WHAT IS LIFE?

It is relatively easy to determine that a human being, an oak tree, and a
grasshopper are living whereas rocks are not. Yet it remains difficult to
define life. At one time it was believed that a living system could be distinguished from a nonliving system by its possession of a special ”vital force."
Now, after centuries of searching, we understand that there is no single
substance or force that is unique to living things. Perhaps the best we can
do toward defining life is to list the features that living things have in
common. When we do this, we find that the characteristics that distinguish
most living things from nonliving things include a precise kind of organization, a variety of chemical reactions we term metabolism, the ability to
maintain an appropriate internal environment even when the external environment changes (a process referred to as homeostasis), movement, responsiveness, growth, reproduction, and adaptation to Environmental
change. We consider each of these characteristics in the following section