A leaf is a flat solar panel that makes its own fuel. It takes carbon dioxide out of the air, water out of the ground and energy out of sunlight, and assembles sugar — the thing almost every other organism on Earth eventually eats. Change the light, the carbon dioxide and the temperature below, and watch which one is holding the leaf back.
The drawing is schematic — cells are not to scale with each other and each moving dot stands for a very large number of molecules. The numbers in the panel are computed live from the model described below the graph.
Photosynthesis is two separate chemical factories sharing a building. The first runs on light and makes nothing you can eat; the second cannot use light at all, and makes the sugar. Step through and watch what crosses between them.
RuBisCO, the enzyme that grabs CO₂ out of the air in the Calvin cycle, fixes about three molecules a second. A typical enzyme handles a thousand or more. Plants compensate by making enormous amounts of it — up to half of all the soluble protein in a leaf.
RuBisCO cannot reliably tell CO₂ from O₂, and evolved when there was almost no oxygen about. In today's air a C3 plant wastes roughly a fifth to a quarter of its captured carbon undoing the mistake — a process called photorespiration, which gets worse as it gets hotter.
One turn of the Calvin cycle fixes one carbon atom. Six turns, costing 18 ATP and 12 NADPH, produce enough three-carbon sugar to build one molecule of glucose — while five sixths of the output is recycled straight back into the acceptor molecule that started it.
There is no separate stock of "living" carbon. The atoms in your muscle were in the atmosphere recently, and will be again. Click a station on the loop to follow one of them round — and then take the long way.
Chlorophyll absorbs hard in the blue around 430 nm and again in the red around 662 nm, and barely touches the middle of the spectrum. The green light it cannot use is reflected straight back at you, which is the only reason a leaf looks green at all — you are seeing the part of the sunlight the plant rejected.
It is also why commercial grow lights are usually magenta: drop the green and you are left with red plus blue, which is the light the plant actually absorbs.
Peak wavelengths are real, measured values. The curves themselves are drawn as sums of Gaussian peaks — the right shape and the right positions, not laboratory data.
Fixing carbon means opening the stomata, and opening the stomata means losing water. Three strategies evolved, each suited to a different climate.
C4 plants are about 3% of species and roughly a quarter of all land photosynthesis. CAM plants stockpile carbon in the dark as malic acid, then shut tight all day.
Cyanobacteria started splitting water about 2.4 billion years ago, and oxygen — a corrosive waste product with nowhere to go — built up in an atmosphere that had almost none. It rusted the dissolved iron out of the oceans, destroyed the methane keeping the planet warm, and killed most of what was alive at the time.
Of the sunlight landing on a crop over a season, roughly 1% ends up as harvestable chemical energy. Most is the wrong wavelength, misses the leaves, or is lost to photorespiration and the plant's own respiration. The theoretical ceiling is about 4.6% for C3 and 6% for C4.
Jan Baptist van Helmont grew a willow in a weighed pot of soil for five years. The tree gained 74 kg; the soil lost 57 g. He concluded the mass came from water. He was most of the way there — the dry mass of a tree is about half carbon, and every one of those carbon atoms arrived as a gas, through a hole in a leaf smaller than this full stop.
Related: Lab 05 — The Cell draws the chloroplast and the mitochondrion that run the two halves of this equation, and lets you take the rest of the cell apart around them.