Every living organism needs energy to survive and they extract that energy from the food they consume. Based on how they obtain energy, living organisms are classified into two groups: autotrophs and heterotrophs.c

The autotrophs can prepare their own food using inorganic raw materials, which they extract from outside sources. Organisms like plants, archaea, protists, algae and a few bacteria belong to this class.c The heterotrophs, on the other hand, do not posses the ability to manufacture their own food. They get the necessary organic nutrients from outside sources. Animals are included in this category.c

Autotrophs obtain the required energy to create their own food directly from the sun. Autotrophs absorb solar energy and convert it into glucose, or sugar, through a complex process called photosynthesis.c Photosynthesis is the process of transforming light energy into chemical energy, which then gets stored as sugar bonds. In this process, glucose and oxygen are produced using carbon dioxide (CO2), water (H2O) and light energy. Photosynthesis occurs in chloroplast cells with the help of the green pigment called chlorophyll. The chemical reaction can be represented as 6CO2 + 6H2O (+ solar energy)-->C6H12O6 + 6O2.c

Autotrophs are extremely important for the environment. Only autotrophs can absorb energy directly from the sun, which makes them the foundation of the ecosystem.c

All animals and the majority of microorganisms (the exceptions are the blue-green bacteria and microscopic algae) are categorized as heterotrophs. Unlike the autotrophs, heterotrophs need to ingest biomass to obtain nutrition and energy.c The heterotrophs are again classified into herbivores, carnivores, omnivores and saprobic organisms based on the type of biomass they consume.

Animals that consume only plants are called herbivores. The carnivores are animals that only consume meats of other animals. Omnivores are animals that eat both plants and animals; human beings are omnivores. Saprobic organisms like fungi absorb nutrition from the environment.c