Solidified Sky

Here is a fact that should be more famous than it is. A giant oak weighs many tonnes, 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?

It came, mostly, from thin air. Every year a tree pulls carbon dioxide gas out of the sky and quietly rebuilds those carbon atoms into trunk, branch, and leaf. The physicist Richard Feynman loved this: the tree is, in a real sense, solidified sky. Drag the guess below and see how wrong the intuition of “plants eat soil” really is.

To see how the sky gets turned into sugar, we have to go small - smaller than seems reasonable. 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. These are adjustable mouths. Carbon dioxide drifts in through them; oxygen and water vapor drift out. Inside each cell, once we get there, we will find dozens of tiny green machines. Those are our destination.

This is a chloroplast, and it is where the whole story happens. It is a green lozenge about five micrometres long, and a single leaf cell packs in dozens of them. Click any part to learn its name, or press Take the tour to be walked through.

Two regions matter. The thylakoids are flat green sacs stacked like coins into piles called grana - this is where sunlight is caught. Around them is the stroma, the fluid filling the rest of the chloroplast - this is where sugar gets built. Two rooms, two jobs. We will visit both.

Press into the wall of a thylakoid and the machinery comes alive. This is the light-dependent stage, and it is running in front of you the whole time. Sunlight arrives as photons. Green pigment molecules called chlorophyll catch them, and each catch kicks an electron up to a high energy. That energized electron is the spark that runs everything downstream.

Watch the sequence. At Photosystem II, to replace the electrons it keeps losing, the machine rips apart water. That is the crucial step: splitting water is what releases the oxygen you are breathing right now. The freed electrons hop down a chain of carriers; their fall pumps protons across the membrane, and that pressure spins a molecular turbine, ATP synthase, that charges up the cell's batteries: ATP and NADPH.

Turn up the sun and the whole scene speeds up: more photon strikes, faster electron hops, more oxygen bubbling off, the battery meters climbing quicker. Turn it to night and everything stops - no light, no reaction. Notice what is not here yet: no sugar. The light reactions only make energy carriers. Sugar is built next door.

Now the payoff. Step out of the thylakoid into the surrounding stroma, 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 the button 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.

The atom's path: in through a stoma, into the stroma, and onto a five-carbon molecule by the enzyme rubisco - the most abundant protein on Earth. That splits into two three-carbon pieces, which the energy carriers convert into a sugar called G3P. Most G3P stays in the cycle to keep it turning; a fraction escapes, and after six carbons have been captured, the escapees join into one glucose. Six turns of the wheel, one sugar. Keep your eye on the bright atom.

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. That is photosynthesis winning.

But here is the correction to a myth almost everyone carries: plants respire around the clock, just like you. They are always burning a little sugar for their own energy, taking in O₂ and giving off CO₂. During the day photosynthesis vastly outruns that, so the net flow is CO₂ in, O₂ out. At night, with no light, only respiration remains - and the leaf quietly breathes the other way. Watch the counters slow, halt, and reverse as you turn toward midnight.

One loose end. If chlorophyll lives on sunlight, why does it throw away the green? 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 light bounces off the leaf and into your eye. A leaf looks green because green is the color it refuses to eat. Drag the wavelength slider across the spectrum and watch the leaf darken where chlorophyll drinks and brighten where it reflects.