Single-celled organisms of the kingdom Monera are broken down into two subgroups: archaea and bacteria. Heterotrophic bacteria can obtain their food from a variety of sources. Some take their food directly from other organisms, such as bacteria that cause diseases in human beings or bacteria that live in dead or decaying material. Others get their nutrients through fermentation, which is when bacteria break down carbohydrates (with alcohol as a byproduct of this process).
The kingdom Protista is somewhat of a catch-all kingdom; almost all of its members are single-celled (though there are some multicellular exceptions), but their cells are more complex than those of bacteria, hence their own kingdom. Like bacteria, some protists, including algae, are autotrophic. Heterotrophic protists effectively envelop their food (which often consists of other protists) to consume it. Heterotrophic protists move towards their food in different ways, such as with a flagellum (which is like a primitive tail) or pseudopods (Greek for "false foot").
Despite their similarities to plants in both appearance and cells, all organisms in the kingdom Fungi are heterotrophs. Fungi consume other organisms by directly absorbing their nutrients. Some fungi grow off decaying matter (such as moss spores on a tree log), while others infect living organisms (for example, ringworm in humans). The fungus's hyphae break down and digest absorbed materials. The cells of fungi are made out of chitin, which makes the cell hard and able to withstand attachment to other organisms.
All organisms in the kingdom Animalia are defined by their multicellular and heterotrophic nature. Animals are further subdivided by the types of organisms they eat: herbivores only eat non-animals, carnivores only eat other animals and omnivores eat all types of organisms. Most animals are herbivores, primarily because the majority of animals are insects. Some animals, such as mammals, have developed extensive digestive tracts for breaking down other organisms, whereas others (such as jellyfish) are almost protist-like in their consumption of food.