Final answer:
Unicellular organisms are made up of only one cell that carries out all life processes. Multicellular organisms, on the other hand, consist of multiple specialized cells. Unicellularity can be a derived state and has evolved multiple times in the history of life.
Step-by-step explanation:
When an organism is described as unicellular or single-celled, it means that the entire organism is comprised of just one cell. This single cell is responsible for all life processes, such as growth, reproduction, and metabolism. In contrast, multicellular organisms are made up of multiple cells that are often specialized for different functions and may be organized into tissues and organs. Some examples of unicellular organisms include bacteria and amoebae.
An interesting aspect of unicellular life forms is that they exhibit a variety of developmental stages even though they consist of a single cell, which includes processes like cell division and growth. These developmental changes can sometimes lead to different cell types, but these are usually related to reproductive cycles and do not make the organism multicellular. Additionally, the concept of unicellularity is not strictly tied to primitive life forms, as some modern unicellular organisms, like yeasts, can be derived from multicellular ancestors.
The origin of multicellularity in the evolution of life has occurred multiple times, with 'simple multicellularity' appearing at least twenty-five times and 'complex multicellularity' evolving even in instances where there are no specialized tissues involving multiple cell types. However, the term multicellular generally refers to organisms that have specialized cells and tissues, a level of organization beyond what is seen in unicellular organisms.