FIGURE 1-4 Living systems such as the human body consist of closely interrelated elements. (a) The surface of the hand is covered by a living organ, skin, that is composed of several layers of tissue. (b) An outer covering of hard, dead skin cells protects the body from injury, infection, and dehydration. This layer is constantly renewed by living epidermal cells, which also give rise to hair and fur in animals. Deeper layers of muscle and connective tissue give skin its tone and firmness. (c) Tissues are formed through subcellular adhesion structures (desmosomes and hemidesmosomes) that join cells to one another and to an underlying layer of supporting fibers. (d) At the heart of cell-cell adhesion are its structural components: phospholipid molecules that make up the cell-surface membrane, and large protein molecules. Protein molecules that traverse the cell membrane often form strong bonds with internal and external fibers made of multiple proteins.