How does a plant make sugars?

A tree grows a thicker trunk without eating a pile of wood. Much of the carbon in that new wood once drifted through the air as carbon dioxide. A leaf helps turn this invisible ingredient into sugars that supply both building material and stored energy.

The process is photosynthesis. Water enters through roots and travels up the plant. Carbon dioxide enters leaves through tiny openings called stomata. Inside many leaf cells are chloroplasts, small structures containing the green pigment chlorophyll. Chlorophyll absorbs light, supplying energy for a series of chemical changes.

A useful comparison is a workshop that needs both materials and power. Water and carbon dioxide supply atoms, the tiny pieces from which substances are built. Light supplies energy for rearranging those atoms. It does not become an extra pile of material. Through several connected reactions, the plant makes sugars and releases oxygen. The oxygen released during photosynthesis comes from water.

Those sugars do not all stay in the leaf. Dissolved sugars travel through the plant to growing stems, roots and fruits. Some become starch, a food reserve. Others provide the pieces for cellulose, the tough material in plant cell walls. A sweet fruit and a sturdy stem can therefore begin with the same sugar-making process, followed by different chemical changes.

Plant cells also use sugars in respiration to make energy available for their work. Respiration continues in daylight and darkness; photosynthesis needs light. Roots obtain water and minerals from soil, but soil is not a store of ready-made sugar meals. The next time a sapling becomes taller, part of the explanation is right above it: ordinary air has supplied carbon for its growing body.