What is the basic unit of life that all organisms are composed of according to cell theory?

• A. The atom
• B. The cell
• C. The tissue
• D. The organ system

Answer: (B)

Cells are the basic unit of life because they are the smallest structures that can carry out all the activities that define life, such as getting and using energy, growing, responding to the environment, and reproducing. According to cell theory, all living things are made of one or more cells, and new cells come from existing cells. An atom is the building block of matter, not life itself; a tissue is a group of similar cells working together, and an organ system is a higher level of organization made of organs and tissues. So the cell is the correct answer because it is the smallest unit capable of performing the life processes that keep organisms alive.