You Must Know

3 stages of cell communication
 Reception, transduction, & response
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

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How G-protein-coupled receptors receive cell
signals & start transduction
How receptor tyrosine kinase receive cell
signals & start transduction
How a cell response in the nucleus turns on
genes while in the cytoplasm it activates
enzymes
What apoptosis means & why it is important to
normal functioning of multicellular organisms
11.1
External signals are converted into
responses within the cell
 Animal cells communicate:

 by direct contact
 by secreting local regulators (growth factors
or neurotransmitters)

3 stages of cell signaling:
 1) Reception:
○ The target cell’s detection of a signal molecule
coming from outside the cell
 2) Transduction:
○ Conversion of the signal to a form that can
bring about a specific cellular response
 3) Response:
○ Specific cellular response to the signal
molecule
11.2
Reception: a signal molecule binds to a
receptor protein, causing it to change
shape
 The binding between a signal molecule
(LIGAND) & a RECEPTOR is highly
specific
 A change in the shape is the initial
transduction of the signal


Receptors are found in 2 places:
 1) Intracellular
○ Inside membrane in the cytoplasm or nucleus
○ Signal molecule MUST cross the membrane
(hydrophobic) : examples – steroids & NO
 2) Plasma membrane receptors
○ Bind to water-soluble ligands

3 types of plasma membrane receptors:
 1) G-Protein-Coupled receptor
 2) Receptor tyrosine kinase
 3) Ligand-gated ion channels

1) G-Protein-Coupled receptor
 Step 1
○ The ligand (signaling molecule) has bound to the Gprotein-coupled receptor
○ Causes a conformational change in the receptor so it
can bind to an inactive G-protein
○ This causes a GTP to displace the GDP
○ This activates the G-protein
 Step 2
○ The G protein binds to a specific enzyme & activates it
○ When activated, it triggers the next step in a pathway
leading to cellular response
○ All shape changes are temporary
○ To continue, new molecules are required

2) Receptor tyrosine kinase
 Step 1
○ Shows binding of signal molecules to the
receptors & formation of a dimer
○ Each tyrosine kinase adds a phosphate from an
ATP
 Step 2
○ Fully activated receptor protein that initiates a
unique response
○ The ability of a single ligand to activate multiple
responses is the difference between #2 & #1

3) Ligand-gated ion channels
 Caused by specific signal molecules
 Opens & closes to regulate the flow of
specific ions - Na+ or Ca2+
11.3
Transduction – Multiple molecular
interactions relay signals from receptors
to target molecules
 Involve a phosphorylation cascade

 Usually a multistep pathway
 Leads to greater amplifying of the signal
 Enzymes called protein kinases
phosphorylate & activate proteins
 Allows for a greater cellular response

Not all components are proteins
 May include small, nonprotein water-soluble
molecules or ions called SECOND
MESSENGERS
 Calcium ions & cyclic AMP are examples
 Once activated, they can initiate a
phosphorylation cascade
11.4
Response – Cell signaling leads to
regulation of transcription or cytoplasmic
activities
 Many pathways regulate protein
synthesis

 Done by turning specific genes on or off in
the nucleus
 The final activated molecule in a signaling
pathway functions as a transcription factor

In the cytoplasm
 Signaling pathways often regulate the
activity of proteins rather than their synthesis
 Example: final step in signaling pathway
may affect the activity of enzymes or cause
cytoskeleton rearrangement
11.5

Apoptosis – integrates multiple cell
signaling pathways
 Controlled cell suicide
 The cell is systematically dismantled &
digested
 Protects neighboring cells from damage that
would occur if a dying cell leaked out its
enzymes

Apoptosis
 Triggered by signals that activate a cascade
of suicide proteins in the cells
 In vertebrates, a normal part of
development:
○ Normal nervous system
○ Operation of immune system
○ Normal morphogenesis of hands & feet in
humans