Examine the following sections:
1.82 Introduction
Receptor Proteins and Signaling Between Cells. (Page 126)
Types of Cell Signaling. (Page 127)
7.2 Proteins in the cell and on its surface receive signals from other cells. (Page 128)
http://www.mhhe.com/biosci/genbio/espv2/data/cells/003/index.html