Photosynthesis

Animals eat to get food. Plants do something stranger: they make their own food out of sunlight.

It happens inside their leaves. Leaves are green because they are packed with a substance called chlorophyll, and chlorophyll is very good at catching light. A plant uses that captured light energy to stick together two very ordinary ingredients: water, pulled up from the roots, and carbon dioxide, a gas taken from the air. Out of those it builds sugar, which is the plant's food and the material it grows with.

There is a leftover from all this. When the plant splits water apart to use it, it throws away the oxygen. That waste gas is the oxygen we breathe. Every breath you take is running on a plant's rubbish.

Trace the energy in almost any living thing back far enough and you arrive here, at a leaf catching sunlight. The grass, the cow that eats the grass, and you: all of it is powered, in the end, by this.

You can write the whole thing as one tidy reaction. Six molecules of carbon dioxide plus six of water, driven by light, become one molecule of glucose plus six of oxygen:

\[ 6\,CO_2 + 6\,H_2O \;\xrightarrow{\text{light}}\; C_6H_{12}O_6 + 6\,O_2. \]

That single arrow hides two very different stages. The first is the light-dependent reactions, and they happen in stacked membranes inside the chloroplast called thylakoids. Chlorophyll there absorbs photons, and the energy is used to split water. This is the key point people often get wrong: the oxygen released comes from the water, not from the carbon dioxide. Splitting water also charges up two energy carriers, ATP and NADPH.

The second stage is the Calvin cycle, sometimes called the light-independent reactions, and it runs in the fluid around the thylakoids (the stroma). It spends the ATP and NADPH from stage one to grab carbon dioxide out of the air and lock it into sugar. The enzyme that does the grabbing is RuBisCO, probably the most abundant protein on Earth.

Because it is a chain of supply and demand, the rate is capped by whatever is in shortest supply: light, carbon dioxide, or temperature. Give a plant more light and it speeds up, but only until carbon dioxide becomes the bottleneck, and so on. The model above lets you feel that trade-off directly.

One last thing worth sitting with. Early Earth had almost no oxygen in its air. Photosynthesis, carried out by tiny organisms for hundreds of millions of years, is what filled the atmosphere with it, an event so large we call it the Great Oxidation. The air is a by-product of life.

The light reactions, in detail. The thylakoid membrane holds two photosystems working in series, and confusingly they are numbered in the order they were discovered, not the order electrons flow through them. Photosystem II absorbs a photon, and the excited chlorophyll passes an electron down an electron transport chain. To replace the electron it just lost, PSII rips it from water, and it is here, at the oxygen-evolving complex of PSII, that \(2\,H_2O \to O_2 + 4H^+ + 4e^-\) and molecular oxygen is set free. The electron then travels to photosystem I, gets re-energised by a second photon, and ends up reducing NADP⁺ to NADPH. Plot the electron's energy against each step and you get the characteristic zig-zag known as the Z-scheme.

Where the ATP comes from. As electrons move down the chain, protons are pumped into the thylakoid space, building an electrochemical gradient. That gradient is potential energy, and the enzyme ATP synthase lets protons flow back through it, using the flow to phosphorylate ADP into ATP. This coupling of a proton gradient to ATP synthesis is chemiosmosis, the same trick your own mitochondria use in reverse-flavour to burn that sugar later.

Why RuBisCO is a bottleneck. RuBisCO fixes carbon dioxide onto a five-carbon sugar to start the Calvin cycle, but it is slow and not very picky: it also binds oxygen, in a wasteful side reaction called photorespiration that undoes some of the fixation. On hot, dry days, when leaves close their pores to save water and oxygen builds up inside, this loss grows. Some plants evolved workarounds. C4 plants (maize, sugarcane) concentrate carbon dioxide near RuBisCO first; CAM plants (cacti, succulents) open their pores only at night. Both are patches on the same slow enzyme.

Why leaves are green. Chlorophyll a and chlorophyll b absorb strongly in the blue and the red parts of the spectrum and poorly in the middle, so green light is largely reflected rather than used. The colour of a forest is, in a sense, the wavelength the machinery found least useful. Accessory pigments widen the catch, and the quantum efficiency of the whole capture is remarkably high, though the fraction of total sunlight energy that ends up as sugar is only a few percent.

Why it matters beyond the leaf. The oxygen now in the atmosphere is almost entirely biological in origin, and its slow accumulation reshaped the planet's chemistry and made room for large, energy-hungry life, including us. In that sense photosynthesis is not just how plants eat but part of the deep story of how a barely-living planet became a living one, a thread that reaches back to the origin of life itself.