class 17 - cell signaling basics and receptors

signaling cell

source of a signaling molecule

signaling molecule

• also called ligand

• substance secreted by signaling cell

• examples: peptides, lipids, gases, hormones, neurotransmitters, etc

secretion

process where signaling cell releases signaling molecules

receptor protein

on responding cell, binds to signaling molecule

• is an integral/transmembrane protein because it takes extracellular signals and causes intracellular change

responding cell

the cell that receives information from the signaling molecule

what happens if the signaling molecule and receptor can’t bind to each other?

the cell doesn’t detect/respond to the signal, even if the signal is present

what happens if the receptor’s shape changes?

receptor and signaling molecule won’t bind properly → prevents/changes signal activation

what happen if the internal signal proteins are altered?

signal transduction pathway changes/fails → incomplete/incorrect cellular response

receptor activation

when the ligand/signaling molecule binds to the receptor on responding cell, turning the receptor on

what are the responses to receptor activation?

they vary:

1. enzymes: activation → shape/activity of enzyme changes

2. channel proteins: activation → opens/closes in response

3. some receptors activate other proteins in the cell

signal transduction

• the chain reaction (one molecule sets of another, etc)

• activation → transmits through cytoplasm and connects to intracellular proteins → signal transduction cascade

• the signal is amplified at every step of the pathway

  • low signal concentration → amplification → large impact on responding cell

cellular response

change in cellular behavior due to the signal

• ex: cell division, DNA replication, DNA expression (gene expression), protein synthesis, change in enzyme activation, etc

termination

stopping the signal transduction

methods to terminate

1. ligand detaches - receptor changes shape to inactive form → receptor becomes inactive

2. dephosphorylation - G proteins convert GTP → GDP, turning off and stopping further activation

3. any downstream protein could be activated

most signaling pathways are counteracted at one or more points to terminate a cell’s response

explain cell signaling in a flight or flight response

• signaling cell: cells of adrenal glands

• signaling molecule: adrenaline (travels by endocrine signaling)

• receptor protein: adrenaline receptors 

• responding cell: heart muscles

• cell response: causes heart to beat faster, leading to more effective oxygen delivery

explain cell signaling in bacteria

• signaling cell: pneumococcal cells

• signaling molecule: 17 amino acid peptide

• receptor protein: peptide receptor on surface of pneumococcal cells

• responding cell: pneumococcal cells 

• cell response: when enough receptors are bound (known by quorum sensing), an expression of genes for DNA uptake is triggered

quorum sensing

detects population density and responds by turning on specific genes across entire community

• used to control + coordinate different types of bacterial behaviors

• low cell density = low probability of receptor binding, and high cell density = high probability of receptor binding. more receptor binding = more DNA uptake

DNA uptake

when bacteria take in foreign DNA

endocrine signaling

• between cells far apart from each other

• carried by circulatory system (bloodstream)

• ex: adrenaline

paracrine signaling

• molecule travels to neighboring cell by diffusion

• ex: neurons release neurotransmitters that diffuse across a synapse

  • if responding cell = neuron: response releases more neurotransmitters + a nerve impulse

  • if responding cell = a muscle cell: muscle may contract

synapse

junction where nerve cell transmits a signal to another cell during paracrine signaling

growth factor

small soluble signaling molecule that tells cells to grow, divide, or survive (prevent cell death)

autocrine signaling

• signaling between different parts of one cell

• signaling cell = responding cell (they’re the same)

contact-dependent signaling

• direct contact between two cells

• typically a ligand + receptor on another cell

• how cells respond to threat

• ex: gap junctions in animal cells