The word "biology"
means the scientific study of life and living organisms. But what,
exactly, is life? There are many definitions, some of which are
hotly debated by scientists, philosophers, and theologians. One
definition is that life is any system that has three basic
properties: Metabolism, Growth, and Reproduction
Metabolism is the sum of all
the chemical reactions in an organism. Growth is the process of
increasing in size. Reproduction is the process of producing new
individuals of the same kind. Rocks, for example, aren't alive
because they don't metabolize or reproduce. Rocks can grow by adding
mineral deposits to their surface, but they must have all three
properties in order to be considered "alive."
Earlier in the course, we
learned that the cell is the simplest level of biological
organization. By that, we mean it's the smallest unit of life that
has all three of the properties listed above. We can think of it as
the "atom" of biology. Some organisms consist of just a
single cell, while most are far more complex. But even the most
complex organisms begin their lives as single cells — including
humans. By the time an embryo becomes an adult person, that single
initial cell divided enough times to become 100 trillion cells!
In this Lesson, we're going to
explore the basic structure and function of cells. We'll develop a
better understanding of what cells are, then look at the two major
classes of cells: prokaryotes and eukaryotes. We'll conclude by
investigating several important tools biologists use to study cells.