What is the smallest unit of life?

The smallest unit of life is the cell. There's no smaller entity that can independently carry out all the functions necessary for life.

While viruses are smaller than cells, they are not considered alive because they:

• Cannot reproduce independently: They require a host cell to replicate.
• Lack the necessary cellular machinery: They don't have ribosomes, cytoplasm, or other organelles essential for metabolism and independent function.
• Don't maintain homeostasis: They don't regulate their internal environment.

Cells, on the other hand, exhibit all the characteristics of life:

• Organization: They have a complex internal structure.
• Metabolism: They carry out chemical reactions to obtain and use energy.
• Growth: They increase in size and complexity.
• Adaptation: They can evolve over time.
• Response to stimuli: They react to changes in their environment.
• Reproduction: They can create copies of themselves.

There are two main types of cells: prokaryotic and eukaryotic. Prokaryotic cells (like bacteria and archaea) are simpler and lack a nucleus and membrane-bound organelles. Eukaryotic cells (like those in plants, animals, fungi, and protists) are more complex and possess a nucleus and other specialized organelles. Even the smallest prokaryotic cells are still considered the smallest units of independent life.