Unicellular organisms, such as bacteria and amoebas, consist of a single cell that performs all essential functions, including obtaining nutrients, metabolism, reproduction, and responding to stimuli. In contrast, multicellular organisms, like plants and animals, are composed of specialized cells organized into tissues, organs, and systems. Each cell type performs specific functions, allowing for division of labor and increased complexity in tasks such as nutrient absorption, locomotion, and reproduction, enabling multicellular organisms to thrive in diverse environments.