Photosynthesis

A tree gains tonnes of mass over its life — where does that material actually come from?

An oak weighs a few grams as an acorn and tens of tonnes at two hundred years old. Matter is not created, so every one of those tonnes came from somewhere. Ask most people where and they will say the soil — the tree is rooted in it, it feeds on it, the answer seems obvious.

But look at the ground around an old tree. There is no crater. The soil level has not dropped by a tree's worth. So where did the mass come from?

Someone actually tested this, around the 1640s. Jan Baptist van Helmont planted a small willow — about five pounds — in a pot holding two hundred pounds of dried soil, gave it nothing but water, covered the pot to keep dust out, and left it for five years. The willow ended at around 164 pounds. He then dried the soil again and weighed it: it had lost about two ounces.

Sit with that result before drawing the conclusion. A hundred and sixty pounds appeared, and the soil supplied perhaps two ounces of it. Whatever the answer is, the soil is not it. Van Helmont concluded the mass was water, which was reasonable given his inputs and turned out to be mostly wrong — but the experiment was right, and it disposed of the intuitive answer permanently.

So what is left? Water, and the air. The air is the surprising candidate because it seems to weigh nothing, but carbon dioxide is a real substance with real mass, and a leaf is in constant contact with an enormous volume of it. If a plant could take carbon dioxide and pull the carbon out, it would be building itself from something invisible.

That is what happens, and the accounting is precise. Dry wood is roughly half carbon by weight, and every one of those carbon atoms arrived through a pore in a leaf as part of a carbon dioxide molecule. Most of the oxygen bound into the wood arrived the same way. What water supplies to the structure is chiefly hydrogen — a few percent of the dry mass — and the minerals from the soil are the trace fraction left as ash if you burn the log. Van Helmont's two ounces.

Now the harder question. Carbon dioxide is stable and going nowhere; pulling it apart and stitching the carbon into sugar costs energy. Where does a leaf get it? From light — and it is worth noticing that this is why the process needs light at all, rather than light being incidental warmth.

The work splits into two halves that are worth keeping separate. In the first, pigments in the membranes inside the chloroplast absorb light and use the energy to rip apart water molecules, stripping electrons from them. The leftover oxygen atoms pair up and leave as O2 gas. Every breath you take is that waste product. The electrons run down a chain that charges the leaf's two energy carriers.

In the second half — which needs no light directly, only what the first half produced — an enzyme grabs a carbon dioxide molecule from the air and attaches it to a five-carbon sugar already in the chloroplast, and the resulting compound is worked round a cycle that both builds sugar and regenerates the acceptor molecule, so the cycle can run again.

Two clarifications matter more than they look.

The first is where the oxygen you breathe comes from. The intuitive reading of the summary equation is that plants take in carbon dioxide and hand back its oxygen. They do not. The released O2 comes from the water, which is why the water-splitting step exists at all — a result established with isotopically labelled water in the 1940s, not deduced from the equation.

The second is that the carbon-fixing enzyme, RuBisCO, is remarkably bad at its job. It is slow — a handful of reactions per second, where many enzymes manage thousands — which is much of why plants must make so much of it that it is plausibly the most abundant protein on Earth. Worse, it cannot reliably tell carbon dioxide from oxygen, and when it grabs oxygen instead the cell has to spend energy salvaging the product, releasing carbon dioxide in the process. This is photorespiration, and it is a genuine loss, not a textbook curiosity. It gets worse as it gets hotter and as pores close to save water.

That inefficiency explains a whole category of plants. Maize and sugarcane use a C4 pathway: they capture carbon dioxide first with a different, more discriminating enzyme, then deliver it concentrated into an inner compartment where RuBisCO sits, so it rarely meets oxygen. Cacti and pineapples use CAM, opening their pores at night and storing the carbon until daylight. Both are workarounds for the same defect — and C4 has evolved independently in plants dozens of times over, which tells you how strong the pressure to fix it has been.

• Why efforts to engineer a better RuBisCO, or move C4 machinery into rice, have proved so difficult.
• How rising atmospheric carbon dioxide changes the balance between fixation and photorespiration.

Every term the collection defines is gathered in the glossary.