All organisms are made of cells. In

All organisms are made of cells. In the hierarchy of biolog-
ical organization, the cell is the simplest collection of matter
that can be alive. Indeed, many forms of life exist as single-
celled organisms. More complex organisms, including plants
and animals, are multicellular; their bodies are cooperatives
of many kinds of specialized cells that could not survive for
long on their own. Even when cells are arranged into higher
levels of organization, such as tissues and organs, the cell re-
mains the organism’s basic unit of structure and function.
All cells are related by their descent from earlier cells. How-
ever, they have been modified in many different ways during
the long evolutionary history of life on Earth. But although
cells can differ substantially from one another, they share
common features. In this chapter, we’ll first examine the tools
and techniques that allow us to understand cells, then tour
the cell and become acquainted with its components.