BIO 311C Chapter Notes - Chapter 11: Melanoma, Endocrine System, Cell Division
CHAPTER 11: CELL COMMUNICATION
11.1
● Scientist think that signaling mechanisms first evolved in ancient prokaryotes and
single-celled eukaryotes like yeasts and then were adopted for new uses by their
multicellular descendents.
● Bacterial cells secrete molecules that can be detected by other bacterial cells. Sensing
the concentration of such signaling allows bacteria to monitor their own local cell density,
a phenomenon called quorum sensing.
○ Ex. biofilms - an aggregation of bacterial cells adhered to a surface
○ Ex. the secretion of toxins by infectious bacteria.
■ This one can have serious medical implications because sometimes
treatment by antibiotics doesn’t work with such infections due to antibiotic
resistance that has evolved in a particular strain of bacteria.
● Direct contact is one type of local signaling.
● Both animals and plants have cell junctions that, where present, directly connect the
cytoplasms of adjacent cells. In these cases, signaling substances dissolved in the
cytosol can pass freely between neighboring cells.
● Animal cells may communicate via direct contact between membrane-bound cell surface
molecules, in a process called cell-cell recognition. This sort of local signaling is
especially important in embryonic development and the immune response.
● Paracrine Signaling - a signalling cell acts on nearby target cells by secreting
molecules of a local regulator (a growth factor, for example)
●Growth factors are compounds that stimulate nearby target cells to grow and divide.
●Synaptic signaling (local signaling that occurs in the animal nervous system): an
electrical signal along a nerve cells triggers the secretion of neurotransmitter molecules.
These molecules acts as chemical signals, diffusing across the synapse, triggering a
response in the target cell.
● Both animals and plants use hormones for long distance signaling aka endocrine
signaling.
● The ability of a cell to respond is determined by whether it has a specific receptor
molecule that can bind to the signaling molecule.
● Read about Sutherland’s work in 1971 investigating how the animal hormone
epinephrine triggers the “fight-or-flight” response
○ The animal hormone epinephrine triggers the “fight or flight” response in animals
by stimulating the breakdown of the storage of polysaccharide glycogen within
liver cells and skeletal muscle cells.
○ Glycogen breakdown releases the sugar glucose 1-phosphate, which the cell
converts to glucose 6-phosphate. The liver or muscle cell can then use this
compound, an early intermediate in glycolysis, for energy production.
● Cellular communication can be dissected into three stages:
○ Reception: the target cell’s detection of a signaling molecule coming from the
outside of the cell
○ Transduction: the binding of the signaling molecule changes the receptor protein
in some way; this stage converts the signal to a form that can bring about a
specific cellular response
○ Response: the transduced signal finally triggers a specific cellular response
11.2
● Reception of the signal depends on the receiver.
○ The signal emitted by an a mating type yeast cell are “heard” only by its
prospective mates, a
cells.
● The signaling molecule acts as a ligand, the term for a molecule that specifically binds
to another (often larger) molecule.
○ For many receptors, this shape change directly activates the receptor, enabling it
to interact with other cellular molecules.
○ For other kinds of receptors, the immediate effect of ligand binding is to cause
the aggregation of two or more receptor proteins, which leads to further
molecular events inside the cell.
● Cell-surface transmembrane receptors; three major types:
○ G protein-coupled receptor (GPCR)
■ Works with the help of a G protein, a protein that binds the energy-rich
molecule GTP
■ Specific loops between the helices form binding sites for signaling
molecules
■ Plays roles in embryonic development and sensory reception
■ Are involved in many human diseases, including bacterial infection. The
bacteria that cause cholera, pertussis, and botulism, among others, make
their victims ill by producing toxins that interfere with G protein function.
○ Receptor tyrosine kinases (RTKs)
■ Characterized by having enzymatic activity
■ RTK is a protein kinase-- an enzyme that catalyzes the transfer of
phosphate groups from ATP to another protein.
■ Thus, RTKs are membrane receptors that attach phosphates to tyrosines.
■ Upon binding a ligand such as a growth factor, one RTK may activate ten
or more different transduction pathways and cellular responses.
● Key difference between RTKs and GPCRs, which generally
activate a single transduction pathway
■ !!Abnormal RTKs that function even in the absence of signaling molecules
are associated with many kinds of cancer!!
○ Ion Channel Receptors
■ Can act as a “gate”, opening or closing the channel when the receptor
changes shape
■ The channel opens or closes, allowing or blocking the flow of specific ions
■ Very important in the nervous system
■ Some gated ion channels are controlled by electrical signals instead of
ligands; “voltage-gated” ion channels
■ Some ion channels are present on membranes of organelles, such as the
ER.
● Intracellular Receptors
○ Intracellular receptor proteins are found either in the cytoplasm or nucleus of
target cells.
○ To reach such a receptor, a signaling molecule passes through the target cell’s
plasma membrane.
■ A number of important signaling molecules can do this because they are
either hydrophobic enough or small enough to cross the hydrophobic
interior of the membrane.
● The hydrophobic signaling molecules include both steroid
hormones and thyroid hormones of animals.
● Also nitric oxide (NO) which is a gas
○ Once a hormone has entered a cell, its binding to an intracellular receptor
changes the receptor into a hormone-receptor complex that is able to cause a
response-- in many cases, the turning on or off of particular genes.
○ Ex. Aldosterone (representative example of how steroid hormones work)
■ The hormone is secreted by cells of the adrenal gland. Aldosterone then
travels through the blood and enters cells all over the body. However, a
response only occurs in kidney cells, which contain the receptor
molecules for aldosterone.
● With aldosterone attached, the active form of the receptor protein
then enters the nucleus and turns on specific genes that control
water and sodium flow in kidney cells, ultimately affecting blood
volume.
○ Recall that genes in a cell’s DNA function by being transcribed and processed
into messenger RNA (mRNA), which leaves the nucleus and is translated into a
specific protein by ribosomes in the cytoplasm.
■ Special proteins called transcription factors
control which genes are
turned on-- that is, which genes are transcribed into mRNA.
● Aldosterone - by acting as a transcription factor, the aldosterone
receptor itself carries out the transduction part of the signaling
pathway.
11.3
● When receptors for signaling molecules are plasma membrane proteins, the transduction
stage of cell signaling is usually a multistep pathway involving many molecules.
○ Steps often include activation of proteins by addition or removal of phosphate
groups or release of other small molecules or ions that act as signaling
molecules.
● Multistep pathways provide more opportunities for coordination and control than do
simpler systems.
Document Summary
Scientist think that signaling mechanisms first evolved in ancient prokaryotes and single-celled eukaryotes like yeasts and then were adopted for new uses by their multicellular descendents. Bacterial cells secrete molecules that can be detected by other bacterial cells. Sensing the concentration of such signaling allows bacteria to monitor their own local cell density, a phenomenon called quorum sensing . Ex. biofilms - an aggregation of bacterial cells adhered to a surface. Ex. the secretion of toxins by infectious bacteria. This one can have serious medical implications because sometimes treatment by antibiotics doesn"t work with such infections due to antibiotic resistance that has evolved in a particular strain of bacteria. Direct contact is one type of local signaling. Both animals and plants have cell junctions that, where present, directly connect the cytoplasms of adjacent cells. In these cases, signaling substances dissolved in the cytosol can pass freely between neighboring cells.
