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BIOL 1010U Study Guide - Chemical Polarity, Cell Membrane, Alpha Helix


School
UOIT
Department
Biology
Course Code
BIOL 1010U
Professor
vicari

Page:
of 2
Chapter 9-Cell Communication
Ligand binds GDP replaced
with GTP and G protein
activates signal transmitted
Receptor associated with G proteins (cytoplasm)
G proteins bound to GDP and GTP)
G protein + GTP = active
Cell Surface Receptor General Structure:
1. Ligand binding site
2. Extracellular domain
3. Transmembrane domain
4. Cytoplasmic domain
Types of cell-surface receptors:
There are thousands of different
receptor proteins on the surface of any
given cell.
Most can be placed into one of three
groups, according to the way they are
activated.
G Protein-Coupled
Receptors
1. Ligand Binding site-
Extracellular
2. Transmembrane region- 7
alpha helices
3. G protein binding site-
If the nonpolar signal molecule is a steroid:
Steroids are hydrophobic; pass through
plasma membrane
Receptor + ligand = steroid–receptor
complex
Either in cytoplasm or nucleus
Active steroid-receptor complexes act as
transcriptional regulators and control gene
Location of a particular receptor in a cell mainly
depends on whether the signal molecule is polar or
nonpolar
Polar signaling molecules:
Cannot cross plasma membrane
Rely on cell-surface receptors
Non-polar signaling molecules:
Small and pass freely through
plasma membrane
Paracrine Signaling:
Signaling molecules
travels short distances
between cells
Up to 20 cell diameters
or a few hundred
micrometers
Signaling molecule
usually small and soluble
Receptor: protein that receives and interprets information carried by
signaling molecule
When ligand binds to binding site on receptor, conformational shape
change occurs in receptor
Shape change activates receptor
Receptor Types:
Cell-surface receptors
Juxtacrine Signaling
No signaling molecule
involved
Occurs due to direct
contact between
adjacent cells
Contact-dependent
Autocrine Signaling
Signaling cell and
responding cell are
the same
Example: Pneumococcus
Endocrine Signaling
Examples of endocrine signaling
molecules:
Estrogen and testosterone
These hormones must travel
from the ovaries or testes
through the bloodstream to
Types of Signaling
In prokaryotes and unicellular
eukaryotes communication is
between individuals
In multicellular eukaryotes,
Prokaryotic example
DNA uptake from the environment
observed in many bacteria
How? peptide(ligand) + receptor
allows gene expression to allow
DNA uptake
Steps in Cell Signaling
Receptor activation The Signal binds
to a receptor which is then activated.
Signal Transduction- The signal is
transmitted to the interior of the cell by
a signal transduction pathway.
Response- The cell responds by
Communication involves:
1. Signaling Cell
2. Signaling molecule Ligand-
bonds non- covalently to
receptor
3. Receptor molecule
4. Receptor cell
All cells process information from
the environment
Required for coordination of
activities
Cells most often communicate via
chemical signals that bind to
specific receptors
Hormones,
neurotransmitters, CO2, H+
Signals can come from outside the
organism, or from neighboring cells
Examples of molecules that act a
signals:
Plants- Ethylene
Receptor Kinase
A kinase is an enzyme that adds a
phosphate group to another molecule
phosphorylation
Phosphatases have the opposite
effect i.e. remove phosphate
group.
Ligand binds receptor kinase active
phosphorylates other proteins signal
transmitted from outside to inside cell
Signal Transduction, Response and Termination
Signal transduction (and sometimes amplification), response and
termination are the steps that occur after a signaling molecule
binds to receptor
Although the ligands are different, the subsequent steps
are similar
Ligand-Gated Ion Channels
These are receptors that alter flow
of ions across plasma membrane
(channels)
Conformational shape change
opens channel
Allows flow of ions in
and/or out
Location Receptor Type Ligand Characteristics Example
Cell surface Transmembrane
receptor
Large
polar
molecule
s
Binding sites
face away
from host cell;
signal directed
inward
-Ligand-
gated Ion
Channels
-Protein
Kinase
Receptors
-G
Protein-
coupled
receptors
Intracellula
r
Cytoplasmic
receptors
Small,
non Polar
Found inside
the cell
Steroid
hormones
G proteins composed
of three subunits:
α (alpha)
β (beta)
γ (gamma)
α (alpha) subunit
binds either GDP
Termination of G Protein Signal
The amount of time a signaling
molecule remains bound to its
receptor depends on how tightly the
receptor holds on to it, its binding
affinity for the signaling molecule.
Most ligands do not bind
permanently to receptors
Amplification of Adrenaline Signal
Amplification occurs at several
places
Small amount of signal large
response
The adrenaline signal was
amplified in three places
Receptor Kinase Activation/ Transduction
Dimerization activates the cytoplasmic
kinase domains causing them to
phosphorylate each other at multiple sites on
their cytoplasmic tails.
These phosphorylated areas provide a place
for other proteins to bind and become active.
Ligand-Gated Ion Channels
Do not activate enzymes in cytoplasm,
instead they regulate permeability of
cell membrane to ions
Still follow same pattern: ligand
binding receptor activation cellular
Where is receptor kinase signaling used?
Formation and elongation of limb buds that become our arms and legs
Insulin signaling
allows us to transport glucose across the plasma membrane into the cytosol
Wound healing
when we cut ourselves, platelet derived growth factor (PDGF) released from
A receptor kinase, Kit, responsible for the
production of pigment in skin
Mutations in the Kit receptor kinase causes
patterns of incomplete pigmentation of
feathers, scales and hair
Integration of Signaling Pathways
Signaling pathways do not operate independently – signaling is very complex!
In one organism - many different signaling molecules each with own receptors
Cellular responses to a signaling molecule can vary from cell type to cell type
Effects depend on the various proteins present in different cells
Chapter 9-Cell Communication