Final answer:
Unicellular organisms consist of a single cell with organelles performing specialized functions, while multicellular organisms consist of multiple specialized cells that form organized tissues and organs.
Step-by-step explanation:
Both unicellular and multicellular organisms are composed of cells, which are the fundamental units of life. Unicellular organisms are single-celled, with a single uninucleate cell performing all functions necessary for life. Despite their simplicity, unicellular organisms can be complex, with internal structures like organelles performing specialized functions. For instance, the Euglena has an eye spot and contractile vacuole analogous to organs in multicellular organisms, despite being a single cell.
Multicellular organisms, on the other hand, exhibit complex multicellularity. This complexity is characterized by cells that differentiate into various types and form a highly organized structure, including tissues and organs. These specialized cells work together to perform the organism's multiple functions, with different cell types fulfilling different roles. Development in multicellular organisms involves the organization of these cell types in a three-dimensional pattern, leading to the formation of tissues and organs.
Lastly, colonial composition represents a form of simple multicellularity where the organism is made of similar cells without distinct tissues and organs. This contrasts with complex multicellularity, which involves specialized cells and the architectural arrangement of tissues and organs.