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Lecture:1

Cell signaling

Ms
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If so, can cells detect what's going on around them or respond from their neighbors and
environment
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Cell signaling is the process of cellular communication within the body driven by cells releasing and
receiving hormones and other signaling molecules
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Cell signaling enables coordination
within multicellular organisms
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Cell signaling is needed by organisms to
coordinate a wide variety of functions
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Cells typically communicate
using chemical signals
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These signal molecules are often secreted from the
cell and released into the extracellular space
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Examples include Nerve cells that communicate with muscle cells to create movement,
immune cells must avoid destroying cells of the body, and cells must organize during the development
of a baby
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This signal
is then received by a “target” cell
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Non-target cell: cell does not have a receptor for the ligand (though it may have other
kinds of receptors)
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Stages of Cell Signaling: Not all cells can receive a particular chemical message
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When a signaling
molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a change inside
of the cell
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The message carried by a ligand is often carried through a chain of chemical messengers inside the
cell
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Thus, the original intercellular (between-cells)
signal is converted into an intracellular (within-cell) signal that triggers a response
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Ruqiyya

Three Stages of Cell Signaling:
Signal transduction have three stages:
1) First, reception in which the signal molecule (ligand) binds the receptor
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Forms of signaling: In multicellular organism, a cell may need to signal to other cells that are at various
distances from the original cell
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The main
difference between the different categories of signaling is the distance that the signal travels through
the organism to reach the target cell
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This type of signaling, in which cells communicate over relatively short distances, is
known as paracrine signaling
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Such signaling is found in responses to allergens, tissue repair, the formation of scar tissue,
and blood clotting but especially important during development, when they allow one group of cells
to tell a neighboring group of cells what cellular identity to take on like spinal cord development
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The image shows
a signaling molecule produced by one cell diffusing a short distance to a neighboring cell
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This process is named for the synapse (the junction between two nerve cells where signal transmission
occurs)
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When the impulse reaches the synapse, it triggers the release of ligands
called neurotransmitters, which quickly cross the small gap between the nerve cells
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The neurotransmitters that are released into the chemical synapse are quickly degraded or taken back

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Lecture:1

Cell signaling

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This "resets" the system so they synapse is prepared to respond quickly to the
next signal
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It diffuses across the small gap between sending and target neurons and binds to receptors on the
target neuron
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An example of an autocrine
agent is the cytokine interleukin-1 in monocytes
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Autocrine signaling is important during development, helping cells to take on correct identities
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Autocrine signaling: a cell targets itself, releasing a signal that can bind to receptors on its own surface
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Signals in form of hormones are produced in one part of
the body and travel through the circulation to reach far-away targets
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For example, the pituitary releases growth hormone (GH) which promotes growth of the skeleton and
cartilage
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Ruqiyya

Endocrine signaling: a cell targets a distant cell through the bloodstream
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d) Cell-cell signaling: It occurs between neighboring cells that are in physical contact with each other
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It
may then interact with a receptor on the membrane of an adjacent cell
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Cell-cell signaling in multIcellular organisms:
-Plasmodesmata: As plant cells are surrounded by cell walls, they don’t contact one another through
plasma membrane as the way animal cells can
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Image of two cells connected by a plasmodesma
Plasmodesmata are lined with plasma membrane that is continuous with the membranes of the two
cells
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Molecules below a certain
size move freely through the plasmodesmal channel by passive diffusion
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They are
channels between neighboring cells that allow for the transport of ions, water, and other substances
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In vertebrates, gap junctions develop when a set of six membrane proteins called connexins form an
elongated, donut-like structure called a connexon
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Gap junctions are
particularly important in cardiac muscle: the electrical signal spreads rapidly between heart muscle
cells as ions pass through gap junctions, allowing the cells to contract
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Ruqiyya

Image of the plasma membranes of two cells held together by gap junctions
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Cell-cell signaling in unicellular organisms:
In multicellular organisms (such as human, animals) cell-cell signaling allows cells to coordinate their
activities, ensuring that tissues, organs, and organ systems function correctly
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Bacteria, for example, use chemical signals to detect population density (how
many other bacteria are in the area) and change their behavior accordingly while yeast produce
chemical signals that allow them to find mates
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-Quorum sensing in bacteria: Many types of bacteria engage in a mode of cell-cell signaling
called quorum sensing
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When the signaling reaches a
threshold level, all the bacteria in the population will change their behavior or gene expression at the
same time
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A
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" The squid gives the bacteria food and in return gets
bioluminesce (emit light)
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When V
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Bacteria uses quorum sensing to decide
when to produce bioluminescence
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Autoinducers let bacteria sense population
density and change their behavior in a synchronized fashion when the density reaches a certain
threshold
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AHL is the autoinducer made by V
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Because AHL is small and hydrophobic, it can diffuse freely across the membranes
of the bacterial cells
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When more bacteria are present, a larger amount of AHL will be produced
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The active receptor acts as a transcription factor attaching to specific sites on the bacterium’s DNA
and changing the activity of nearby target genes
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Ruqiyya

In V
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Lecture:1

Cell signaling

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The ligand does
not directly enter the cell
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There are three
general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors,
and enzyme-linked receptors
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To form a channel, this type of cellsurface receptor has an extensive membrane-spanning region
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Voltage-gated ion
channel is a good example that enable the passage of selected inorganic ions across cell membranes
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This ion distribution leads to a negative resting
membrane potential
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Gated ion channels form a pore through the plasma membrane that opens when the signaling
molecule binds
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2) G-protein-coupled receptors: G-protein-coupled receptors bind a ligand and activate a membrane
protein called a G-protein
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Before the ligand binds, the inactive G-protein can bind to a site on a
specific receptor
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One or both of these subunits may be able to activate
other proteins as a result
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Ruqiyya

When a signaling molecule binds to a G-protein-coupled receptor in the plasma membrane, a GDP
molecule associated with the G-protein is exchanged for GTP
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Hydrolysis of GTP to
GDP terminates the signal
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In some cases, the intracellular domain of the receptor
itself is an enzyme
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When a ligand binds to the extracellular domain, a signal is transferred
through the membrane, activating the enzyme
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A great example of a cell signaling pathway is seen in the balancing actions of insulin
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The high
glucose levels in the pancreas stimulate the release of insulin into the bloodstream
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This sets off a signal transduction pathway
within each cell that causes the glucose channels to open
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The cells will use the glucose to create ATP energy or
the cells store it as fats and starches for later use
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Insulin Acting on a Cell

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Lecture:1

Cell signaling

Ms
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L
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L
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Lehninger principles of biochemistry (5th ed)
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H
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Lodish, H
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(Ed

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