Define unicellular and multicellular organisms. Give examples.

Unicellular organisms are life forms consisting of a single cell that performs all the functions necessary for life independently—including nutrition, respiration, excretion, growth, reproduction, and response to stimuli. Despite being structurally simple, these organisms are functionally complete, with that one cell containing all the genetic information and cellular machinery needed for survival. Examples include bacteria (like Escherichia coli), protozoans (like Amoeba and Paramecium), some algae (like Chlamydomonas), and yeast (a fungus). These organisms can be remarkably successful; bacteria, for instance, inhabit virtually every environment on Earth and outnumber all other organisms combined.

Multicellular organisms are composed of many cells that work together in a coordinated manner, exhibiting cellular specialization and division of labor. Different cells are specialized for specific functions—muscle cells for movement, nerve cells for communication, epithelial cells for protection—and are organized into tissues, organs, and organ systems. Examples include all animals (humans, elephants, insects), plants (trees, flowering plants, ferns), most fungi (mushrooms, molds), and some algae (kelp, seaweed). This cellular cooperation allows for larger body sizes, greater complexity, longer lifespans, and more sophisticated interactions with the environment. The transition from unicellular to multicellular life was one of the most significant evolutionary innovations, enabling the diversity of complex life forms we observe today, though it's worth noting that multicellularity evolved independently multiple times across different lineages.