Photosynthesis.

Step 1 of 5

From thin air

All the energy in your food, in bread, meat and chocolate, once came from the Sun, by way of a plant. Plants, algae and a few bacteria are the only living things that can build sugar from simple substances, and they do it by photosynthesis:

$6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} \xrightarrow{\text{light}} \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2}$⁠

Carbon dioxide from the air and water from the roots become, with the energy of light, glucose and oxygen. The Scene shows a chloroplast: light packets fall on it from above, carbon dioxide molecules arrive from the left and water from below, sugar piles up inside it, and oxygen bubbles rise out.

Step 2 of 5

Light is energy

Light does not turn into matter. It supplies the energy that makes it possible to build a sugar molecule, which holds far more energy than the carbon dioxide and water it was built from. That energy stays stored in the sugar, until some cell breaks it down by respiration.

Step 3 of 5

Inside the chloroplast

Cut open has opened the chloroplast. Inside are stacks of membrane discs, grana, holding chlorophyll, the pigment that absorbs red and blue light and reflects green. Photosynthesis has two stages: in the discs, light splits water and charges up energy carriers; in the fluid round them, the stroma, that energy is used to join carbon dioxide into sugar.

Step 4 of 5

The scarcer input

Light and Carbon dioxide each arrive at their own rate. Sugar is built fast only when there is enough of both.

Step 5 of 5

The equation

The photosynthesis equation balances: there are as many of each kind of atom on both sides. Atoms are neither made nor lost; they are only rearranged.