Photosynthesis: what it is and how it works, explained simply

The oldest food factory in the world

Every green leaf is a tiny chemical factory. The raw materials come in through two doors: water, absorbed by the roots and carried up the stem, and carbon dioxide (CO₂), entering the leaves through microscopic openings called stomata. The energy that powers the factory comes from the sun.

The final product is glucose, a sugar that is the plant’s food, plus a “waste” by-product that for us is anything but waste: oxygen.

The whole process fits in one formula worth remembering:

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Six molecules of carbon dioxide plus six of water, powered by light, become one molecule of glucose and six of oxygen.

Where it happens: chloroplasts and chlorophyll

Photosynthesis runs inside specialized organelles of plant cells, the chloroplasts. Inside them sits chlorophyll, the pigment that gives plants their green color and works like a natural solar panel: it captures the energy of light and makes it available for chemical reactions.

Fun fact: leaves are green precisely because chlorophyll doesn’t use green light: it absorbs mostly red and blue, and reflects the green component — which is what reaches our eyes.

The two stages of photosynthesis

Stage 1: the light reactions

They only happen in the presence of light. Chlorophyll captures solar energy and uses it to split water molecules: the oxygen is released into the air through the stomata, while the energy is stored in two molecules that act as chemical “batteries”, called ATP and NADPH.

Stage 2: the Calvin cycle (the “dark” stage)

Using the energy from the “batteries” charged in the first stage, the plant turns carbon dioxide into glucose. It’s called the “dark” stage because it doesn’t directly need light — not because it happens at night: during the day the two stages run at the same time, like two departments of the same factory.

	Light reactions	Calvin cycle
Needs light?	Yes, directly	No (uses stored energy)
Consumes	Water and light	CO₂, ATP and NADPH
Produces	Oxygen, ATP and NADPH	Glucose

What the plant does with the glucose

The plant uses glucose in three ways:

1. Immediate energy: it “burns” it through cellular respiration to grow and live, exactly as we burn food.
2. Storage: it stores it as starch (which is why potatoes and cereals are so rich in starch).
3. Building material: it turns it into cellulose, the brick it uses to build stems, branches and leaves.

Why it matters to us

Without photosynthesis there would be (almost) no life on Earth, for three reasons:

• The oxygen we breathe comes from the photosynthesis of plants, algae and cyanobacteria — the latter, in the oceans, produce more of it than forests do.
• All the food we eat derives, directly or indirectly, from the glucose made by plants: even a steak is “solar energy” that passed through grass and then an animal.
• CO₂ absorption: photosynthesis removes the main greenhouse gas from the atmosphere, which is why forests are called “the lungs of the planet”.