Topic 1 Regents Biology Review: Living Organisms

Topic Overview

Earth's living environment is made up of millions of diverse organisms, from towering redwood trees, sleek antelope, and mushrooms that grow in huge circles, to microscopic bacteria, one-celled organisms that turn the tides red, and the students in your class. These living organisms are both similar to and different from each other. They also differ from the nonliving parts of the environment. Although that difference may seem obvious, scientists have not been able to agree upon a simple definition of life.

The Characteristics of Life

Although there is no simple definition of life, most scientists agree that living things share certain characteristics that distinguish them from nonliving things.

• Living things are organized structures. All are made of one or more cells, which are the basic units of structure and function. They maintain their cellular organization throughout life.

• Living things use energy to maintain life and to grow and develop. These activities require that the cells carry out various chemical reactions. The combination of all the chemical reactions that occur in an organism is called metabolism.

• Living things maintain a fairly stable internal environment even when their external environment changes dramatically. The maintenance of this internal stability is known as homeostasis. To maintain homeostasis, organisms must respond and adapt to both their internal and external environments.