9Signal Transduction and Cell Growth
One characteristic of organisms is that they exhibit various behaviors in response to changes in their environment (i.e., the outside world). Cells are equipped with numerous receptor proteins in order to detect changes that occur on an extracellular level. When bound with signaling molecules, receptors change their structure, thereby greatly altering the structure and functions of cells via intracellular signal transduction pathways.
In unicellular organisms, information related to chemicals (such as nutrients and oxygen) and physical stimuli (such as temperature and light) are primarily communicated, whereas in multicellular organisms, signal transduction between cells also takes place. Transduced signals regulate cell functions through protein activation in the short term and through genetic regulation in the long term.
The series of events through which one parent cell replicates its genetic information and is divided into two daughter cells is called the cell cycle. This cycle is arrested in many of the cells of multicellular organisms, and is resumed in response to cell growth signals. In cancer cells, this regulation system is mutated, causing uncontrolled cell growth.