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Cell Communication Ch. 11
*Cell-to-cell
communication is
essential for
multicellular
organisms
*How do you think
cells
communicate?
*
Animal cells communicate in
many ways:
*Direct contact (gap junctions)
*Secreting local regulators
(growth factors, chemical
messengers, neurotransmitters)
*Long distance (hormones)
Local
• Autocrine signaling respond to molecules
they produce themselves. Examples
include many growth factors.
*
In local signaling, animal cells
May communicate via direct contact
Figure 11.3(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.
*In other cases, animal cells
communicate using local regulators
Local signaling
Target cell
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Neurotransmitter
diffuses across
synapse
Secretory
vesicle
Local regulator
diffuses through
extracellular fluid
(a) Paracrine signaling. A secreting cell acts
on nearby target cells by discharging
molecules of a local regulator (a growth
factor, for example) into the extracellular
fluid.
Target cell
is stimulated
(b) Synaptic signaling. A nerve cell
releases neurotransmitter molecules
into a synapse, stimulating the
target cell.
Long Distance
• A hormone is a chemical released
by a cell in one part of the body,
that sends out messages that
affect cells in other parts of the
organism
Long-distance signaling
Endocrine cell
Blood
vessel
Hormone travels
in bloodstream
to target cells
• All multicellular organisms
produce hormones
Target
cell
• Hormones in animals are often
transported in the blood
Figure 11.4
(c) Hormonal signaling. Specialized
endocrine cells secrete hormones
into body fluids, often the blood.
Hormones may reach virtually all
C body cells.
Signal Transduction Pathways
*Convert signals on a cell’s
surface into cellular responses
*Are similar in microbes and
mammals, suggesting an early
origin
* Earl W. Sutherland’s
Three Stages of Cell Signaling
- suggested that cells receiving
signals went through three
processes
*Reception
*Transduction
*Response
Step One - Reception
*Reception occurs when a signal
molecule binds to a receptor protein,
causing it to change shape
Receptor protein is on the cell
surface
*The binding between signal molecule
(ligand) and receptor is highly specific
*A conformational change in a receptor is
often the initial transduction of the signal
Step Two - Transduction
*The binding of the signal molecule alters the
receptor protein in some way
*The signal usually starts a cascade of reactions
known as a signal transduction pathway
*Multistep pathways can amplify a signal
Step Three - Response
*Cell signaling leads to regulation of
cytoplasmic activities or transcription
*Signaling pathways regulate a variety
of cellular activities
Overview of cell signaling
EXTRACELLULAR
FLUID
1 Reception
Plasma membrane
CYTOPLASM
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signal
molecule
Figure 11.5
Example of Pathway
Steroid hormones bind to intracellular receptors
Hormone
EXTRACELLULAR
(testosterone) FLUID
Plasma
membrane
Receptor
protein
Hormonereceptor
complex
1 The steroid
hormone testosterone
passes through the
plasma membrane.
2 Testosterone binds
to a receptor protein
in the cytoplasm,
activating it.
3 The hormone-
receptor complex
enters the nucleus
and binds to specific
genes.
DNA
4
The bound protein
stimulates the
transcription of
the gene into mRNA.
mRNA
NUCLEUS
Figure 11.6
CYTOPLASM
New protein
5 The mRNA is
translated into a
specific protein.
Receptors in the Plasma
Membrane
There are three main types of
membrane receptors:
*G-protein coupled
*Tyrosine kinases
*Ligand gated Ion channel
G protein
• G proteins ( guanine nucleotide-binding protein) act
as molecular switches inside cells, and are involved
in transmitting signals from outside a cell to its
interior.
• These proteins change between an active
conformation when bound to GTP, and an inactive
conformation when bound to GDP.
• IN the absence of a signal they are bound to GDP.
• Signal results in the release of GDP and the binding
of abundant GTP. After a short period of time they
hydrolyze GTP and come back to their “off” state.
Protein Kinases
• Upon activation they add phosphate groups to
themselves and/or other proteins at either
serine/threonine, or at tyrosine residues.
• Their activity can be regulated by second messengers,
interaction with other proteins, or by phosphorylation
itself. They are opposed by phosphatases that remove
phosphate groups from specific phosphorylated
proteins.
Ion channel receptors
Signal
molecule
(ligand)
Gate closed
Ligand-gated
ion channel receptor
Ions
Plasma
Membrane
Gate open
Cellular
response
Gate close
Intracellular receptor model
• For a signal molecule to bind with an intracellular
receptor it must be able to pass through the cellular
membrane.
• Generally signal molecules that enter the cell are
nonpolar and fat soluble.
• These signal molecules can pass through the lipid
bilayer of the cell membrane.