Photosynthesis

A plant needs three main inputs to make food: light, water, and carbon dioxide. Nearly all of the work happens in the leaves, inside organelles called chloroplasts that hold the green pigment chlorophyll. Chlorophyll captures light energy. That energy drives the reaction that builds glucose from water and carbon dioxide, releasing oxygen as a by-product. Plants release far more oxygen than they use, which is why forests are called the planet's lungs.

Write the recipe as carbon dioxide plus water plus light, making glucose plus oxygen. One important detail: the carbon in the glucose comes from carbon dioxide, not from the water. The leaf is not a mouth; it is a factory. It takes simple molecules from air and water and builds the rich carbon compounds that make up stems, leaves, seeds and roots. Every plant cell then uses that glucose as its fuel and its building material.

Here is the surprising part: most of a plant's dry matter is carbon. Roots do bring in water and dissolved minerals, but those are only a small part of a tree's mass. The wood, the leaves and the fruit are mostly carbon and water built from carbon dioxide pulled out of the air. Plant growth is really atmospheric carbon being packed into solid form, season after season.

The soil's real jobs are anchorage, water, and a few mineral nutrients. If the soil supplied most of a plant's matter, a growing field would slowly sink. Weighed-tree experiments show the soil barely loses mass while the tree gains plenty. The missing mass comes from carbon dioxide. Feed a bean sprout sunlight and air, and it grows heavy from what it took out of the sky, not from what it took out of the pot.