Carbon is so important to living things that the Earth's inhabitants are sometimes referred to as "carbon-based life." Autotrophs are those organisms that are able to extract raw carbon from the atmosphere and turn it into energy-rich compounds; by contrast, heterotrophs are those organisms that cannot produce their own carbon-based food and must obtain it by consuming other materials — very frequently, the same ones produced by the autotrophs.
Autotrophic organisms are called "producers" because they create their own food; the word "autotrophic" in Greek means "self-feeding." A small number of bacteria, including the ancient Archaea group, are capable of generating food from sulfur or other chemical reactions, but the majority of autotrophs rely on sunlight. As a result, they are known as "phototrophs," a group that encompasses the remaining autotrophic bacteria as well as plants.
One of the most common autotrophic behaviors is called "photosynthesis." In this process, specialized molecules capture carbon from the air and bind it to water using energy produced from sunlight. Following the standard scientific terminology that molecules using water are known as "hydrates," the resulting carbon compound is known as a "carbohydrate." Because it removes free-floating atmospheric carbon and converts it into solid form, this photosynthetic process is known as "carbon fixation." The ability to fix carbon is the primary difference between autotrophs and heterotrophs.
Most types of life, including most bacteria, cannot fix carbon and must obtain their energy by consuming either organic compounds produced by autotrophs or by relying on sulfur or hydrogen reduction. Animals, including people, fall into the former category, along with fungi and those single-celled organisms that do not possess a cell nucleus. Many autotrophs are capable of consuming the carbohydrates produced by autotrophs, and are therefore part of a larger carbon cycle that encompasses most forms of life.
Not all organisms fit neatly into a division between heterotroph and autotroph. If an organism must produce its own carbon compounds rather than consuming those produced by others, it is known as "obligate" autotroph. Some bacteria and other microorganisms, though, can either obtain carbon from autotrophic activity or rely on other organic material for it. These organisms have more complex scientific names based on the exact nature of their energy production but fall into the general category of "mixotrophs," combining heterotrophic and autotrophic activity.