Photosynthesis

How plants eat sunlight

Junior level — plain language, no maths

Plants pull off something no animal can: they make their own food out of thin air and sunshine. Point a leaf at the Sun and, inside it, an astonishing bit of chemistry runs all day long. The leaf drinks in carbon dioxide from the air and water from the roots, catches sunlight with a green pigment called chlorophyll, and rebuilds those simple ingredients into sugar - its food - giving off oxygen as the leftover. That leftover happens to be the air you breathe.

The green of almost every plant is chlorophyll, the molecule that catches the light. It soaks up the red and blue parts of sunlight and bounces the green straight back at your eyes - which is exactly why leaves look green. The energy it grabs powers the whole rebuild, turning low-energy ingredients (carbon dioxide and water) into high-energy sugar the plant can burn later or use to build roots, stems and fruit.

This one reaction quietly runs the entire living world. Nearly all the food on Earth traces back to it: the grass a cow eats, the wheat in your bread, the plankton feeding the oceans. And nearly all the oxygen in the atmosphere was breathed out by plants and algae doing photosynthesis over billions of years. Every breath you take is, in a real sense, borrowed from a leaf.

The light and dark reactions, and limiting factors

Student level — the core equations

The overall equation looks deceptively tidy: \(6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2\) - six carbon dioxides and six waters become one glucose and six oxygens, powered by light. But it hides two linked stages, both inside the chloroplast.

First, the light-dependent reactions in the thylakoid membranes: chlorophyll absorbs photons, which excite electrons and split water - this is where the \(O_2\) comes from - banking the energy as ATP and NADPH. Second, the light-independent Calvin cycle in the stroma spends that ATP and NADPH to fix \(CO_2\) into sugar, stitching carbon from the air onto an existing molecule with the enzyme rubisco, the most abundant protein on Earth.

The overall rate is set by whatever is in shortest supply - a limiting factor. In dim light, adding light speeds things up; in bright light the rate plateaus and something else (CO₂ concentration or temperature) becomes the bottleneck. Growers exploit this precisely: pump extra CO₂ into a greenhouse and, as long as light and warmth allow, the plants grow faster. Push any single factor and the rate climbs only until the next one runs out.

From photons to sugar: the machinery and its limits

Scholar level — full mathematical depth

01Two photosystems and the Z-scheme

Light strikes Photosystem II and Photosystem I in series. PSII (its reaction centre, P680) becomes a strong enough oxidant to split water, \(2H_2O \to O_2 + 4H^+ + 4e^-\); the electrons flow down an electron-transport chain - the "Z-scheme" - pumping protons, then are re-energized at PSI (P700) to reduce NADP⁺ to NADPH. Water is the electron source, and \(O_2\) is its by-product.

02Chemiosmosis makes the ATP

The proton gradient built across the thylakoid membrane drives ATP synthase, exactly as it does in respiration's mitochondria - the same rotary enzyme, the same trick, on the opposite kind of membrane. Here light charges the gradient; in respiration, the oxidation of food does.

03The Calvin cycle and rubisco's flaw

In the stroma, rubisco fixes \(CO_2\) onto ribulose-1,5-bisphosphate; three turns net one G3P, and six turns (18 ATP, 12 NADPH) yield one glucose. But rubisco is sloppy: it also grabs \(O_2\), triggering wasteful photorespiration. This costly ambiguity - a relic of evolving when the air was almost oxygen-free - caps the efficiency of most plants.

04C4 and CAM: engineering around the flaw

Hot, dry climates make photorespiration worse, so some plants concentrate \(CO_2\) around rubisco to suppress it. C4 plants (maize, sugarcane) separate carbon capture and fixation into different cells; CAM plants (cacti, pineapples) separate them in time, opening their stomata only at night. They are independent evolutionary solutions to the very same problem.

05Efficiency and the planetary balance

Photosynthesis converts only about 1-2% of the sunlight that lands on a leaf into biomass, yet at planetary scale it fixes roughly 120 gigatonnes of carbon a year and, over billions of years, filled the sky with oxygen - the Great Oxidation Event of ~2.4 billion years ago. It remains the ultimate power source for very nearly all life on Earth.