In simple terms

Photosynthesis is how plants, algae and some bacteria make their own food. They use energy from sunlight to turn carbon dioxide and water into sugars.

Oxygen comes out as a byproduct. Almost every breath you take depends on it.

How it works

Three inputs are needed: carbon dioxide, water and light. Carbon dioxide enters through tiny holes in a plant leaves and stems, water arrives through the roots, and sunlight supplies the energy.

The product is glucose, a simple sugar. From there the plant builds starches, fats and proteins, along with the wood, leaves and roots that make up its body. Organisms that feed themselves this way are called autotrophs.

The rate is not fixed. It rises and falls with light, temperature and rainfall, which is why tropical forests grow faster than land near the poles. Nutrients matter too, especially nitrogen and phosphorus, and a shortage of either slows growth even in bright sun.

The total organic matter that photosynthesizing organisms make in a place is called its productivity, and ecologists use that figure to compare one ecosystem with another.

This is not only a land process. Drifting plankton in the ocean surface layer photosynthesize in enormous numbers, invisible to the naked eye.

Why it matters

Photosynthesis is the entry point for energy into almost every food chain. Herbivores eat the producers, carnivores eat the herbivores, and the energy traces back to sunlight.

It also built the atmosphere we live in. The first photosynthesizing single celled bacteria appeared more than 3.5 billion years ago, and the rise in atmospheric oxygen about a billion years later reshaped the course of life and formed the ozone layer.

It draws carbon dioxide out of the air, which ties it directly to the climate. Fossil fuels are the buried remains of photosynthetic organisms, and burning them returns that carbon far faster than photosynthesis can take it back.

Trusted sources differ on the ocean share. NOAA puts it at roughly half of oxygen production on Earth, while Berkeley’s Understanding Global Change project puts ocean algae at about 70 percent of atmospheric oxygen. Either way, it is very large.

Where you will see it

• School science lessons and plant biology courses.
• Crop yield and food security reporting.
• Satellite maps of vegetation greenness and ocean plankton.
• Carbon cycle and climate change explanations.

Example

The ocean bacterium Prochlorococcus is the smallest photosynthetic organism known, and NOAA reports that this one species produces up to 20 percent of the oxygen in the biosphere.

Often confused with

Photosynthesis makes sugar and releases oxygen. Cellular respiration does the reverse, using oxygen to release energy from sugar. Plants do both, continuously.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 27, 2026Researched and drafted with AI assistanceReport an issue