Concept 11.1: External signals are converted to responses within the cell Flashcards
There are two sexes, or mating types, called
a and α
Each type secretes a specific factor that binds only to
receptors on the other type of cell.
When exposed to each other’s mating factors, a pair of cells of opposite type change
shape, grow toward each other, and fuse (mate).
The new a/α cell contains all the genes of both original cells, a combination of
genetic resources that provides advantages to the cell’s descendants, which arise by subsequent cell divisions.
The unique match between mating factor and receptor is key to ensuring mating only among cells of the same species of
yeast.
How does the binding of a mating factor by the yeast cell surface receptor initiate a signal that brings about the cellular response of mating? This occurs in a series of steps called a
signal transduction pathway.
Scientists 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 descendants.
Sensing the concentration of such signaling molecules allows bacteria to monitor their own local cell density, a phenomenon called
quorum sensing.
Quorum sensing allows bacterial populations to coordinate their behaviors in activities that require a given number of
cells acting synchronously
One example is formation of a __________, an aggregation of bacterial cells adhered to a surface.
biofilm
is a common bacterial species found on the surface of healthy skin that can turn into a serious pathogen if introduced into tissue through a cut or abrasion.
Staphylococcus aureus (S. aureus)
Once inside the body, a population of _______________ that reaches a certain density will start to secrete a toxin, killing body cells and contributing significantly to inflammation and damage
S. aureus
Because about 1 in 100 people carry a strain of S. aureus that is resistant to common antibiotics, a minor infection can turn permanently
harmful or even deadly.
Like bacteria or yeast cells, cells in a multicellular organism usually communicate via signaling
molecules targeted for cells that may or may not be immediately adjacent.
eukaryotic cells may communicate by direct contact, which 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.
Moreover, animal cells may communicate via direct contact between membrane-bound cell-surface molecules, in a process called
cell-cell recognition
In many other cases of local signaling, signaling molecules are secreted by the
signaling cell.
Some molecules travel only short distances; such local regulators influence cells in the vicinity. This type of local signaling in animals is called
paracrine signaling
One class of local regulators in animals, _______________, are compounds that stimulate nearby target cells to grow and divide
growth factors
Numerous cells can simultaneously receive and respond to the growth factors produced by a single cell in their
vicinity.
A more specialized type of local signaling called __________________occurs in the animal nervous system
synaptic signaling
An electrical signal along a nerve cell triggers the secretion of
neurotransmitter molecules.
These molecules act as chemical signals, diffusing across the synapse—the narrow space between the nerve cell and its target cell—triggering a response in the
target cell.
Both animals and plants use molecules called
hormones for long-distance signaling.
In hormonal signaling in animals, also known as _________________________, specialized cells release hormones, which travel via the circulatory system to other parts of the body, where they reach target cells that can recognize and respond to them
endocrine signaling
Plant hormones (often called __________________________) sometimes travel in plant vessels (tubes) but more often reach their targets by moving through cells or by diffusing through the air as a gas
plant growth regulators
The ability of a cell to respond is determined by whether it has a specific receptor molecule that can bind to the
signaling molecule.
Our current understanding of how signaling molecules act via signal transduction pathways had its origins in the pioneering work of
Earl W. Sutherland, whose research led to a Nobel Prize in 1971.
Sutherland and his colleagues at Vanderbilt University were investigating how the animal hormone _____________________________ triggers the “fight-or-flight” response in animals by stimulating the breakdown of the storage polysaccharide glycogen within liver cells and skeletal muscle cells.
epinephrine (also called adrenaline)
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
Alternatively, the compound can be stripped of phosphate and released from the cell into the __________________ , which can fuel cells throughout the body.
blood as glucose
Thus, one effect of epinephrine is the mobilization of fuel reserves, which can be used by the animal to either ________________________________________________, as the impala in Figure 11.1 is clearly doing.
defend itself (fight) or escape whatever elicited a scare (flight)
Sutherland’s research team discovered that epinephrine stimulates glycogen breakdown by somehow activating a
cytosolic enzyme, glycogen phosphorylase
However, when epinephrine was added to a cell-free mixture containing the enzyme and its substrate, glycogen,
no breakdown occurred
Glycogen phosphorylase could be activated by epinephrine only when the hormone was added to
intact cells.
This result told Sutherland two things.
First, epinephrine does not interact directly with the enzyme responsible for glycogen breakdown; an intermediate step or series of steps must be occurring in the cell. Second, an intact, membrane-bound cell must be present for transmission of the signal to take place.
Sutherland’s work suggested that the process going on at the receiving end of a cellular communication can be dissected into three stages:
reception, transduction, and response
is the target cell’s detection of a signaling molecule coming from outside the cell
Reception
The binding of the signaling molecule changes the receptor protein in some way, initiating the process of
transduction.
The transduction stage converts the signal to a form that can bring about a specific
cellular response.
Transduction sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules—a
signal transduction pathway
The molecules in the pathway are often called
relay molecules.
In the third stage of cell signaling, the transduced signal finally triggers a specific cellular
response.
The response may be almost any imaginable cellular activity—such as
catalysis by an enzyme (for example, glycogen phosphorylase), rearrangement of the cytoskeleton, or activation of specific genes in the nucleus.
