Photosynthesis (from the Greek word photo, meaning “light,” and synthesis, from the Greek word syntithenai, meaning “to put together”) is the process by which plants use energy derived from light in order to make food molecules from carbon dioxide and water. It is basically a two-step process: Light energy derived from sunlight is converted to chemical energy, with oxygen (O2) produced as a waste product of this process. The second step is carbon-fixation reactions (or the conversion of carbon dioxide [CO2] into organic compounds) known as the Calvin cycle—a series of reactions that assemble sugar molecules from carbon dioxide (CO2) and the energy-containing products of the light reactions. (For more about the Calvin cycle, see the chapter “Basics of Biology.”)

Ultimately, photosynthesis is the process that provides food for the entire world. It is estimated that each year more than 250 billion metric tons of sugar are created through photosynthesis. Photosynthesis is a source of food not only for plants, but also all organisms that are not capable of internally producing their own food—including humans—which is why it’s wise to eat your vegetables.