Long before humans figured out how to harness solar power, photosynthesizers beat us to it. Our first attempt came less than 3,000 years ago, when the ancient Greeks built magnifying glasses to concentrate light for starting fires. By then, other lifeforms had already been converting those same rays into chemical energy for upwards of 3.5 billion years. There is truly nothing new under the sun.

This process, called photosynthesis, is fundamental to almost all life on Earth. Primary producers (plants, most familiarly, but also algae and cyanobacteria) use it to make their own food, then they become food for the organisms higher up the chain.

In terms of importance, cellular biologist Geoffrey McFadden wrote in a study published in Plant Physiology that “the origin of oxygenic photosynthesis must rank just after the origin of life itself.”

What Is Photosynthesis?

Simply put, photosynthesis is how plants build their bodies. Instead of running around chasing down meals, like other living things, they just bask in the sun. To high-maintenance creatures such as ourselves it seems like magic, as if they’re bootstrapping themselves into existence.

Even more astonishing, photosynthesis requires only three ingredients. It starts with water (which plants draw up through their roots) and carbon dioxide (which they bring in through pores in their leaves). Once the H2O and CO2 come together, the plant just adds a dash of sunlight, and voila — inanimate matter comes to life.

The first hint of photosynthesis came from experiments by English clergyman and scientist Joseph Priestly. In 1774, he burned a candle inside an airtight bell jar until it ran out of its invisible — and until then unknown — fuel. Then he put a sprig of mint inside, and found that after a few days the air could once again support combustion. He deduced that plants produce some vital substance, which we now call oxygen. Later that decade the Dutch physician Jan Ingen-Housz proved that light was essential to this operation.

Read More: Why Some Plants Close Their Leaves at Night

Where Does Photosynthesis Take Place?

Plant cells are filled with organelles called chloroplasts, which are, in turn, filled with a pigment called chlorophyll (the same one that gives plants their green color). It’s these chlorophyll molecules that absorb sunlight and apply it to photosynthesis, diverting energy from photons in a biologically useful direction. Most photosynthesis happens in the leaves, since they receive maximum sunlight, but it can also occur in stems, flowers and even tree bark.

Genetic testing shows that chloroplasts aren’t your typical organelles. They’re the descendants of ancient cyanobacteria, which long ago took up residence within the precursors of modern plant cells, forming what McFadden calls “an extraordinary partnership.” (This phenomenon, one organism living inside another to the benefit of both, is called endosymbiosis.)

“Equipped with chloroplasts,” McFadden wrote in the study, “the early plants were ready to colonize the land and green the planet.”

What Are the Products of Photosynthesis?

For plants, the most important product of photosynthesis is sugar. The whole process is aimed at transforming water and carbon dioxide into energy-rich glucose molecules, which plants can then use to fuel their growth and other cellular activities.

For us and for other animals, however, there’s a second fortunate result: Oxygen. The main element of the air we breathe is a byproduct of photosynthesis — something to be gotten rid of, as far as a plant is concerned. Zillions of oxygen molecules, released from zillions of chloroplasts around the planet, become the life-sustaining atmosphere that supports Earth’s vast ecological communities.

Read More: Is Plant Communication a Real Thing?

What Is the Equation for Photosynthesis?

Photosynthesis is incredibly complicated, but can be represented by a straightforward equation of inputs and outputs: It starts with 6 molecules of carbon dioxide and another 6 of water, then uses sunlight to turn them into 6 oxygen molecules and a single sugar. Written as a chemical formula: 6CO2 + 6H2O → C6H12O6 + 6O2.

Getting a bit more technical, here’s how the conversion works. There are two stages. First, in the “light-dependent reaction,” a chloroplast captures the energy from a photon, then uses it to split a water molecule into oxygen atoms and hydrogen ions. As already mentioned, the oxygen floats off to replenish the atmosphere, and you can forget about it now. The hydrogen ions, however, go on to help create ATP and NADPH, molecules that provide energy and electrons for the next stage.

Second, in the Calvin cycle or “light-independent reaction” (because it doesn’t require photons), ATP and NADPH join forces with carbon dioxide. After a few mind-numbing steps, this reaction makes yet another molecule called G3P. Two of them combine to form a sugar — one more brick in the edifice that is a plant. Meanwhile, a bunch of G3P is left over to restart the cycle.

Why Is Photosynthesis Important?

It’s no exaggeration to say we wouldn’t exist without photosynthesis. If another dino-killing asteroid struck Earth tomorrow, its immediate impact would be the least of our worries; famine would multiply the death toll many times over as dust clouds deprived crops of sunlight. Only certain anaerobic bacteria — which don’t rely on oxygen and get their sustenance from geochemical products — would survive.

Besides keeping us and our ancestors alive for the past several billion years, photosynthesis has recently become useful in distinctly modern ways. Some plant scientists want to optimize its mechanisms to engineer more productive crops, for example, and beyond that it sheds light on the workings of complex environmental systems.

According to the Japanese researchers Ayumi Tanaka and Amane Makino, “photosynthesis is now an integral component of simulation models used to predict the future of our planet.”

Read More: Physicists See Quantum Effects in Photosynthesis

Article Sources

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Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor’s degree in journalism and media production from Washington State University.