00A Tree Is Made of Air
A tree weighs many tons. So where did all that weight come from?
A giant oak weighs many tons, and almost none of that weight came out of the ground. Dig up the soil under a full-grown tree and weigh it - barely anything is missing.
So where did all that wood come from? Mostly, from thin air. Every year a tree pulls carbon dioxide out of the sky and quietly rebuilds those carbon atoms into trunk, branch, and leaf. The tree is, in a real sense, solidified sky.
Richard Feynman loved this: dig up the soil, weigh it, put the tree back - the ground barely lost anything. The wood came from the air.
Where does a tree's mass come from?
Set your guess for how much of a tree's dry mass comes from the soil, then reveal what the classic willow experiment actually found.
In the 1600s, a Flemish doctor named van Helmont wanted to know what a growing plant is actually made of.
The obvious guess was the soil. A plant sits in dirt and gets bigger, so surely it eats the ground.
So he built a sealed test. He dried a pot of soil, weighed it exactly, and planted a small willow - about 2.3 kg.
For five years he added nothing but water, and kept the pot covered so no stray dust could drift in and skew the weight.
Then he weighed it all again. The willow had gained about 74 kg. The soil had lost only about 60 grams.
If the tree had eaten the soil, the soil should have lost 74 kg too. It barely moved - so the mass came from somewhere else.
Van Helmont said water. He was half right: it wasn't the soil, but the real source is the CO₂ in the air, with water supplying only the hydrogen.
Carbon from the sky, built into wood.
01Zoom Into a Leaf
A tree is built out of thin air. But where does that actually happen?
Not in the trunk, not in the roots. It happens inside machines so small that a single leaf holds billions of them - and almost every meal you have ever eaten traces back to one.
So let's shrink down and go find one. A leaf is a thin, flat solar panel. Slide the zoom below and fall through its layers - the whole tree, then one leaf, then the spongy green tissue inside called the mesophyll, then a single living cell.
Notice the pores on the underside - the stomata. Adjustable mouths. CO₂ drifts in through them; oxygen and water vapor drift out. Inside each cell, dozens of tiny green machines. Those are our destination.
Tree - a few meters. Leaf - a few centimeters. Mesophyll cell - about 30 micrometers. Chloroplast - about 5 micrometers. Every step down, ×100 or so.
One continuous zoom: tree → leaf → cell
Drag the slider to fly inward. Labels fade in as each new scale appears; watch gases move through the stomata near the end.
02Meet the Chloroplast
We keep calling it "the machine." What does it actually look like inside?
The chloroplast was not always part of the plant. It began as a free-living bacterium that got swallowed whole - and never left. Every leaf you have seen runs on captured solar factories with their own ancient machinery.
This is a chloroplast, and it is where the whole story happens. A green lozenge about five micrometers long. A single leaf cell packs in dozens of them.
Inside, two regions matter - and they map cleanly to the two halves of photosynthesis. Click any part of the model, or press Take the tour to be walked through.
The whole rest of the story is just: what happens in the thylakoids, and what happens in the stroma. Keep this table handy.
| Region | Job | What it makes |
|---|---|---|
| Thylakoids | catch light, split water | ATP · NADPH · O₂ |
| Stroma | build sugar from CO₂ | G3P → glucose |
A chloroplast you can take apart
Move your mouse to tilt it in 2.5D. Click the thylakoid stacks, the stroma, or the outer envelope to label them.
Deep dive: a chloroplast was once a bacterium
The chloroplast started as a free-living cyanobacterium - the same kind of microbe that first invented water-splitting photosynthesis. Somewhere around 1.5 billion years ago, a larger cell swallowed one and never digested it. The bacterium kept doing what it did best (making sugar from sunlight); the host got the sugar. Neither ever left.
Chloroplasts still carry their own tiny circle of DNA, still divide on their own schedule, and still run the same water-splitting reaction their ancestors ran in the ocean. That reaction is what filled the sky with the oxygen you are breathing (we'll come back to it in the last station's Great Oxidation Event).
03The Light Reactions
How do you turn light - no weight, nothing to hold - into something a plant can actually use?
This is the step that catches sunlight and tears water apart. The oxygen in the breath you just took was made right here, in a leaf, not long ago.
Press into the wall of a thylakoid and the machinery comes alive. Sunlight arrives as photons. Green pigment molecules called chlorophyll catch them, and each catch kicks an electron up to a high energy.
That excited electron is the spark that runs everything downstream. The whole light-reactions stage turns it into two portable energy carriers the sugar factory can use.
- Light kicks an electron loose. A photon hits chlorophyll inside Photosystem II. An electron jumps to a high energy and leaves.
- Water gets split to replace it. Photosystem II is down an electron, so it grabs one from a water molecule - and that is what releases the oxygen you are breathing.
- The electron's fall powers everything. As it falls down the chain, it pumps H⁺ across the membrane. Those H⁺ flow back through ATP synthase and it makes ATP. At the end of the chain, Photosystem I makes NADPH.
The O₂ you breathe comes from splitting water, not from CO₂. Ruben and Kamen proved it in 1941 by labeling water's oxygen with a heavy isotope and watching that exact oxygen come out as gas. Never say “CO₂ is split to release oxygen.” The carbon of CO₂ goes into sugar; the oxygen of O₂ came from water.
The energy doesn't live in the moving electron - it lives in the H⁺ piled up on one side of the membrane, like water piled up behind a dam. Let them flow back through the turbine (ATP synthase) and you get ATP. This trick is called chemiosmosis, and your mitochondria use the exact same idea to power you.
O₂ released, ATP made, NADPH made - all three climb together in bright sun. Turn the dial to night and they all freeze. That is what “light-dependent” literally means.
The thylakoid membrane, running
Slide the sun. Brighter light drives more electrons, splits more water, and fills the ATP and NADPH meters faster. Flip to night to freeze it.
Deep dive: linear vs cyclic electron flow (why the ATP:NADPH ratio works)
The path we just walked - Photosystem II to Photosystem I, both firing - is called linear flow. It makes ATP and NADPH in a roughly 3 : 2 ratio.
The Calvin cycle uses a bit more ATP per NADPH than that. So plants also run cyclic flow around Photosystem I alone: the electron loops back into the chain, pumps more H⁺, and makes ATP without making NADPH. Dial in cyclic flow when you need extra ATP; back off when you need more NADPH. Same machinery, tunable ratio.
04Follow One Carbon Atom
CO₂ is an invisible gas. How does it become something solid you can hold - wood, sugar, a whole leaf?
This is the exact moment air turns into matter, one carbon atom at a time. The "solidified sky" the whole site is named for happens right here.
Step out of the thylakoid into the surrounding stroma. This is where the Calvin cycle spends the ATP and NADPH from next door to build sugar out of air.
Rather than describe it, let us ride it. Press play and we will grab a single CO₂ molecule from outside the leaf and follow one of its carbon atoms all the way into a sugar. Keep your eye on the bright atom.
The Calvin cycle is not the “dark reactions.” It doesn't need light directly - but it burns the ATP and NADPH the light reactions make. Cut the light and the cycle stalls within seconds. It runs by day, next door to Station 03.
Six turns of the wheel build one glucose - here is the first turn, step by step.
CO₂ is a very stable, low-energy molecule. Turning it back into sugar takes both energy (ATP) and high-energy electrons (NADPH) - exactly what the light reactions just made next door.
- CO₂ drifts in through a stoma.
- Rubisco welds the carbon onto RuBP (a 5-carbon sugar skeleton).
- The 6-carbon result immediately splits into two 3-carbon 3-PGA pieces. Your atom lands in one of them.
- ATP and NADPH pay in energy and electrons to turn 3-PGA into G3P - the real product of photosynthesis.
- Five out of every six G3P recycle back into new RuBP, keeping the wheel turning. One escapes.
- After six turns, six escaped G3P combine into one glucose. Six carbons captured. One sugar built.
Ride a carbon from air to sugar
Press play and the camera locks onto one carbon atom, tracing its journey through rubisco and the Calvin cycle until it becomes part of glucose.
05The Whole Cycle, Running
Everyone knows plants give off oxygen. But do they ever use it up - and what happens after dark?
Plants breathe around the clock, exactly like you do. Whether a leaf adds oxygen to the air or takes it back flips between day and night - and most people get this backwards.
Pull all the way back to the whole leaf and let a day pass. Turn the dial from dawn to noon to dusk to midnight and watch both counters.
In daylight the leaf is a net producer: it takes in CO₂ and breathes out O₂ far faster than it burns any. Photosynthesis winning.
But the myth-buster: plants respire around the clock, just like you. Always burning a little sugar, taking in O₂, giving off CO₂. At night, with no light, only respiration remains - and the leaf quietly breathes the other way.
| Day | Night | |
|---|---|---|
| Photosynthesis | fast | off |
| Respiration | steady | steady |
| Net O₂ | out ↑ | in ↓ |
| Net CO₂ | in ↓ | out ↑ |
Takeaway: a leaf isn't a photosynthesis-only machine. It is always doing both.
A leaf over one full day
Turn the time dial. The sky, the reaction speed, and the two net counters all follow the sun. Cross into night and watch the flow reverse.
Around dawn and dusk there is a moment when photosynthesis exactly cancels respiration - the leaf breathes in and out at the same rate, net zero. Turn the dial slowly through those hours to see the counters cross.
“Dark reactions happen at night.” No - the Calvin cycle needs the ATP and NADPH the light makes, so it runs in daylight and stalls at night. “Photosynthesis only happens in leaves.” It happens anywhere there are chloroplasts, including green stems and unripe fruit.
06Why Green?
A leaf lives on sunlight. So why throw away the green - the color sitting right in the middle of the rainbow?
The green of every tree, plant, and blade of grass is the one color a leaf refuses to eat. All that green you see is rejected light.
Sunlight is a mix of every visible color. Chlorophyll grabs blue light hard (around 430 nm) and red light hard (around 660 nm), but it barely absorbs the green in the middle. So green bounces off the leaf and into your eye. A leaf looks green because green is the color it refuses to eat.
Drag the wavelength across the visible spectrum. The leaf darkens where chlorophyll drinks (blue, red) and brightens through the green gap in the middle.
The color a leaf refuses
Sweep the wavelength from violet to red. The curve is chlorophyll's absorption; the leaf swatch darkens where absorption is high. Note the green gap in the middle.
Deep dive: rubisco is bad at its job, and how corn and cacti cheat
Rubisco, the enzyme that grabs CO₂, is famously sloppy: it also grabs O₂ by mistake, wasting energy in a process called photorespiration. This gets worse when it is hot and dry and the stomata are shut, letting O₂ build up inside the leaf.
C4 plants (corn, sugarcane) concentrate CO₂ around rubisco in a special inner cell so it almost never grabs oxygen. CAM plants (cacti, pineapple) open their stomata only at night, storing CO₂ until morning so they lose less water in the desert heat. Same chemistry, two clever workarounds for the same flawed enzyme.
Nearly every oxygen molecule you have ever breathed was split off a water molecule by chlorophyll, somewhere, at some time. Ancient cyanobacteria running this exact reaction filled the sky with oxygen over two billion years ago - the Great Oxidation Event. Every forest is still, quietly, turning sky into wood and water into breath.
Three questions
The whole story, in five lines
- The O₂ you breathe came from splitting water - never from CO₂.
- More than 90% of a tree's dry mass came from air, not soil.
- Six turns of the Calvin cycle = one glucose.
- Chlorophyll grabs red and blue, refuses green - that is why leaves look green.
- Plants respire day and night. They breathe just like you.