One-celled organisms, such as bacteria and protozoa, are unicellular, meaning they are made up of a single cell. In contrast, many-celled organisms, such as animals and plants, are multicellular, meaning they are made up of many cells. The cells of a one-celled organism must be capable of carrying out all the necessary functions of life, such as obtaining nutrients, eliminating waste, and reproducing, on their own. In many-celled organisms, cells are specialized to perform specific functions. For example, muscle cells are specialized for contraction, while nerve cells are specialized for transmitting signals. Additionally, many-celled organisms have more complex systems for communication and coordination between cells, such as the nervous and endocrine systems. Overall, the differences between the cells of one-celled and many-celled organisms reflect the different strategies these organisms have evolved to survive and thrive in their respective environments.