxide molecules react with six water molecules to give us six oxygen molecules and a pretty complicated carbohydrate. That beefy molecule is what the plant uses for fuel, and is not dissimilar from what we animals use as fuel, hence the reason we can eat plants; when you’re eating a plant that carbohydrate is what you’re burning for energy.

By the end of the 19th century the theory of photosynthesis was fully developed, which suggested that plants get their mass from absorbed water bonded with carbon, and that the elusive source of that carbon is the air itself!

Yes, plants get a good portion of their mass literally out of thin air. When a log is burned in a campfire it leaves behind a black powdery substance that we call charcoal, which is almost pure carbon. It is this carbon that gives a plant its rigid structure.

Scientists arrived at this answer via a chain of experiments conducted over hundreds of years by very intelligent and accomplished scientists, but there is one thing missing, and that is unquestionable proof. It could not be provided until a new experimental technique was developed in the 1940s by Martin Kamen, who went on to win the Nobel Prize in chemistry in part for his contribution to the very question we’ve been pondering.

In order to prove that carbon from the air (in the form of carbon dioxide) was being absorbed and incorporated into a plant it was necessary to track carbon atoms. The problem is that all carbon atoms are identical and so small in size and large in number (in any given plant) that it’s fundamentally impossible. What was needed was a form of carbon chemically identical to carbon but somehow traceable.

A typical atom has three components: electrons, protons, and neutrons. The nucleus – or core – of the atom contains protons and neutrons, whereas the electrons orbit around the nucleus. All of the chemical properties of an atom are determined by the number of protons and electrons in the atom; the neutrons just kind of hang around adding mass. It was discovered in the early 20th century at the dawn of the nuclear age that it was sometimes possible to knock an extra neutron into or out of a nucleus using the right kind of radiation. When this happens it’s possible to end up with an atom that is either very rare or doesn’t occur in nature at all. We can differentiate between these different variants of atoms – called isotopes – by adding up all the protons and neutrons in the nucleus and sticking that number next to the name of the atomic element. The most common form of carbon on earth (99% of it, in fact) is carbon-12. Since an atom of carbon is defined by having six protons in the nucleus that means that carbon-12 must contain six neutrons as well, since 6+6=12. Around 1% of the remaining carbon on earth is carbon-13, which contains seven neutrons in its nucleus.

There is one more type of carbon found on earth, but it exists only in remarkably small quantities. Carbon-14 (which contains eight neutrons in its nucleus) accounts for one atom out of every 1,000,000,000,000 carbon atoms in our atmosphere. There’s something incredibly special about carbon-14 though. Unlike the two other forms of carbon, this one is radioactive. If a plant was fed CO2 made with carbon-14 then the location of the carbon could be traced into the plant by various methods that measure radiation.

As I’ve said, carbon-14 is incredibly rare, but does occur naturally. In order to trace the carbon, the carbon-14 CO2 we’re adding would have to be in a much higher abundance than the naturally occurring carbon-14 CO2. What was needed was a way to produce carbon-14 in significant quantities.

This is what Martin Kamen did in 1940. Fully aware of the possible implications to biology he subsequently created carbon-14 CO2 and successfully proved that plants take in carbon in the form of carbon dioxide and incorporate it into their structure. If the carbon in the plant was coming from any other source it wouldn’t show an unusual abundance of carbon-14.

So if you didn’t know where plants got their mass, don’t feel bad. It took hundreds of years and some incredibly accomplished scientists to discover the truth, and in the process they unlocked photosynthesis, the secret of life for our green and leafy companions here on planet Earth.

You know, all this talk about radioactive carbon-14 has got me thinking about an archaeological technique called carbon dating, but that’s a topic for another day.

About Andrew Porwitzky

Dr Andrew Porwitzky is a professional scientist, comic book junkie, and freelance writer. He is also on Twitter way too much.

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