Essential Cell Biology 5th edition

CHAPTER SIXTEEN
16
Cell Signaling
Individual cells, like multicellular organisms, need to sense and respond
to their environment. A free-living cell—even a humble bacterium—must
be able to track down nutrients, tell the difference between light and
dark, and avoid poisons and predators. And if such a cell is to have any
kind of “social life,” it must be able to communicate with other cells.
When a yeast cell is ready to mate, for example, it secretes a small protein
called a mating factor. Yeast cells of the opposite “sex” detect this
chemical mating call and respond by halting their progress through the
cell-division cycle and reaching out toward the cell that emitted the signal
(Figure 16–1).
GENERAL PRINCIPLES OF
CELL SIGNALING
G-PROTEIN-COUPLED
RECEPTORS
ENZYME-COUPLED RECEPTORS
In a multicellular organism, things are much more complicated. Cells
must interpret the multitude of signals they receive from other cells to
help coordinate their behaviors. During animal development, for example,
cells in the embryo exchange signals to determine which specialized
role each cell will adopt, what position it will occupy in the animal, and
whether it will survive, divide, or die. Later in life, a large variety of signals
coordinates the animal’s growth and its day-to-day physiology and
behavior. In plants as well, cells are in constant communication with one
another. These cell–cell interactions allow the plant to coordinate what
happens in its roots, stems, and leaves.
In this chapter, we examine some of the most important mechanisms
by which cells send signals and interpret the signals they receive. First,
we present an overview of the general principles of cell signaling. We
then consider two of the main systems animal cells use to receive and
interpret signals. Finally, we describe a few signaling mechanisms that
work in a different way—including one that operates in plants—before