Final answer:
In a single-celled organism, life functions are performed by the organelles within the cell. These organisms, including prokaryotes and eukaryotes, are capable of handling all life processes independently, contrasting with multi-cellular organisms that have specialized organs and systems.
Step-by-step explanation:
In a single-celled organism, these functions are performed by the various organelles within the cell. In a multi-cellular organism, cells combine to form tissues, which then form organs and organ systems, each with specialized functions. However, in single-celled organisms, such as prokaryotes and eukaryotes, the entire process of life, from reproduction to digestion, is carried out within just one cell that lacks the complexity of organs and organ systems. These single-celled organisms are also known as microorganisms.