The molecules of living materials are organized into complex structures known as cells. Cells are the basic structural units of living matter, but they are too small to see with the unaided eye. As Figure 1.8 illustrates, cells are bag-like structures surrounded by a barrier called a membrane. 

Membranes allow cells to accumulate certain substances and exclude other substances. In this way, cells can maintain a different composition from their environment.

Imagine a wooded area on an early summer day. Plants provide shelter and food for a variety of birds. A rabbit has created a burrow Figure 1.11 Organisms along and in this nearby and is feeding on stream interact and depend on one another. wild berries. Not far away, a fox just left her den in search of food for her young. This community of different, but connected, living systems illustrates the seventh unifying principle of biology: the interactive and interdependent nature of life.