Ch 16 Textbook Material Pt1 (general principles of cell signaling)

Why do cells in multicellular organisms use extracellular signal molecules?

to communicate and coordinate activities with other cells

What is endocrine signaling?

a “public” form of communication where signal molecules (hormones) are secreted into the bloodstream and carried throughout the body

What are hormones?

extracellular signal molecules secreted by endocrine cells that travel through the bloodstream

What cells produce hormones in animals?

endocrine cells

What is paracrine signaling?

a local form of signaling where signal molecule diffuse through the extracellular fluid to nearby cells

What are signal molecules that act locally called?

local mediators

Where is paracrine signaling often used?

to regulate inflammation or control cell proliferation in wound healing

What is autocrine signaling?

when a cell responds to the local mediators it produces itself

How do cancer cells sometimes use autocrine signaling?

they promote their own survival and proliferation

What is neuronal signaling?

a long-distance but specific form of communication carried out by neurons

How is neuronal signaling different from endocrine signaling?

its private and targeted, not broadcast widely

Where does a neuron’s axon deliver its message?

to a specific target cell at a synapse

What are neurotransmitters?

extracellular signal molecules released by neurons that cross the synaptic gap to reach target cells

What is contact-dependent signaling?

a direct, short-range communication where cells physically touch using membrane-bound signal and receptor proteins

Does contact-dependent signaling involve secreted molecules?

no, it requires direct physical contact between cells

What determines whether a cell will respond to a signal molecule?

whether the cell possesses the specific receptor for that signal

What are the two major classes of extracellular signal molecules?

• large/hydrophilic molecules: rely on cell-surface receptors

• small/hydrophobic molecules: pass through membranes and bind to intracellular receptors

What type of receptor do large or hydrophilic signals use?

cell-surface receptors

What type of receptor do small or hydrophobic signals use?

intracellular receptor proteins inside the cytosol or nucleus

What are effector proteins?

proteins that directly change cell behavior

• metabolism, movement, or gene expression

How can cells generate complex responses with few signals?

by combining signals in different ways to produce unique outcomes

What happens when intracellular signaling systems interact?

the presence of one signal can modify the effects of another, tailoring the cells reponse

What are three possible outcomes from different signal combinations?

• survival

• differentiation

• division

What happens if cells don’t receive proper survival signals?

they undergo apoptosis (programmed cell death)

What determines how quickly a cell responds to an extracellular signal?

it depends on what changes the signal triggers inside the cell

What causes some extracellular signals to act rapidly?

they alter the activity of proteins already present in the cell

Example of slow responses?

cell growth and cell division (can take hours)

What are most extracellular signal molecules made of?

proteins, peptides, or small hydrophilic molecules

Where do most extracellular signal molecules bind?

to cell-surface receptors that span the plasma membrane

What do transmembrane receptors do?

they detect signals outside the cell and relay messages inside

What is the first step in signal transduction?

the receptor recognizes the signal and creates and intracellular signal molecule

What can be the final outcomes (cellular responses) of a signaling pathway?

• metabolic enzyme activation

• cytoskeleton rearrangement

• gene switched on or off

List key functions of intracellular signaling components

• relay the signal onward

• amplify the signal

• integrate multiple signals

• distribute the signal to multiple effectors

• engage in feedback regulation

What is the role of scaffold proteins in signaling?

they organize and bring together signaling components to propagate the signal efficiently

What is feedback regulation in cell signaling?

when downstream components regulate earlier steps in the signaling pathway

What does positive feedback do?

enhances the response to the signal

• can produce all-or-none, switchlike effects

What does negative feedback do?

inhibits earlier components

• can cause oscillating on/off responses

Why is feedback important in biological systems?

it allows fine-tuning and control of cellular responses to signals

Acetylcholine

• slows heart pacemaker cell firing

• stimulates saliva secretion in salivary cells

• triggers contraction in skeletal muscle

What are molecular switches in intracellular signaling?

proteins that toggle between inactive and active states in response to signals, controlling other proteins in signaling pathways

Why is the switching-off process important in signaling pathways?

it resets proteins to their original state

• allows the pathway to recover and be ready for new signals

What are the two main classes of molecular switches?

• proteins regulated by phosphorylation

• GTP-binding proteins

How are phosphorylation-controlled proteins switched on and off?

• on: by protein kinases (add phosphate)

• off: by protein phosphatases (remove phosphate)

What are the two main types of protein kinases in signaling pathways?

• serine/threonine kinases

• tyrosine kinases

How do GTP-binding proteins act as switches?

• they are active when bound to GTP

• inactive when bound to GDP

How do GTP-binding proteins switch themselves off?

they hydrolyze their bound GTP to GDP using intrinsic GTPase activity

What are the two main types of GTP-binding proteins?

• large, trimeric G proteins

• small, monometric GTPases

What do trimeric GTP-binding proteins (G proteins) relay messages from?

G-protein-coupled receptors (GPCRs)

What do small, monomeric GTPases help relay signals from?

enzyme-coupled receptors (and other cell-surface receptors)