Here's a breakdown of why:

* Cells are the fundamental unit of life: Every living thing is made up of one or more cells.

* Cellular processes drive life functions: Individual cells carry out all the essential processes that keep an organism alive, including:

* Metabolism: Breaking down nutrients for energy and building complex molecules.

* Growth and Development: Increasing in size and complexity.

* Reproduction: Creating new cells to replace old ones or to create offspring.

* Response to Stimuli: Reacting to changes in the environment.

* Homeostasis: Maintaining a stable internal environment.

While cells are the fundamental unit, they often work together in organized systems to perform more complex functions:

* Tissues: Groups of similar cells working together (e.g., muscle tissue, nervous tissue).

* Organs: Different tissues working together to perform a specific function (e.g., heart, lungs, brain).

* Organ Systems: Multiple organs cooperating to carry out a major bodily function (e.g., circulatory system, digestive system).

So, while the basic functions of life are carried out at the cellular level, the complex workings of a multicellular organism are made possible by the coordinated efforts of many cells organized into tissues, organs, and organ systems.