LIFESCI 7C Chapter Notes - Chapter til 36: Quorum Sensing, Signal Transduction, Conformational Change
9.1 Principles of Cell Communication
I. Cellular communication consists of a signaling cell, a signaling molecule, a receptor
protein, and a responding cell
A. The signaling cell is the source of a signaling molecule
1. Fight or flight: signal goes from signaling cells to responding cells in the
stomach, heart, and other organs
B. Signaling molecules carry information from one cell to the neck
1. Vary immensely and include peptides, lipids, and gases
2. FOF: adrenaline (epinephrine)
C. Binds to a receptor protein on or in the responding cell
1. FOF: adrenaline is released from the adrenal glands and acts on other
cells - heart beats more strongly and quickly
D. Causes a change in activity or behavior of the responding cell
1. FOF: able to deliver oxygen more effectively to the rest of the body
II. Quorum sensing
A. Process by which bacteria are able to determine whether they are at low or high
population density and then turn on specific genes across the entire community
III. What happens when a signaling molecule binds to receptor on a responding cell?
A. Receptor activation
1. The receptor is turned on, or activated, by causing a conformational
change in the receptor
2. Some receptors bind to and activate other proteins located inside the cell
3. Others are enzymes and binding of the signal changes the shape and
activity of the enzyme
4. Others are channels that open or close in response to binding a signaling
molecule
B. Signal transduction
1. One molecule activates the next molecule, which activates the next, etc.
2. Signal is often amplified at each step in the pathway
a) A low signal concentration can have a large effect on the
responding cell
C. Cellular response
1. Signaling pathways can activate enzymes involved in metabolic
pathways, or turn on genes that cause the cell to divide, change shape, or
signal other cells
D. Termination
1. Cellular response is stopped
2. The response can be terminated at any point along the signaling pathway
3. Protects the cell from overreacting to existing signals and helps the cell to
have an appropriate level of response
4. Allows the cell to respond to new signals
9.2 Cell Signaling Over Long and Short Distances
find more resources at oneclass.com
find more resources at oneclass.com
I. In multicellular organisms, the distance between communicating cells varies
considerably
A. When the two cells are far apart, the signaling molecule is transported by the
circulatory system
1. Signaling molecules released by a cell that may have to travel great
distances to reach receptor cells in the body
a) Carried in the circulatory system
b) Signaling by means of molecules that travel through the
bloodstream is called endocrine signaling
B. When they are close, the signaling molecule simply moves by diffusion
(paracrine signaling)
1. Usually a small, water-soluble molecule such as a growth factor (type of
signaling molecule that causes the responding cell to grow, divide, or
differentiate) or a neurotransmitter
2. Many cells are physically attached to one another
a) Without diffusion or circulation of the signaling molecule, but
through direct contact
(1) A transmembrane protein on the surface of an adjacent
cell acts as the receptor
(2) The signaling molecule is not released from the cell, but
instead remains associated with the plasma membrane of
the signaling cell
C. Signaling molecules may be released by a cell and then bind to receptors on the
very same individual cell
1. Autocrine signaling
a) Maintain cell differentiation
b) Used by cancer cells to promote cell division
9.3 Cell-Surface and Intracellular Receptors
I. Receptors are proteins that receive and interpret information carried by signaling
molecules
A. The signaling molecule is referred to as a ligand
B. The signaling molecule binds to a specific part of the receptor protein called the
ligand-binding site
1. The bond is non covalent and highly specific the signaling molecule binds
only to a receptor with a ligand-binding site that recognizes the molecule
II. The binding of a signaling molecule to the ligand-binding site of a receptor causes a
conformational change in the receptor
III. Many signaling molecules are small, polar proteins that cannot pass through the
hydrophobic core of the plasma membrane
A. The receptor proteins for these signals are on the outside surface of the
responding cell
find more resources at oneclass.com
find more resources at oneclass.com
B. Receptor proteins for growth factors and other polar ligands are transmembrane
proteins with an extracellular domain, a transmembrane domain, and a
cytoplasmic domain
1. The entire molecule undergoes the change and carries the message
across the hydrophobic core
IV. Some nonpolar signaling molecules don’t need a receptor on the cell surface
A. Steroids are hydrophobic are pass easily through the core
B. Bind to receptor proteins located in the cytosol or in the nucleus, where they act
to control the expression of specific genes
C. Receptors are already bound to DNA and can turn on gene expression with their
steroid counterpart
V. Many receptors act as binary molecular switches, existing in two alternative states,
either “on” or “off”
A. Cell-surface receptors
1. G protein-coupled receptor
a) When a ligand binds to a G protein-coupled receptor, the receptor
couples to a G protein
2. Receptor kinases
a) Enzymes that are activated when the receptor binds its ligand
b) Kinase is an enzyme that catalyzes the transfer of a phosphate
group from ATP to a substrate
(1) Binds both ATP and substrate to catalyze
(a) Called phosphorylation
(b) Typically activates
(c) Addition of a phosphate group to a protein alters its
shape or provides a new site for other proteins to
bind
(d) Phosphatases remove a phosphate group
(dephosphorylation)
3. Ion channels
a) Alter the flow of ions across the plasma membrane
(1) Some open to changes in voltage across the membrane
(voltage-gated ion channels)
(2) Others open when bound by their ligand (ligand-gated)
9.4 G Protein-Coupled Receptors and Short-Term Responses
I. G protein-coupled receptors are two characteristics
A. Have a similar structure, consisting of a single polypeptide chain that has seven
transmembrane spanning regions, with the ligand-binding site on the outside of
the cella dn the portion that binds to the G protein on the inside of the cell
B. When activated, they associate with a G protein
1. The are able to transmit the signal from the outside to the inside of the
cell
C. A G protein is able to bind to GTP and GDP
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Quorum sensing: process by which bacteria are able to determine whether they are at low or high population density and then turn on specific genes across the entire community. 9. 2 cell signaling over long and short distances. The binding of a signaling molecule to the ligand-binding site of a receptor causes a conformational change in the receptor. What determines the response of the cell: types of receptor on the surface, determine which signals the cell is able to respond to, set of proteins that is found in it. Most ligands do not bind to their receptors permanently: binding affinity: the length of time a signaling molecule remains bound to its receptor depends on how tightly the receptor holds on to it. The cellular responses that result from receptor kinase activation tend to involve changes in gene expression, which in turn allow cells to grow, divide, differentiate, or change shape.
