Cell Communication
The Cellular “Internet”
 Within multicellular organisms, cells must
communicate with one another to coordinate
their activities
 A signal transduction pathway is a series of
steps by which a signal on a cell’s surface is
converted into a specific cellular response
 Signal transduction pathways are very similar in
all organisms, even organisms as different as
unicellular yeasts and multicellular mammals
Local (Short-Distance) Signaling
 Cells may communicate by direct contact
 Plasmodesmata in plant cells
 Gap junctions in animal cells
 Animal cells can also use cell-cell recognition
 Membrane-bound surface molecules can interact and
communicate
Local (Short-Distance) Signaling
 Messenger molecules can also be secreted by the signaling cell
 Paracrine signaling:
 One cell secretes (releases) molecules that act on nearby “target” cells
 Example: growth factors
 Synaptic Signaling:
 Nerve cells release chemical messengers (neurotransmitters) that
stimulate the target cell
Long-Distance Signaling
 Endocrine (hormone)
signaling
Specialized cells
release hormone
molecules, which travel
(usually by diffusion
through cells or
through the circulatory
system) to target cells
elsewhere in the
organism
Hormones in Plants
In plants hormones can travel in vessels or
diffuse through the air as a gas
Ethylene = gas that helps regulate growth
and promotes ripening
The Three Stages of Cell Signaling
 There are 3 stages at the “receiving end” of a
cellular conversation:
1. Reception
2. Transduction
3. Response
Stage 1: Reception
 The target cell “detects” that there is a signal molecule
coming from outside the cell
 The signal is detected when it binds to a protein on the cell’s
surface or inside the cell
 The signal molecule “searches out” specific receptor proteins
 The signal molecule is a ligand
• It is a molecule that specifically binds to another one (think enzymes!)
Stage 2: Transduction
This stage converts the signal into a form
that can bring about a specific cellular
response
One signal-activated receptor activates another
protein, which activates another molecule, etc.,
etc.
These act as relay molecules
Often the message is transferred using protein
kinases, which transfer phosphate groups from
ATP molecules to proteins
Stage 2: Transduction
Stage 3: Response
 The signal that was
passed through the signal
transduction pathway
triggers a specific cellular
response
 Examples: enzyme action,
cytoskeleton
rearrangement, activation
of genes, etc., etc.
 Diagram example:
transcription of mRNA
The Specificity of Cell Signaling
 The particular proteins
that a cell possesses
determine which signal
molecules it will respond
to and how it will respond
to them
 Liver cells and heart cells,
for example, do not
respond in the same way
to epinephrine because
they have different
collections of proteins