because they are the simplest units of structure and function capable of life, cells are often called the ______

of life.

building blocks?

I would reserve building blocks for amino acids. But one could apply it to cells.

Cells are often called the "building blocks" of life. The reason for this is because cells are the smallest fundamental units of structure and function in living organisms. They are responsible for carrying out essential processes such as metabolism, growth, and reproduction. Without cells, life as we know it would not be possible. Therefore, cells can be thought of as the basic building blocks that make up all living organisms.