Eating Sunlight: How Photosynthesis Works

The trick that feeds the world

Nearly every calorie you have ever eaten traces back to a single chemical trick performed by plants, algae, and certain bacteria: turning sunlight, water, and carbon dioxide into sugar. Written as an overall equation, six molecules of carbon dioxide and six of water, driven by light, become one molecule of glucose and six of oxygen. That tidy summary hides a two-part process, and getting the two parts straight is the key to understanding the whole thing. The first part captures light and converts it into chemical energy; the second part uses that energy to build sugar. They are often called the light reactions and the Calvin cycle, and they happen in different places inside the same tiny organelle, the chloroplast.

The light reactions: turning photons into fuel

The light reactions take place in a set of stacked internal membranes inside the chloroplast called the thylakoids, which are studded with the green pigment chlorophyll. Chlorophyll’s job is to absorb light, and it absorbs red and blue wavelengths well while reflecting green, which is why leaves look green to us. The absorbed light energy excites electrons in the chlorophyll to a higher energy state, and the cell has built machinery to grab those energized electrons before they fall back and capture their energy.

Here is the step that ought to be more famous than it is. To replace the electrons it keeps sending off, the system pulls them from water, physically splitting water molecules apart. When water is split, it releases electrons, hydrogen ions, and oxygen. That oxygen is a waste product from the plant’s point of view, and it is the oxygen we breathe. The entire breathable atmosphere is, in a real sense, exhaust from photosynthesis. Take a breath and you are inhaling the leftovers of water that some plant or alga tore apart to get at its electrons.

The energized electrons are passed down a chain of proteins, and as in respiration, that downhill flow is used to pump hydrogen ions across the membrane, building a gradient that drives an ATP-making enzyme. The electrons end their journey by being loaded, along with hydrogen, onto a carrier molecule called NADPH. So the light reactions produce two things: ATP and NADPH, both forms of portable chemical energy. They do not make any sugar. They are the power supply for the part that does.

The Calvin cycle: building sugar in the dark

The second stage does not directly need light, which is why it is sometimes misleadingly called the dark reactions, though it usually runs in daylight using the products the light reactions just made. It happens in the fluid surrounding the thylakoids, the stroma, and its job is to take carbon dioxide from the air and assemble it into sugar, spending the ATP and NADPH as fuel.

The central event is carbon fixation, the moment an inorganic carbon dioxide molecule from the atmosphere is attached to an organic molecule and becomes, for the first time, part of a living thing’s substance. The enzyme that does this is named RuBisCO, and it deserves a mention for sheer abundance: it is thought to be the most plentiful protein on Earth, precisely because plants need so much of it. RuBisCO grabs carbon dioxide and bonds it to a five-carbon molecule, beginning a cycle of reactions that, powered by ATP and NADPH, produces a small sugar and regenerates the starting molecule so the cycle can run again. Run it enough times and the small sugars combine into glucose. Melvin Calvin worked out the steps in the 1950s using radioactive carbon to trace the path, which is why the cycle carries his name.

A flaw in the machine

RuBisCO is essential but not very good. It evolved long ago, when the atmosphere held little oxygen, and it has a clumsy habit of sometimes grabbing oxygen instead of carbon dioxide, which wastes energy in a process called photorespiration. On hot, dry days, when plants close the pores in their leaves to save water and the internal carbon dioxide runs low, this waste gets worse. Some plants have evolved workarounds. Corn and sugarcane use a system, called C4, that concentrates carbon dioxide around RuBisCO to keep it working efficiently, and desert plants like cacti use a related trick that lets them open their pores only at night. These are different solutions to the same imperfection in an ancient, indispensable enzyme.

Why it matters beyond biology class

Photosynthesis is not just how plants feed themselves. It is the foundation of nearly every food chain, since the animals that eat plants, and the animals that eat those animals, are all living on energy that a plant first captured from sunlight. It built our atmosphere: the oxygen that makes animal life possible was pumped out by photosynthetic bacteria starting roughly two and a half billion years ago, in an event so transformative it is called the Great Oxidation. And it is the original source of fossil fuels, which are nothing more than the bodies of ancient photosynthetic organisms, the captured sunlight of hundreds of millions of years ago, which we are now releasing all at once. Understanding the two-stage trick inside a leaf turns out to be the key to understanding the energy economy of the entire planet.