I. Cells must communicate to coordinate their activities in multicellular organisms.
II. Cell signaling evolved early in the history of life.
A. One topic of cell “conversation” is sex.
1. Saccharomyces cerevisiae, the yeast of bread, wine, and beer, identifies
potential mates by chemical signaling.
2. There are two sexes, a and , each of which secretes a specific signaling
molecule, a factor and factor, respectively.
3. These factors each bind to receptor proteins on the other mating type.
4. Once the mating factors have bound to the receptors, the two cells grow
toward each other and undergo other cellular changes.
5. The two cells fuse, or mate, to form an a/ cell containing the genes of
B. The process by which a signal on a cell’s surface is converted into a specific
cellular response is a series of steps called a signal-transduction pathway.
1. The molecular details of these pathways are strikingly similar in yeast and
animal cells, even though their last common ancestor lived more than a
billion years ago.
2. Signaling systems of bacteria and plants also share similarities.
3. These similarities suggest that ancestral signaling molecules evolved long
ago in prokaryotes and have since been adopted for new uses by single-
celled eukaryotes and multicellular descendents.
III. Multicellular organisms release signaling molecules that target other cells even if the
communicating cells are far apart.
A. Cells may communicate by direct contact.
B. Both animals and plants have cell junctions that connect to the cytoplasm of
C. Signaling substances dissolved in the cytosol can pass freely between adjacent
D. Animal cells can communicate by direct contact between membrane-bound cell
1. Such cell-cell recognition is important to such processes as embryonic
development and the immune response.
E. In other cases, messenger molecules are secreted by the signaling cell.
1. Some transmitting cells release local regulators that influence cells in the
2. One class of local regulators in animals, growth factors, includes
compounds that stimulate nearby target cells to grow and multiply.
a. This is an example of paracrine signaling, which occurs when
numerous cells simultaneously receive and respond to growth
factors produced by a single cell in their vicinity.
b. In synaptic signaling, a nerve cell produces a neurotransmitter that
diffuses across a synapse to a single cell that is almost touching the
sender. 3. Plants and animals use hormones for long-distance signaling.
a. In animals, specialized endocrine cells release hormones into the
circulatory system, by which they travel to target cells in other
parts of the body.
b. Plant hormones, called growth regulators, may travel in vessels but
more often travel from cell to cell or move through air by
F. In order for a signal to affect a cell, the signal must be recognized by a specific
receptor molecule, and the information it carries must be changed into another
form, or transduced, inside the cell before the cell can respond.
IV. The three stages of cell signaling are reception, transduction, and response.
A. In reception, a chemical signal binds to a cellular protein, typically at the cell’s
surface or inside the cell.
B. In transduction, binding leads to a change in the receptor that triggers a series of
changes in a series of different molecules along a signal-transduction
pathway. The molecules in the pathway are called relay molecules.
C. In response, the transduced signal triggers a specific cellular activity.
D. A signal molecule binds to a receptor protein, causing the protein to change shape.
1. The cell targeted by a particular chemical signal has a receptor protein on
or in the target cell that recognizes the signal molecule.
2. Recognition occurs when the signal binds to a specific site on the receptor
that is complementary in shape to the signal.
3. The signal molecule behaves as a ligand, a small molecule that binds with
specificity to a larger molecule.
4. Ligand binding causes the receptor protein to undergo a change in shape.
5. This may activate the receptor so that it can interact with other molecules.
6. Most signal receptors are plasma membrane proteins, whose ligands are
large water-soluble molecules that are too large to cross the plasma
7. Some signal receptors are dissolved in the cytosol or nucleus of target
a. To reach these receptors, the signals pass through the target cell’s
b. Such chemical messengers are either hydrophobic enough or small
enough to cross the phospholipid interior of the plasma membrane.
c. Hydrophobic messengers include the steroid and thyroid hormones
d. The cytosol of target cells contains receptor molecules that bind
testosterone, activating the receptor.
e. These activated proteins enter the nucleus and turn on specific
genes that control male sex characteristics.
E. These activated proteins act as transcription factors to turn on
genes. Transcription factors control which genes are turned on—that is, which
genes are transcribed into messenger RNA. F. Other intracellular receptors (such as thyroid hormone receptors) are found in the
nucleus and bind to the signal molecules there.
G. Most signal molecules are water-soluble and too large to pass through the plasma
membrane. They influence cell activities by binding to receptor proteins on the
1. Binding leads to changes in the shape of the receptor or to the aggregation
2. These cause changes in the intracellular environment.
3. There are three major types of membrane receptors: G-protein-linked
receptors, receptor tyrosine kinases, and ion-channel receptors.
a. A G-protein-linked receptor consists of a receptor protein
associated with a G protein on the cytoplasmic side.
b. The G protein acts as an on/off switch.
c. If GDP is bound to the G protein, the G protein is inactive.
d. When the appropriate signal molecule binds to the extracellular
side of the receptor, the G protein binds GTP (instead of GDP) and
e. The activated G protein dissociates from the receptor and diffuses
along the membrane, where it binds to an enzyme, altering its
f. The activated enzyme triggers the next step in a pathway leading to
a cellular response.
g. The G protein can also act as a GTPase enzyme to hydrolyze GTP
to GDP. This change turns the G protein off.
h. Now inactive, the G protein leaves the enzyme, which returns to its
i. The whole system can be shut down quickly when the extracellular
signal molecule is no longer present.
4. The tyrosine-kinase receptor system is especially effective when the cell
needs to trigger several signal transduction pathways and cellular
responses at once.
a. This system helps the cell regulate and coordinate many aspects of
cell growth and reproduction.
b. The tyrosine-kinase receptor belongs to a major class of plasma
membrane receptors that have enzymatic activity.
c. A kinase is an enzyme that catalyzes the transfer of phosphate
groups. The cytoplasmic side of these receptors functions as a
tyrosine kinase, transferring a phosphate group from ATP to
tyrosine on a substrate protein.
d. An individual tyrosine-kinase receptor consists of several parts:
(1) An extracellular signal-binding site.
(2) A single alpha helix spanning the membrane.
(3) An intracellular tail with several tyrosines.
e. The signal molecule binds to an individual receptor. f. Ligands bind to two receptors, causing the two receptors to