AP Biology: Cell Signaling

cells communicate directly via...

cell junctions

juxtacrine signaling

signaling between cells in direct contact with each other

gap junctions

in animal cells, these junctions allow ions and small molecules to pass quickly

plasmodesmata

in plant cells, these are tunnels of cytoplasm between cells that allow movement of ions, amino acids, sugars, small proteins, and microRNA

synaptic signaling

local electrochemical signaling occurring between nerve cells or nerve and muscle cells

neurotransmitters

short-lived chemical signals that play a key role in synaptic signaling

paracrine signaling

local communication; cells secrete substances that affect only nearby cells, like growth factors during early animal development

endocrine signaling

long-distance communication; distributes signals throughout a multicellular organism (hormones)

signal transduction pathway

series of steps by which a signal on a cell's surface is converted to a specific cellular response

three stages of cell signaling

reception, transduction, response

reception (step 1)

a ligand from outside the cell binds to a receptor protein and the receptor changes shape

transduction (step 2)

series of molecule interactions that relay signals from receptor to target molecule, typically within the cell

response (step 3)

the specific cellular response to the signal molecule, usually transcription or cytoplasmic activities

ligand

signaling molecule

membrane receptors

anchored to the cell membrane. the ligand is water-soluble/hydrophilic/polar/large

second messengers

molecules inside the cell that relay signals from the receptor to the target molecule

protein kinase

enzyme that phosphorylates and activates proteins at the next level

phosphorylation

the addition of a phosphate group, usually to active a molecule or transfer energy

phosphorylation cascade

a reaction that enhances and amplifies a signal

apoptosis

controlled cell suicide, where the cell is systematically dismantled and digested, protecting neighboring cells from damage

autocrine signaling

signals diffuse from one part of a cell to another part of the same cell

sequence of a signal transduction pathway

signal (1st messenger) >>> receptor >>> proteins or other 2nd messengers >>> cellular responses

if a ligand is HYDROPHOBIC/LIPOPHILIC, what type of receptor will it bind to?

intracellular

if a ligand is HYDROPHILIC/LIPOPHOBIC, what type of receptor will it bind to?

membrane

when a ligand binds to a receptor, what happens to the receptor?

it changes shape, initiating some sort of response

receptor proteins

molecules with binding sites for signaling molecules (ligands)

gated ion channel receptors

transmembrane protein with a gated channel that opens and closes in response to a specific signal; when open, a specific ion can pass through

ligand-gated ion receptor

a ligand, or molecular signal, triggers the channel to open/close to allow an ion to pass

voltage-gated ion receptor

open or close in response to voltage differences across a membrane

typical ligand-gated ion sequence

signal received >>> ligand binds to receptor >>> receptor changes shape >>> channel opens and ions pass >>> ions initiate chemical response inside cell >>> ligand detaches from receptor and deactivates channel

example of a ligand-gated ion channel

an active neuron releases acetylcholine (Ach) into synapse >>> Ach binds to ligand-gated receptor molecule of next neuron >>> receptor molecules open >>> Na+ enters cell >>> inside of cell becomes more positive >>> change in voltage initiates a nerve impulse

G-protein coupled receptor (GPCR)

a transmembrane protein that activates a G protein, which in turn activates another membrane protein, which triggers a cell response or activates a second messenger

why is a G-protein receptor called that?

it has a GTP or a GDP attached to it instead of ATP or ADP (the difference is guanine instead of adenine)

typical G-protein coupled receptor sequence

ligand binds to receptor and it changes shape >>> receptor uses GTP to activate G protein >>> G protein binds to effector protein >>> effector is activated >>> effector initiates response >>> GTP is converted to GDP >>> receptor is deactivated

protein kinase receptors

transmembrane protein that is a kinase (enzyme that adds phosphate groups to a protein)

protein kinase receptors are most important in...

cell reproduction and regulation

G-protein coupled receptors are most important in...

most things - it is the largest family of signal receptors and includes vision, taste, airborne signals, hormones, neurotransmitters, immune system

types of cell surface receptors

ion channel receptors, G-protein coupled receptors, and protein kinase receptors

ion channel receptors are most important in...

muscle contraction and nerve impulses

best understood protein kinase receptor

receptor tyrosine kinase (RTK)

typical protein kinase receptor sequence

ligand binds to receptor >>> receptor forms a dimer with another receptor >>> the receptors phosphorylate each other and activates the complex >>> relay protein attaches to receptor and is phosphorylated >>> relay proteins are released >>> relay proteins activate cellular response

how is a protein kinase receptor deactivated?

dephosphorylation of kinases OR when the membrane folds and encircles the receptor protein in a vesicle (endocytosis)

examples of hydrophobic chemical messengers for intracellular/cytoplasmic receptors

steroid hormones (testosterone, estrogen), thyroid hormones, nitric oxide gas (NO), second messengers

typical intracellular receptor signaling pathway

ligand enters cytoplasm >>> ligand binds to and activates receptor in cytoplasm or nucleus >>> complex acts as a transcription factor, binding to DNA and affecting gene expression

second messenger

molecules that relay a signal from the inside face of the receptor protein to other molecules that may initiate a cellular response or may act as additional second messengers

characteristics of a second messenger

small, nonprotein, and hydrophilic/hydrophobic/gaseous

cascade effect/signaling cascade

a series of enzymatic reactions where a small number of enzymes can activate more and more to initiate a large-scale response

protein phosphatase

dephosphorylates a substrate (removes a phosphate group to deactivate it)

characteristics of signaling pathways

characterized by a signal, transduction, and a response. highly specific and regulated. one signal molecule can cause a cascade effect, releasing thousands inside the cell. these pathways evolved millions of years ago in a common ancestor.

advantages of signaling pathways

amplification (widespread), control (very accurate), and multiplicity (a single signaling molecule can activate many different pathways)