Ch. 11 - Cell Communication

Ch. 11 (AP Biology)

signal transduction pathway

a series of steps by which a signal from outside the cell is converted/transducer into a functional change within the cell

order of actions that result in the activation of a cellular response

reception (ligand binding to receptor) —> transduction —> response

yeast cell a produces _____ that binds to _____

a factor; yeast cell alpha's receptors

yeast cell alpha produces _____ that binds to _____

alpha factor; yeast cell a's receptors

example of signal transduction pathway?

yeast mating (binding, fusion, new cell)

quorum sensing definition + example

phenomenon that allows bacteria to sense the concentration of such signaling molecules, which then allows it to sense their local cell density
+secretion of toxins by infectious bacteria: interfering with signaling pathway used in quorum sensing can treat it

what does quorum sensing allow bacteria to do?

it allows bacterial populations to coordinate their behaviors in activities that require a given number of cells acting synchronously

biofilm

an aggregation of bacterial cells adhered to a surface

examples of biofilm

-slimy coating on a fallen log
-leaves lying on forest path
-film on teeth every morning

fruiting body

The reproductive structure (spore-forming structure) of a fungus/bacteria that produces spores (thick-walled cells capable of surviving until the environment improves)

what do cell junctions do?

they allow molecules, including signaling molecules, to pass readily between adjacent cells without crossing plasma membranes

name of cell junctions in animal cells

gap junctions

name of cell junctions in plant cells

plasmodesmata

what does cell-cell recognition do?

it allows two cells in an animal to communicate by interaction between molecules protruding from their surfaces

description of paracrine signaling

a secreting cell acts on nearby target cells by discharging molecules of a local receptor into the extracellular fluid

example of paracrine signaling

growth factors

paracrine signaling; local or long distance?

local

growth factors (in animals) are...

compounds that stimulate nearby target cells to grow and divide; numerous cells can simultaneously receive and respond to the growth factors produced by a single cell in their vicinity

description of synaptic signaling

a nerve cell releases neurotransmitter molecules (chemical signals) into a synapse, stimulating the target cell; the electrical signal along a nerve cell triggers the secretion of neurotransmitter molecules

example of synaptic signaling

neurotransmitters (neurons; brain cells)

synaptic signaling; local or long distance?

local

description of endocrine/hormone signaling

specialized endocrine cells secrete hormones into body fluids, often blood; hormone may reach virtually all body cells

example of endocrine signaling (human)

pancreas cells secreting insulin

endocrine signaling; local or long distance?

long distance

example of endocrine signaling (plant)

ethylene (plant hormone/growth regulator) - gas that promotes fruit ripening and helps regulate growth

#1 of cell signaling

reception

#2 of cell signaling

transduction

#3 of cell signaling

response

what is reception?

the target's detection of a signaling molecule coming from outside the cell; chemical signals detected when the ligand binds to a receptor protein located at the cells' surface (or possibly inside the cell)

what is transduction?

the conversion of the signal (from the binding of the signaling molecule that changes the receptor protein in some way) to a form that can bring about a specific cellular response; usually requires a sequence of changes (signal transduction pathway) with molecules in the pathway called relay proteins

what is response? (step of cell signaling)

when the transduction finally triggers a specific cellular response
ex) catalysis by an enzyme, rearrangement of the cytoskeleton, activation of genes in the nucleus

ligand (signaling molecule)

A molecule that binds specifically to another (often larger) molecule

where are the GPCR/RTK/ion channel receptors located?

plasma membrane

most common receptor and most common for diseases? 60~70%

GPCR (G coupled-protein receptor)

GPCR - what happens?

binding of ligand to GCP receptor —> G protein activation (GTP replaces GDP on protein) —> G protein activates enzyme —> triggers production of other second messengers

RTK - what happens?

binding of ligand to binding site of RTK proteins (x2 for both proteins) —> two proteins combine to become a unphosphorylated dimer (dimerization) which activates the regions —> each tyrosine (3 and 3) gain a phosphate group from 6 ATP, which results in 6 ADP —> relay proteins bind to specific tyrosine —> transduction pathway triggered —> cellular response

what does RTK do?

it regulates cell growth, differentiation, and survival and works with growth factors; no cell growth regulation = cancer (uncontrolled cell growth/mutation)

ion channels - what happens?

ligand binds to receptor of close channel —> channel opens, letting in ions —> concentration of that ion changes —> cellular response
(and then: ligand removed? —> ion channel closes —> ions cannot come into the cell)

what do ion channels do?

allow passage/diffusion of ions across the membrane by opening and closing in response to the binding of a ligand

examples of ion channel receptors

-neurons (synaptic signaling - ions flow in/out, triggering an electrical signal that propagates down the length of the receiving cell)
-ions: Na+(sodium), Cl-(chlorine), Ca2+(calcium), K+(potassium)

ligand-gated ion channels and voltage-gated ion channels are important in which body system?

nervous system (neurotransmitter molecules are released, which bind as ligands to ion channels, etc.)

where are intracellular receptors found?

cytoplasm or nucleus of target cells

requirement for chemical messengers that bond to intracellular receptors?

hydrophobic or very small
ex) nitric oxide

intracellular receptors - what happens? (aldosterone)

hormone passes through plasma membrane —> hormone binds to receptor protein in cytoplasm —> activation of protein —> hormone-receptor complex enters nucleus —> binds to specific genes —> bound protein acts as transcription factor, stimulating transcription of gene into mRNA —> mRNA is translated into a specific protein

transcription factors

special proteins that control which genes are turned on in a particular cell at a particular time; they can carry out the transduction part of the signaling pathway

examples of hormones

insulin, adrenaline, testosterone, aldosterone, and estrogen

2 benefits of multistep pathways

-possibility of greatly amplifying the signal (domino effect)
-more opportunities for coordination and control than simpler systems (regulation & response)

protein kinase

the enzyme that transfers phosphate groups from ATP to protein (usually acts on protein, not themselves, and usually phosphorylates serine/threonine amino acids, rather than tyrosine) "on"

protein phosphatase

the enzyme that can rapidly remove phosphate groups from proteins through dephosphorylation "off"

phosphorylation cascade (definition)

a series of different proteins in a pathway are phosphorylation in turn, each protein adding a phosphate group to the next one in line

phosphorylation cascade (steps)

ligand binds to receptor —> receptor activated —> activated relay molecule —> protein kinase 1 activated by rm —> protein kinase 2 activated by pk 1 —> active pk 2 phosphorylates a different protein —> cellular response to signal by active protein
+protein phosphatases (PP) catalyze the removal if the phosphate groups from the proteins, making the proteins inactive again

second messengers are...

small, nonprotein, water-soluble molecules or ions

first messengers are...

extracellular signaling molecules or ligands

2 most widely used second messengers

cAMP and calcium ions (Ca2+)

what is the enzyme that creates cAMP from ATP?

adenylyl cyclase

process from first messenger to cellular response (with second messenger in GPCR pathway)

first messenger (ex: epinephrine)/ ligand binds to GPCR —> GPCR activated —> bound GTP activates G protein —> G protein binds to adenylyl cyclase —> GTP hydrolyzed —> activated adenylyl cyclase converts ATP to cAMP —> cAMP activates protein kinase A —> cellular response

process of creating AMP

ATP —> adenylyl cyclase (-p & p(i)) —> cAMP —> phosphodiesterase (+H20) —> AMP

what is the important relationship between the second messenger and protein kinase A?

the immediate effect of cAMP is usually the activation of a serine/threonine kinase called protein kinase A; the activated protein kinase A then phosphorylates various other proteins, depending on the cell type

how can cellular responses be inhibited?

through different signals and protein activation; a different signaling molecule will activate a different receptor, which may block a pathway

cholera (toxin)

caused from contaminated water; bacteria form biofilm on the lining of small intestine and produces a toxin (enzyme chemically modifies a G protein involved in regulating water and salt secretion)
-unable to hydrolyze GTP to GDP in active form, which stimulates adenylyl cyclase to make cAMP, resulting in lots of salt into the intestines and water following osmosis

3 pathways induced by calcium ions

-increasing cytosolic concentration of Ca2+ causes responses in animal cells (ex: muscle contraction, secretion, cell division.
-plant cells, through many hormonal and environmental stimuli, can causes increases in Ca2+ (ex: greening in response to light)
-Ca2+ is actively transported out of the cell and imported from the cytosol to ER (sometimes even mitochondria and chloroplasts) by protein pumps

description of calcium concentrations in the cell

cells always contain some Ca2+, but cytosol is normally much lower than the concentration outside the cell; the calcium concentration in the ER is usually much higher than that in the cytosol

reminder: check the models of Ch. 11 - 11.3 is hard to explain

okay :) (hopefully)

what happens when cell signaling causes a response in the nucleus?

-regulation of protein synthesis by turning on/off specific genes
-final activated molecule functions, as a transcription factor, regulates several different genes (last in the signaling pathway)

what happens when cell signaling causes a response in the cytoplasm?

-regulation of protein activity, which directly affects proteins that function outside the nucleus
-signal can open/close ion channel or change in cell metabolism

a single molecule of epinephrine is formed from...

10^8 (10 to the power of 8) molecules of glucose-1-phosphate

usage of different proteins cause...

different cellular responses

Figure 11.17: Cell A (description)

pathway leads to a single response
-signaling molecule binds to receptor —> relay molecules —> response 1

Figure 11.17: Cell B (description)

pathway branches, leading to two responses
-triggered by one signal, not two
-ex) RTK and second messengers
-signaling molecule binds to receptor —> relay molecules —> response 2 & response 3

Figure 11.17: Cell C (description)

cross-talk occurs between two pathways
-pathways triggered by separate signals converge to modulate a single response
-important for regulation and coordination of cell's responses to information coming in from different sources in the body
-2 different signaling molecules bind to 2 different receptors —> different relay proteins —> activates one relay protein together —> response 4

Figure 11.17: Cell D (description)

different receptor leads to a different response
-same ligand/signaling molecule but different receptor protein
-signaling molecule binds to a different receptor —> relay protein —> response 5

scaffolding proteins

large relay proteins to which several other relay proteins are simultaneously attached

example of scaffolding proteins at work

brain cells that permanently hold together networks of signaling pathway proteins at synapses

How do scaffolding proteins enhance a cellular response?

the efficiency of signal transduction is apparently increased by the presence of scaffolding proteins, large relay proteins to which several other relay proteins are simultaneously attached; they can also hold networks of signaling-pathway proteins together

apoptosis (quick definition)

programmed cell suicide

What specifically happens to a cell during the process of apoptosis?

Cellular agents chop up the DNA and fragment the organelles and other cytoplasmic components. The cell shrinks and becomes lobed, and the cell's parts are packaged up in vesicles that are engulfed and digested by specialized scavenger cells, leaving no trace.

what does ced stand for?

cell death

was is ced-3 and ced-4?

apoptosis genes that encode proteins essential for apoptosis

example of apoptosis signal from outside the cell

-neighboring cell releases a signal to another cell
(death-signaling ligand occupies a cell-surface receptor, causing the activation of caspases and other enzymes that carry out apoptosis)

examples of apoptosis signal from inside the cell

-DNA damaged in the nucleus, generating a signal
-excessive rote in misfolding in the ER, generating a signal