Photosynthesis: How Plants Eat Light

Plants do something no animal can. They make their own food out of air, water and light. Put a seed in soil with nothing but water and sunshine and it grows into a tree weighing tonnes. Almost none of that mass came from the soil. It came from the sky, carbon dioxide pulled from the air molecule by molecule and welded into wood using the energy of sunlight. The process is called photosynthesis, and nearly every living thing on Earth, including you, is fed by it.

The recipe

The reaction is simple to write down. Six molecules of carbon dioxide plus six of water, with energy from light, become one molecule of sugar and six of oxygen. The plant keeps the sugar and releases the oxygen as waste. The sugar is both fuel, burned to power the plant's cells, and raw material, joined into chains to make cellulose, the fibre that forms wood, cotton and paper, and starch, the stored energy in grains and potatoes.

Doing it is far from simple. It involves dozens of steps and some of the most elaborate molecular machinery known, assembled over billions of years of evolution and still not fully reproduced in any laboratory.

Catching light

The green in leaves is a pigment called chlorophyll. It absorbs red and blue light strongly and reflects green, which is why plants look the colour they do. Chlorophyll sits in stacks of membranes inside tiny structures called chloroplasts, hundreds of which pack each cell of a leaf. When a particle of light strikes a chlorophyll molecule it knocks an electron loose, and that energetic electron is passed along a chain of proteins like a hot potato, with each handoff used to do work: pumping ions across a membrane and charging up molecules that will drive the next stage.

The electron has to be replaced, and the plant gets a new one by splitting water. That splitting is where the oxygen comes from. The oxygen in every breath you take was once part of a water molecule, torn apart inside a leaf or an ocean alga.

Building sugar

The light reactions produce energy-carrying molecules. The second stage uses them to fix carbon. Carbon dioxide enters the leaf through thousands of microscopic pores on its underside, and an enzyme grabs each molecule and bolts it onto a sugar already in the cell, which is then rearranged and extended, step by step, into glucose. The enzyme that does the grabbing is slow and clumsy, but plants compensate by making enormous amounts of it; it is probably the most abundant protein on Earth.

The pores that let carbon dioxide in also let water out, and a plant can lose hundreds of litres a day through them. That water is pulled up from the roots through the trunk in continuous threads, in some trees a hundred metres high. On hot dry days plants close their pores to save water, and photosynthesis slows.

Where it happens

Half of all photosynthesis takes place not in forests but in the sea, in floating microscopic algae and cyanobacteria too small to see, which together produce as much oxygen as all the land plants combined. Cyanobacteria invented the process about three billion years ago, and the chloroplasts in every plant cell are their descendants, swallowed by a larger cell and kept as live-in food factories around 1.5 billion years ago.

Photosynthesis is also strikingly inefficient by engineering standards. Plants capture only about one or two percent of the energy in the sunlight falling on them; a solar panel manages twenty. But plants build and repair themselves, run on rain, and cover a third of the land surface, which is why they still outproduce every power station on Earth by a wide margin.

Why it matters

Photosynthesis is the gateway through which the Sun's energy enters the living world. Every animal eats plants or eats animals that ate plants. Coal, oil and gas are photosynthesis from hundreds of millions of years ago, buried and compressed. The oxygen you breathe, the food you eat, the wood in your house and the cotton on your back are sunlight, transformed by leaves. Plants also pull about a quarter of the carbon dioxide people emit back out of the air each year; without forests and plankton, climate change would be running far faster. The green chemistry of the leaf is, quite simply, what makes the planet habitable.