Chapter 11 (Notes) Flashcards
Cell-to-cell communication is essential for both
multicellular and unicellular organisms
Biologists have discovered some universal mechanisms of
cellular regulation
Cells most often communicate with each other via
chemical signals.
For example, the fight-or-flight response is triggered by a signaling molecule called epinephrine
External signals are converted to
responses within the cell
Microbes provide a glimpse of the role of
cell signaling in the evolution of life
A signal transduction pathway is a
series of steps by which a signal on a cell’s surface is converted into a specific cellular response
Signal transduction pathways convert signals on a cell’s surface into
cellular responses
Pathway similarities suggest that
ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes
Cells in a multicellular organisms communicate by
chemical messengers
Animal and plant cells have cell junctions that
directly connect the cytoplasm of adjacent cells
In local signaling, animal cells may communicate by
direct contact, or cell-cell recognition
In many other cases, animal cells communicate using
local regulators, messenger molecules that travel only short distances
paracrine signaling-
local regulator
grown factors from one cell stimulate numerous neighboring cells
synaptic signaling-
local regulator
neurotransmitters
In long-distance signaling, plants and animals use
chemicals called hormones
Endocrine signaling-
long-distance signaling. hormones
hormones are released into the circulatory system to reach distance regions of the body
The ability of a cell to respond to a signal depends on whether or not it has a
receptor specific to that signal
Earl W. Sutherland discovered how the hormone
epinephrine acts on cells
Sutherland suggested that cells receiving signals went through three processess:
Reception
Transduction
Response
Reception: a signaling molecules binds to a
receptor protein, causing it to change shape
The binding between a signal molecule (ligand) and receptor is
highly specific
Ligand-
a molecule that specifically binds to a larger molecule
A shape change in a receptor is often the
initial transduction of the signal
Most signal receptors are
plasma membrane proteins
Most water-soluble signal molecules bind to
specific sites on receptor proteins that span the plasma membrane
There are three main types of membrane receptors
- G protein-coupled receptors
- Receptor tyrosine kinases
- Ion channel receptors
Also have intracellular receptors
G protein-coupled receptors (GPCRs) are
the largest family of cell-surface receptors
A G protein-coupled receptor (GPCR) is a
plasma membrane receptor that works with the help of a G protein
The G protein acts as an
on/off switch: If GDP is bound to the G protein, the G protein is inactive
Receptor tyrosine kinases (RTKs) are
membrane receptors that attach phosphates to tyrosines
A kinase is an
enzyme that catalyzes the transfer of phosphate groups
A receptor tyrosine kinase can trigger
multiple signal transduction pathways at once
Abnormal functioning of receptor tyrosine kinases (RTKs) is associated with
many types of cancers
A ligand-gated ion channel receptor acts as a
gate when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the
gate allows specific ions, such as NA+ or CA^2+, through a channel in the receptor
Important in the nervous system
-neurotransmitter release
Intracellular receptor proteins are found in the
cytosol or nucleus of target cells
Small or hydrophobic chemical messengers can
readily cross the membrane and activate receptors
Examples of hydrophobic messengers are
the steroid and thyroid hormones of animals
An activated hormone-receptor complex can act as a
transcription factor, turning on specific genes
Transduction:
cascades of molecular interactions relay signals from receptors to target molecules in the cell
Signal transduction usually involves
multiple steps
Multistep pathways can amplify
a signal: A few molecules can produce a large cellular response
Multistep pathways provide more opportunities for
coordination and regulation of cellular response
(Signal transduction pathways)
The molecules that relay a signal from receptor to
response are mostly proteins
(Signal transduction pathways)
Like falling dominoes, the receptor activates another protein, which
activates another, and so on, until the protein producing the response is activated
(Signal transduction pathways)
At each step, the signal is transduced into a different form, usually a
shape change in a protein
-usually phosphorylation
((Protein phosphorylation and dephosphorylation))
In many pathways, the signal is transmitted by
a cascade of protein phosphorylations
((Protein phosphorylation and dephosphorylation))
Protein kinases transfer phosphates from
ATP to protein, a process called phosphorylation
Protein phosphatases remove the phosphates from
proteins, a process called dephosphorylation
((Protein phosphorylation and dephosphorylation))
This phosphorylation and dephosphorylation system acts as a
molecular switch, turning activities on and off or up or down, as required
Small molecules and ions as
second messengers
The extracellular signal molecule (ligand) that binds to the receptor is a
pathway’s “first messenger”
Second messengers are
small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion
Second messengers participate in pathways initiated by
GPCRs and RTKs
Cyclic AMP and calcium ions are
common second messengers
Cyclic AMP (cAMP) is one of the
most widely used second messengers
Adenylyl cyclase, an enzyme in the plasma membrane, converts
ATP to cAMP in response to an extracellular signal
Many signal molecules trigger formation of
cAMP
Other components of cAMP pathways are
G proteins, G protein-coupled receptors, and protein kinases
cAMP usually activates
protein kinase A, which phosphorylates various other proteins
Further regulation of cell metabolism is provided by G-protein systems that inhibit
adenylyl cyclase
Calcium ions (Ca^2+) act as a
second messenger in many pathways
Calcium is an important second messenger because
cells can regulate its concentration
A signal relayed by a signal transduction pathway may trigger an increase in
calcium in the cytosol
Pathways leading to the release of calcium involved
inositol triphosphate (IP3) and diaclyglycerol (DAG) as additional second messengers
Response:
Cell signaling leads to regulation of transcription or cytoplasmic activities
The cell’s responses to an extracellular signal is sometimes called the
“output response”
Ultimately, a signal transduction pathway leads to a
regulation of one or more cellular activities
The response may occur in the
cytoplasm or in the nucleus
Many signaling pathways regulate the synthesis of
enzymes or other proteins, usually by turning genes on or off in the nucleus
The final activated molecule in the signaling pathway may function as
a transcription factor
Other pathways regulate the activity of
enzymes rather than their synthesis
There are four aspects of fine-tuning to consider
- Amplification of the signal (and thus the response)
- Specificity of the response
- Overall efficiency of response, enhanced by scaffolding proteins
- Termination of the signal
Enzyme cascades amplify the
cell’s response
(((signal amplification)))
At each step, the number of activated products is much greater than in the
preceding step
Amplification stems from the fact that these proteins persist in the active form long enough to
process numerous molecules of substrate before they become inactive again
Few numbers of signal can affect
hundreds of millions of end molecules
Different kinds of cells have
different collections of proteins
These different proteins allow cells to
detect and respond to different signals.
- A liver cell in the presence of epinephrine breaks down glycogen
- A heart cell in the presence of epinephrine contracts
Even the same signal can have different effects in cells with
different proteins and pathways
Pathway branching and “cross-talk” further help
coordinate incoming signals
Scaffolding proteins are
large relay proteins to which other relay proteins are attached
Scaffolding proteins can
increase the signal transduction efficiency by grouping together different proteins involved in the same pathway
In some cases, scaffolding proteins may also help
activate some of the relay proteins
Inactivation mechanisms are an
essential aspect of cell signaling
If ligand concentration falls,
fewer receptors will be bound
Unbound receptors revert to an
inactive state
- signal molecules are reversible (on/off)
- allows the cell to be ready for any new signals
Apoptosis integrates
multiple cell-signaling pathways
Apoptosis is
programmed or controlled cell suicide
Components of the cell are
chopped up and packaged into vesicles that are digested by scavenger cells
The cell shrinks and becomes
lobed (blebbing)
Apoptosis prevents enzymes from
leaking out of a dying cell and damaging neighboring cells
Caspases are the main
proteases (enzymes that cut up proteins) that carry out apoptosis
Apoptosis can be triggered by
- an extracellular death-signaling ligand
- DNA damage in the nucleus
- protein misfolding in the endoplasmic reticulum
Apoptosis evolved early in animal evolution and is essential for
the development and maintenance of all animals
Apoptosis may be involved in some
diseases (for example, parkinson’s and alzheimer’s); interference with apoptosis may contribute to some cancers
