What types of organisms are classified as unicellular?

Unicellular organisms consist of a single cell that performs all necessary functions for life. Bacteria and protozoans are prime examples of unicellular organisms. Bacteria, which are prokaryotic, are the simplest form of life, consisting of a single cell that carries out metabolism, reproduction, and response to the environment independently. Protozoans, which are generally classified as eukaryotic, are also predominantly unicellular and can be found in a variety of environments, playing various roles in ecosystems.

In contrast, fungi, plants, and mammals are multicellular organisms, composed of many cells that differentiate into various tissues and functions. For instance, fungi are composed of networks of hyphae and form complex structures such as mushrooms. Plants have specialized cells forming roots, leaves, and stems, while mammals are composed of diverse tissue types that perform specific functions within a multicellular structure. Therefore, the correct classification of unicellular organisms specifically identifies bacteria and protozoans as those that meet the criteria.