A green leaf builds sugar from two of the simplest molecules there are: carbon dioxide from the air and water from the soil. Sunlight supplies the energy, and oxygen comes out as a by-product.
This is photosynthesis, from Greek words meaning "putting together with light". Carbon dioxide enters a leaf through tiny pores called stomata, mostly on its underside. Water travels up from the roots. The work happens in chloroplasts, the tiny green parts of leaf cells, and a single leaf cell can hold dozens of them.
Glucose, C₆H₁₂O₆, is the sugar ring with five -OH groups that plants join into starch and cellulose. Count the atoms on each side, as you would for methane burning, and they match.
| Atom | 6 CO₂ + 6 H₂O | C₆H₁₂O₆ + 6 O₂ |
|---|---|---|
| carbon | 6 | 6 |
| hydrogen | 6 × 2 = 12 | 12 |
| oxygen | 12 + 6 = 18 | 6 + 12 = 18 |
Energy stored in sugar
Photosynthesis is endothermic: it takes energy in. The C=O bonds in carbon dioxide and the O–H bonds in water are strong. Breaking them takes more energy than making the bonds in glucose and oxygen gives back, so the plant has to supply the difference, and sunlight provides it.
That energy stays in the glucose. A gram of glucose stores about 16 kJ, and burning it gives the same energy back as heat. Your body releases it too, in many small steps, from the sugar and starch in your food. Almost all the energy in food was sunlight first.