AP Biology Cell Signaling

homeostasis

steady internal state/maintenance of internal balance

cells communicate directly via...

cell junctions

where do external signals come from?

biological sources (pathogen, bee sting), physical sources (light, heat), and other cells

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

step 1 of cell signaling: reception

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

step 2 of cell signaling: transduction

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

step 3 of cell signaling: response

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

ligand

signaling molecule

location of receptors

within the cell (cytoplasmic receptors) and on the plasma (cell) membrane

intracellular receptors

inside the cell; found in the cytoplasm or nucleus. the ligand must be hydrophobic/nonpolar/very small

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

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, no cell lysis

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, what type of receptor will it bind to?

intracellular

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

membrane

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

it changes shape (changes confirmation), 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

what can cause a ligand to detach from a receptor?

ligand can be broken down by an enzyme, binding site can be blocked by an allosteric ligand, or the ion passage can be obstructed by a channel blocker

example of a voltage-gated ion channel

after Ach binds and Na+ enters the cytoplasm, the voltage inside the neuron becomes more positive. if strong enough, voltage-gated ion channels for Na+ and then K+ will open, allowing the impulse to travel along the neuron.

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

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

how is an intracellular receptor signaling pathway deactivated?

ligand or receptor proteins are enzymatically degraded

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

scaffold proteins

improve efficiency of a signaling cascade by holding enzymes involved in close proximity - keeps members of one signaling cascade isolated from others

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)

cAMP

most common second messenger

resting potential

The difference in electric charge between the inside and outside of a neuron's cell membrane

action potential

the change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell.

Neuron

a nerve cell; the basic building block of the nervous system

Neurotransmitters

chemical messengers that cross the synaptic gaps between neurons

Synapse

the junction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron

Phosphatase enzymes

remove phosphate groups

plant hormones

substances that act as chemical messengers within plants, travel through the air as a gas

threshold potential

The minimum membrane potential that must be reached in order for an action potential to be generated.

axon hillock

The conical region of a neuron's axon where it joins the cell body; typically the region where nerve signals is generated.

saltatory conduction

the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.