Photosynthesis

Photosynthesis is the process by which plants make their own food from sunlight, water and carbon dioxide. It is one of the most important chemical reactions on Earth: it produces almost all of the oxygen we breathe, it provides the food at the base of almost every food chain, and it has been going on for over 2.4 billion years. The name comes from the Greek words photo (light) and synthesis (putting together): literally "putting things together with light".

• InputsCO&sub2;, water, sunlightAll free and abundant
• OutputsGlucose + oxygenGlucose feeds the plant; oxygen is released
• Where it happensChloroplastsTiny green organelles in plant cells
• Green pigmentChlorophyllAbsorbs red and blue light, reflects green
• First photosynthesisapprox. 2.5 billion years agoIn cyanobacteria
• Annual oxygen outputapprox. 300 billion tonnesFrom all photosynthesis on Earth

The photosynthesis equation

Photosynthesis is a complicated chemical reaction, but its overall summary is simple. In words:

carbon dioxide + water + sunlight → glucose + oxygen

Or with the chemical formulas:

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

In words: take 6 molecules of carbon dioxide from the air, 6 molecules of water from the soil, plus some energy from sunlight, and combine them to make 1 molecule of glucose (sugar) and 6 molecules of oxygen.

Where photosynthesis happens

Photosynthesis takes place inside tiny green structures called chloroplasts, which are found mostly in the leaves (and a few other green parts) of plants. Each chloroplast contains stacks of disc-shaped sacs called thylakoids, which are packed with the green pigment chlorophyll. Chlorophyll absorbs the red and blue parts of sunlight and reflects the green parts back, which is why leaves look green.

The reaction needs three raw materials, and the plant has clever ways of getting each one:

• Carbon dioxide: comes in through tiny pores on the underside of leaves called stomata.
• Water: drawn up from the soil by the roots and carried to the leaves through the xylem.
• Sunlight: absorbed directly by chlorophyll in the leaves.

What the plant does with the food

The glucose produced by photosynthesis is the plant's fuel. It can be used straight away to power the plant's own cells, or it can be stored for later. Different plants store glucose in different forms:

• As starch in seeds, roots and tubers (potatoes, rice, wheat).
• As cellulose in cell walls, leaves and wood.
• As sugar in fruit (apples, grapes), sap (maple syrup) or specialised storage tissue (sugar cane, sugar beet).

Almost every bite of food you eat is plant-stored glucose, either directly (an apple, a slice of bread, a potato) or indirectly (a steak from a cow that ate grass, eggs from a chicken that ate corn).

The oxygen we breathe

One particular by-product of photosynthesis matters more than anything else for life on Earth: the oxygen. Almost every oxygen molecule in the air came from photosynthesis. Land plants produce about half of it. The other half comes from microscopic photosynthetic life in the ocean (mostly phytoplankton). Together, every breath you take is the breath of a plant or an algae somewhere on Earth.

Fact Plants do not get any of their mass from the soil. A famous experiment by the Dutch scientist Jan Baptist van Helmont in the 1600s grew a small willow tree from a sapling in a pot of weighed soil. After 5 years the tree had gained over 70 kg in weight, but the soil had lost only a few grams. Almost all the new mass had come from the air (CO₂) and water through photosynthesis. Trees are largely built from invisible air, packaged up by sunlight.

The opposite process: respiration

Photosynthesis stores energy in glucose. But plants (and every other living thing) need to get the energy back out later when they need it. That happens through a process called cellular respiration, which is basically photosynthesis run backwards:

glucose + oxygen → carbon dioxide + water + energy

Plants do respiration too, just like animals, in the mitochondria of every one of their cells. During the day, photosynthesis runs much faster than respiration in a leaf, so the plant gives off more oxygen than it uses. At night photosynthesis stops (no sunlight) but respiration carries on, so plants give off carbon dioxide instead. This is why people sometimes warn against keeping lots of plants in your bedroom overnight, although the effect is tiny.

Did you know? Photosynthesis is not very efficient. A typical plant only converts about 1 to 3% of the sunlight that hits its leaves into stored chemical energy. The rest is lost as heat or reflected back into the air. Modern solar panels convert about 20% of the light energy they receive into electricity, so technologically we have already beaten plants. But plants have been doing it for 2.4 billion years, building themselves out of thin air. That is still a pretty impressive trick.

Deeper dive: the Great Oxygenation Event

For the first 2 billion years of Earth's history, the atmosphere contained almost no oxygen. All life lived without it, and many ancient bacteria were actually poisoned by oxygen if they ever met it. Then a single group of bacteria (the cyanobacteria) evolved the ability to do photosynthesis using water as a raw material, releasing oxygen as a waste product.

For hundreds of millions of years, the oxygen they produced was instantly soaked up by chemicals already in the oceans, especially dissolved iron. Sediments from this era are full of distinctive iron-rich layers called "banded iron formations", which are the rusted remains of the early oceans gradually being oxidised.

Eventually, around 2.4 billion years ago, the oceans ran out of iron to rust, and oxygen started building up in the air for the first time. Levels rose from almost nothing to a few percent of the atmosphere. For most life of the time this was a disaster: oxygen was a deadly poison. Many species went extinct in the Great Oxygenation Event, possibly the largest mass extinction in Earth's history.

The survivors either retreated to oxygen-free habitats or evolved new chemistry that turned the poison into a fuel. Modern aerobic respiration (the way animals and plants release energy from glucose using oxygen) is a direct descendant of this adaptation. So every breath you take is paying back a debt to ancient bacteria that almost destroyed the world by accident.

For the parts of a plant that do photosynthesis, see parts of a plant. For more on plants in general, see what is a plant.