How photosynthesis actually works

Almost every meal you eat and every breath you take traces back to one quiet chemical process happening inside green leaves. Photosynthesis is how plants, algae and certain bacteria capture the energy of sunlight and lock it into sugar — building food out of little more than air, water and light. It is one of the most important reactions on Earth, and understanding how it works explains why plants are green, why they need sunlight and water, and why life as we know it depends on them.

Turning light into food: the big picture

At its simplest, photosynthesis takes three cheap ingredients — carbon dioxide from the air, water from the soil, and light from the sun — and rearranges them into glucose (a sugar the plant uses for energy) and oxygen, which is released as a by-product. The energy of sunlight is what drives this otherwise uphill transformation, and it ends up stored in the chemical bonds of the sugar.

This all happens inside tiny structures called chloroplasts, found mainly in the cells of leaves. Chloroplasts are packed with chlorophyll, the green pigment that absorbs light — and because chlorophyll absorbs red and blue light while reflecting green, leaves look green to our eyes.

Two stages: capturing energy, then building sugar

Photosynthesis runs in two connected stages inside the chloroplast:

• The light reactions happen in the chloroplast's internal membranes. Chlorophyll absorbs sunlight and uses that energy to split water molecules, releasing oxygen and capturing the energy in two energy-carrying molecules, ATP and NADPH.
• The Calvin cycle (the "light-independent" reactions) happens in the fluid around those membranes. Here the ATP and NADPH power a series of steps that take carbon dioxide from the air and assemble it, carbon by carbon, into glucose.

The first stage captures energy; the second stage spends it to build food. One cannot run usefully without the other.

A leaf is a solar factory that runs on water and air. It does not burn fuel to make energy — it uses light to pack energy into sugar, and hands out oxygen as change.

Why it matters for students and researchers

Photosynthesis sits at the heart of biology, and it connects to some of the most urgent questions in science today — from boosting crop yields to feed a growing world, to engineering more efficient photosynthesis, to designing artificial "leaves" that make clean fuel from sunlight. It draws on plant biology, biochemistry, genetics and environmental science all at once. Following the peer-reviewed literature is how life-science and agriculture students and professionals keep pace with research that touches food security and climate alike.

Frequently asked questions

What is photosynthesis in simple terms?

Photosynthesis is the process by which plants, algae and some bacteria use sunlight to turn carbon dioxide and water into sugar (food) and oxygen. The light provides the energy, and the sugar stores that energy in a form the organism can use later.

What are the two stages of photosynthesis?

The first stage, the light reactions, uses sunlight to split water, release oxygen and capture energy as ATP and NADPH. The second stage, the Calvin cycle, uses that energy to turn carbon dioxide into glucose. The first captures energy; the second uses it to build sugar.

Why are plants green?

Plants are green because of chlorophyll, the pigment that absorbs sunlight for photosynthesis. Chlorophyll absorbs mostly red and blue light and reflects green light, so the green wavelengths reach our eyes and the plant appears green.

Why is photosynthesis important for life?

Photosynthesis produces the oxygen most living things breathe and forms the base of nearly every food chain, since the sugars plants make feed the animals that eat them. Without it, most life on Earth could not exist.