Chapter 8 – How Cells Communicate with Each Other and
with the Environment
Chapter Outline
1. General features of cell
communication
2. Receptor activation
3. Cell surface receptors
4. Intracellular receptors
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8.1 General Features of Cell Communication
Section 8.1 Learning
Outcomes
1. List 2 general reasons cells
need to respond to signals
2. Compare and contrast the 5
ways that cells
communicate with each
other based on the distance
between them
3. Outline the 3-stage process
of cell signaling
4. Describe 3 different
examples of how cells
respond to signals
8.1 General Features of Cell Communication
• All living organisms (bacteria, archaea, protists, fungi, plants, and
animals) require cell communication to survive
• Cell communication (aka cell signaling) involves both
• Responding to incoming signals and
• Producing outgoing signals
• Signals are often
chemical molecules
• Signals can also
be physical
parameters like
light or
temperature
8.1 General Features of Cell Communication
Cells Detect and Respond to Signals from Their
Environment and from Other Cells
Why is cell communication
necessary?
1. To respond to a changing
environment; a cellular
response to the
environment can be
critical for survival
2. To communicate with
other cells (this is
particularly important for
multicellular organisms)
8.1 General Features of Cell Communication
Cell-to-Cell Communication Occurs Between Adjacent
Cells and Between Cells That Are Long Distances Apart
Signals are relayed between cells in 5 common ways:
1. Direct intercellular signaling, where cell junctions allow signals to
directly pass from one cell to another
2. Contact-dependent signaling, where molecules bound to the
surface of cells serve as signals to other cells encountering them
3. Autocrine signaling, where cells secrete signaling molecules that
bind to their own surface receptors
4. Paracrine signaling, where cells release signals that affect nearby
cells
5. Endocrine signaling, where signals (called hormones) are
released into the blood and affect target cells that are far away
8.1 General Features of Cell Communication
Cell-to-Cell Communication Occurs Between Adjacent
Cells and Between Cells That Are Long Distances Apart
8.1 General Features of Cell Communication
Cells Usually Respond to Signals via a 3-Stage Process
• Three common stages of cell signaling are receptor activation,
signal transduction, and cellular response
• Often, a group of proteins form a signal transduction pathway
8.2 Receptor Activation
Section 8.2 Learning
Outcomes
1. Explain how a signaling
molecule, or ligand, binds
to its receptor and causes a
conformational change that
activates the receptor
2. Interpret the affinity,
measured as a dissociation
constant, that a receptor
has for its signaling
molecule, or ligand
8.2 Receptor Activation
Receptors Bind to Chemical Signals and Undergo
Conformational Changes
• A ligand (signaling molecule) binds noncovalently to its receptor
with high specificity
• Ligand binding changes the receptor structure; this conformational
change transmits the signal across the membrane
• Once the ligand is
released, the
receptor reverts
to the inactive
conformation
8.2 Receptor Activation
Receptors Have a Measureable Affinity for Their Ligands
• The complex that forms between the ligand and its receptor is
stable for a finite period of time; the complex will dissociate
kon
[Ligand] + [Receptor] ⇌ [Ligand-Receptor complex]
koff
• Brackets [ ] refer to concentration
• kon is the rate at which binding occurs
• koff is the rate at which the ligand-receptor complex dissociates
• Generally, binding and release between a ligand and receptor is
relatively rapid and equilibrium is reached
8.2 Receptor Activation
Receptors Have a Measureable Affinity for Their Ligands
8.3 Cell Surface Receptors
Section 8.3 Learning
Outcomes
1. Compare and contrast the 3
general types of cell surface
receptors
2. Sketch a schematic diagram
of each type of cell surface
receptor and explain how it
functions
8.3 Cell Surface Receptors
• Most signaling molecules are small and hydrophilic or too large to
pass through the plasma membrane
• Cell surface receptors that are embedded in the plasma membrane
detect these types of extracellular signals
• An individual cell may have hundreds of different cell surface
receptors
• Most receptors can be classified as either
1. Enzyme-linked receptors
2. G-protein-coupled receptors
3. Ligand-gated ion channels
8.3 Cell Surface Receptors
• Enzyme-linked receptors
typically have 2 important
domains: an extracellular
signal-binding domain
and an intracellular
catalytic domain
• Most enzyme-linked
receptors function as
protein kinases; they
transfer a phosphate
group from ATP to a
specific amino acid in
target proteins
• Phosphorylation alters
the structure and
function of a protein
8.3 Cell Surface Receptors
• G-protein-coupled receptors (GPCRs) interact with intracellular
proteins called G proteins; G proteins can bind GTP and GDP
• Receptor activation causes the α subunit and β/γ dimer of the G
protein to dissociate, then the subunits interact with other proteins
8.3 Cell Surface Receptors
• When a signaling molecule binds a ligand-gated ion channel, the
channel opens and allows the flow of ions across the membrane
• In animals, ligand-gated ion channels are important in
communication between neurons and in communication from
neurons to muscle cells
8.4 Intracellular Receptors
Section 8.4 Learning
Outcomes
1. Describe intracellular
receptors, using the
estrogen receptor as an
example
2. Sketch a schematic diagram
of an intracellular receptor
and explain how it
functions
8.4 Intracellular Receptors
• Intracellular receptors, found in the cytosol or nucleus, bind signal
molecules that are small and hydrophobic
• Such signals can diffuse across the plasma membrane
• Activation of an
intracellular receptor
typically leads to a
change in gene
expression
• Steroid hormones like
estrogen and
testosterone utilize
intracellular receptors
8.4 Intracellular Receptors
Chapter 8 Summary
8.1 General features of cell communication
• Cells detect and respond to signals from their environment and
from other cells
• Cell-to-cell communication occurs between adjacent cells and
between cells that are long distances apart (5 common forms)
• Cells usually respond to signals via a three-stage process
(receptor activation, signal transduction, cellular response)
8.2 Receptor activation
• Receptors bind to chemical signals and undergo conformational
changes
• Receptors have a measureable affinity for their ligands (Kd)
8.3 Cell surface receptors
• Most cell surface receptors fall into one of three categories:
enzyme-linked receptors, G-protein-coupled receptors, and
ligand-gated ion channels
Chapter 8 Summary
8.4 Intracellular receptors
• Intracellular receptors, found in the cytosol or nucleus, bind
signal molecules that are small and hydrophobic